Ram buffer memory circuit system for train printer-data link processor
FIELD OF THE INVENTION 
     This disclosure relates to the field of devices called peripheral-controllers and specifically applies to data link processors used as an I/O controller for a host computer in order to operate a peripheral unit known as a train printer mechanism. 
     CROSS REFERENCES TO RELATED APPLICATIONS AND PATENTS 
     The following cases which involve the use of an input-output subsystem connecting a main host computer and various peripheral units are included herein by reference: 
     U.S. Pat. No. 4,162,520, inventors Darwen Cook and Donald Millers, II. The case described the peripheral-controller known as a line control processor which controlled and handled data transfers between a given peripheral terminal unit and the main host system. 
     U.S. Pat. No. 4,074,352 entitled &#34;Modular Block Unit for I/O Subsystem&#34;, inventors Darwen Cook and Donald Millers, II. This case described a base module unit which housed and supported a group of eight peripheral-controllers and interfaced them to a main host computer system. 
     U.S. Pat. No. 4,106,092, entitled &#34;Interface System Providing Interfaces to Central Processing Unit and Modular Processor-Controllers for I/O Subsystem&#34;, inventor Donald Millers II. This patent described a unit in the main host system, designated as an I/O translator or &#34;IOT&#34;, which controls and regulates data transfer between the main host system and a plurality of base modules and their peripheral-controllers. 
     U.S. Pat. No. 4,189,769, entitled &#34;Input/Output Subsystem for Digital Data Processor System&#34;, Inventors Darwen Cook and Donald Millers, II. This case describes a subsystem wherein a plurality of peripheral-controllers are organized in base modules for data communications with a main host system. The peripheral-controllers and the base module form an input-output subsystem for controlling data transfers to/from a large number of peripheral units to the main host computer system. 
     U.S. Pat. No. 4,322,792, issued Mar. 30, 1982, entitled &#34;Common Front End Control for Peripheral Controller&#34;, Inventor Kenneth Baun. 
     U.S. Pat. No. 4,313,162, issued Jan. 26, 1982 entitled &#34;I/O Subsystem Using Data Link Processors&#34;, Inventors Kenneth Baun and Donald Millers II. 
     The above issued patents form a foundation and background for the present application and are included by reference into this specification. 
     SUMMARY OF THE INVENTION 
     The basic functions of a data link processor (or intelligent I/O interface control unit as it was previously called in the cited patents) is to provide the specific processing and control functions for data transfers between a specific peripheral and a main host computer system. 
     There has newly been developed an improved peripheral controller (data link processor) which, instead of being individually designed and oriented for one particular type of peripheral, now has isolated certain common function elements for all types of peripheral-controllers to form a data link processor of improved capability. The common-to-all unit is designated as the common front end (CFE) which forms a basic part of the data link processor. This common front end, when used in conjunction with a peripheral dependent logic board (PDB) (which adapts the data link processor to a particular peripheral unit) is known as a Common I/O data link processor. 
     The line control processor (peripheral-controller) in U.S. Pat. No. 4,162,520 was described as an input-output interface data transfer control unit. While many of the general overall operating functions remain the same in regard to following the commands of I/O descriptors from the main system and the returning of Result Descriptors to the main system, the improved peripheral-controller now designated as the &#34;data link processor&#34; (DLP) provides a PROM with micro-code words for use of the required control functions rather than the complexities of processor logic and plurality of multiplexors used in the peripheral-controller line control processor described in U.S. Pat. No. 4,162,520. 
     A peripheral-controller, for controlling the printout of data via a train-printer mechanism, has a specialized RAM buffer memory with dedicated portions for control data and dedicated portions for two special buffer storage areas. One area is named PIB or print image buffer. It stores the data (received from the host system) to be subsequently printed by the train-printer. 
     The second special area is the TIB or train image buffer which stores codes representing the character data on the train blocks of a rotating train. The special buffers make it possible for comparison scans in each buffer to be compared for a match which indicates a desired character is juxtaposed to a print-hammer which can now be fired to print the character in the appropriate column of the paper. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall system diagram showing the relationship of the data link processor to the main host system and the peripheral terminal unit, designated as the train printer. 
     FIG. 2 is a block diagram of a plurality of typical base modules which house and support a plurality of data link processors. 
     FIG. 3 is a schematic drawing of the printed circuit cards which make up the data link processor. 
     FIGS. 4A1 and 4A2 constitute a circuit diagram of the common front end (CFE) card of the data link processor; FIG. 4B is a circuit of the common front end clear circuitry; FIG. 4C shows the clock control circuitry for the CFE; FIG. 4D shows the connection logic circuitry of the common front end; FIG. 4E is a timing diagram showing how data transfers are effectuated during certain clock periods. 
     FIGS. 5A1-5 constitute a circuit block diagram for the entire data link processor used to control the train printer peripheral; FIGS. 5B1-2 constitute a schematic drawing of the RAM buffer memory storage; FIGS. 5C1-2 illustrate portions of memory designated print image buffer and train image buffer; FIGS. 5D1-2 constitute a timing and signal diagram for a typical 1100 LPM train printer; FIG. 5E is a simplified basic timing diagram for basic functions of the train printer-data link processor; 
     FIG. 5F is a flow chart illustrating use of the train image buffer memory area; FIG. 5G is a schematic drawing of the column done register; FIG. 5H is a block diagram illustrating how data bus characters are error-checked; FIG. 5I is a diagram of the vertical parity generation check circuit; FIG. 5J shows the delimiter detection circuit; FIG. 5K is a block diagram of the stop bit generator circuit; FIGS. 5L1-2 constitute a simplified block diagram of the compare logic circuit; FIG. 5M is a diagram of the line-turn logic circuitry. 
     The overall environment or system configuration in which a data link processor, such as a train printer data link processor (TP-DLP) operates, is shown in FIG. 1. The main host computer system 10 having a host dependent port 10 p , connects message level interface buses 15 i  to specialized peripheral-controllers such as data link processor (DLP) 20 10  and 20 11  through a distribution control card 20 id . Likewise, the message level interface 15 i  connects to a distribution control card (DC) 20 0d , which supports a plurality of other data link processors, such as the train printer-DLP, 20 03 , FIG. 1. 
     In the particularly preferred system embodiment for the train printer-data link processor, the DLP 20 03  (FIG. 1) is connected by bus 101 to a train printer mechanism (peripheral) such as is described in Burroughs Corporation publication form 1094802 copyright 1976 by Burroughs Corporation, Detroit, Michigan 48232. 
     FIG. 2 shows a more generalized situation in which a host computer 10 is connected by a message level interface 15 i  to two base modules 20 0  and 20 1 . Each of these base modules has its own respective distribution cards (DC) respectively designated 20 0d  and 20 1d . 
     Each base module supports a plurality of data link processors (peripheral-controllers). For example, base module 20 0  may have a console data link processor 20 00  which controls a console unit 50 c . Likewise, data link processor 20 01 , 20 02  may respectively handle control and data communication transfers between items such as a card printer, card punch or other peripheral units. Each particular data link processor is specifically oriented for serving the requirement of each type of peripheral unit. 
     Likewise, base module 20 1  has its own distribution card (DC) 20 1d  which can connect to data link processors such as 20 10 , 20 11 , each of which data link processors is specifically tailored to handle the data transfer and control operation for a particular type of peripheral terminal unit. Each base module will be seen to have its own maintenance card unit (20 0m  for base module 20 0  and maintenance card 20 1m  for base module 20 1 ). The maintenance card provides diagnostic and maintenance coperations for the data link processors of a given base module. 
     As previously described in U.S. Pat. Nos. 4,106,092 and 4,162,520, each base module has a series of backplane connectors and grooved slides whereby a large number of printed circuit-component cards may be inserted and slid-in for connection to the backplane of the base module. Thus, the distribution card 20 0d , the maintenance card 20 0m  and other printed circuit cards which constitute the peripheral-controllers or other function cards, may be inserted and connected into the common backplane of the base module. 
     FIG. 3 is a schematic illustration of how a specific data link processor (peripheral-controller) may be physically structured with slide-in printed circuit cards. Thus, a common front end card 10 c  is seen having a series of backplane connections 10 b , for connection to the motherboard of the base module, and having frontplane or foreplane connectors 80 a1 , 80 b1 , 80 c1 , and DLP address jumper pins 83 which are used to set the specific address of the particular data link processor (DLP) involved. 
     Connected to the CFE card 10 c  by means of the foreplane connectors is the peripheral dependent board (PDB) card 80. This card has foreplane connectors 80 a , 80 b , 80 c , which connect to its associated common front end (CFE) card 10 c . The peripheral dependent board 80 also has a foreplane connector 80 d  for connection to the train printer peripheral unit. The PDB card 80 has jumper pins 82 for identification (ID) whereby an encoded identification signal for the board can be effectuated. 
     Thus, the specialized data link processor (peripheral controller) as 20 03  can be seen to comprise two slide-in cards, one of which, the common front end card 10 c , is common to all types of peripheral controllers; while the peripheral dependent board, PDB card 80, is a specialized card providing the circuitry functions required for the control, handling and specialized protocols particularly required for a specific type of peripheral unit. 
     OVERALL SYSTEM DESCRIPTION 
     As previously discussed in U.S. Pat. Nos. 4,162,520 and 4,106,092, use was made of an input/output subsystem operating between a main host computer and one or more peripheral devices. The input/output subsystem used a plurlity of base module cabinets, each of which supported a group of base modules. The base modules consisted of up to eight &#34;Line Control Processors&#34; (peripheral-controllers) which were designated as intelligent input/output processing units which controlled data transfers between any specific peripheral device and the main host system. Since these line control processors (LCP) provided an identification code or &#34;data link&#34; for each task assigned by the main host system, these Line Control Processors came, in subsequent design, to be known as Data Link Processors (DLP) and henceforth will be referred to by the term of data link processors. 
     The data link processor I/O subsystem interfaces a variety of peripheral (I/O) devices with one, or more than one, host system. This setup simplifies the I/O device maintainability by allowing maintenance of the I/O subsystem to be performed without being specifically tailored to the vagaries of main host systems. The data link processor subsystem is distinguished by its adaptability to any host computer system having the MLI (message level interface) capabilities described in U.S. Pat. Nos. 4,162,520 and 4,106,092. 
     The data link processor I/O subsystem can be utilized by host systems having totally different internal characteristics, for example, the same I/O subsystem may be shared by any combination of different Burroughs B 900 systems, as the B 2900, B 3900, B 4900 and the B 6900 system. 
     The message level interface is a connected line discipline by which the host system or host systems communicate with the I/O subsystem. In the preferred embodiment of the latest configuration, each of the host systems utilizes a unit called a Host Dependent Port (HDP) to achieve the message level communication. 
     The peripheral devices which are attached to and operate with a data link processor I/O subsystem operate in a semiautonomous environment, that is, a host system must initiate a peripheral device to perform an operation, by sending the data link processor an I/O descriptor. After the DLP is initiated, it (DLP) performs the operation (OP) independently of the host system. At the conclusion of the operation, the DLP reconnects back to the host system which initiated the operation and returns information called a &#34;result descriptor&#34; (R/D). During this operation, the DLP can &#34;reconnect&#34; to the host system to transfer data via the data link processor. 
     The ability of a DLP to disconnect from a host system and proceed with an I/O transfer operation frees the host system to perform other operations during the time that an I/O transfer operation is in progress within the data link processor subsystem. 
     The ability to reconnect to a host system requires the data link processor to contain a &#34;linking mechanism&#34; to re-establish the communication path between the peripheral device and the host system that initiated that peripheral device. The logic circuit that provides for this reconnection is contained in the data link processor. Alternatively the logic circuit that provides for this connection and reconnection may be contained in a system controller (SC) or a unit such as the IOT (input/output translator unit, described in the aforementioned U.S. Patents). Every peripheral device that is connected to the DLP I/O subsystem uses one of this type of &#34;connection--reconnection&#34; control devices. 
     The message level interface that connects the host system to a DLP I/O subsystem is capable of conducting all communication between the host system and the peripheral subsystem including connection, reconnection and disconnection. 
     Two types of communications are made use of between the host system and the I/O subsystem. These are (a) &#34;control&#34; communications done by control levels and (b) &#34;data transferring&#34; communications. In both types of communication information (either control levels or data) pass between the host system and the peripheral subsystem. 
     When information passes from the main host system to the DLP I/O subsystem, the operation is described as a &#34;Write&#34; operation. When information is passed from the I/O subsystem to the main host system, the operation is described as a &#34;Read&#34; operation. For example, when a peripheral device requests to be reconnected to the main host system or to another available main host system, the communication starts in the I/O subsystem with information passed on to the host system and is, therefore, a &#34;Read&#34; type of operation. When a host system initiates a peripheral device and the communication starts from the host system with information being passed from the host system to the peripheral, this is called a &#34;Write&#34; type of operation. 
     Those peripheral devices of the &#34;input&#34; type will produce only &#34;Read&#34; data for the host system. Those peripheral devices of the &#34;output&#34; type will only receive data from the host system, i.e., a &#34;Write data&#34; operation. Thus, input or output peripheral units require either a &#34;Read data&#34; or a &#34;Write data&#34; operation. Read data and Write data are not the same thing as &#34;read or write&#34; operations because &#34;read or write&#34; only describes the direction of the flow of information on the message level interface. A read function performed by an input peripheral device requires a &#34;Write&#34; operation to initiate the peripheral device first, followed by a &#34;Read&#34; operation to pass the data to the host system. 
     As described in U.S. Pat. No. 4,074,352, the LCP base module is the basic building block for an input-output subsystem. In the case where a universally &#34;Common Front End&#34; card is used in each data link processor (DLP), the subsystem can be designated as the &#34;Common I/O Subsystem&#34;. The DLP base module consists of a backplane which connects slide-in cards, any two of which form a DLP. A base module consists of one to six distribution cards (DC) for communication to/from the system and houses from one to eight data link processors (DLP&#39;s), each of which handles data communications to/from a specific peripheral terminal unit. Each base module also includes one maintenance card (MC), two termination boards and space for additional cards for specialized purposes, but which are not essential to a normal operating situation. 
     Each data link processor (DLP) consists of a common front end card (CFE) and peripheral dependent logic which is located on separate circuit board known as the peripheral dependent board (PDB). Read only memories designated as PROMs located on the common front end card contain micro-code words which are the only elements dependent on the type of peripheral device used, the remaining of the elements being standard independent components. The common front end card and the peripheral dependent board communicate to each other via three 50-pin frontplane connectors seen in FIG. 3. 
     The data link processor (20 03 , FIG. 1) contains the hardware and power required to interface the host dependent port 10 p  of the main host system 10 with a plurality of peripheral devices 53. The hardware of the subsystem consists of a base module as 20 0 , FIG. 2, which include the following: 
     1. a control console (50 c , FIG. 2) 
     2. the message level interface cabling (MLI), 15 i . 
     3. the base backplane 20 0B  for the data link processors, FIG. 3. 
     4. the base modules which fit into the cabinet and connect to the base backplane. 
     5. the data link processors (DLP) which are typically organized such that 8 data link processors are supported by each of the base modules (FIG. 2). 
     As described in the heretofore mentioned patents, the base modules were fitted with one or more distribution card (DC) depending on the number of host systems which could be connected to that base module, plus a termination card and a maintenance card (MC). 
     As described in the afore-cited patents, the &#34;poll test&#34; is defined as the procedure used by a host system and its host dependent port to establish communication with a DLP subsystem. The host system initiates a connection to a data link processor by performing a &#34;poll test&#34; sequence. The result of this sequence is as follows: 
     1. a connection is made from the host system to the data link processor that also includes the status condition of the addressed data link processor. 
     2. a signal indication that a particular data link processor is &#34;not active&#34; in that particular DLP address, that is, the DLP is not physically present, or it has been taken off-line by the maintenance card, or it is unavailable because of a PROM parity error. 
     3. a signal indication that another distribution card (DC) is actively engaged in the particular base module and thus a path to the requested DLP is not available at this time. 
     4. a signal indication that the address requested did not carry the proper parity. 
     To begin a &#34;poll test&#34;, the host system 10 sends a Channel Select to the particular base module that it has selected to be connected. The Channel Select signal provides an indication to the distribution card (DC) that it has been selected for a poll test. The host system sends the address for the DLP on data lines D-8 (MSB, most significant bit) through D-1 (LSB, least significant bit). 
     A LEM (line expansion module) card can be connected to a base module to enable 16 DLPs to be connected to a signal MLI 15 i . A BCC (base control card) can be used to provide identity signals for a base module. 
     If a line expansion module (LEM) is in use, the host system sends the distribution card the address of the requested base module on data lines C-8 (MSB) through C-1 (LSB). If a base control card (BCC) is to be addressed, then bit A-8 is used. The host dependent port (HDP) then sends Address Select to all of its base modules. This causes the distribution cards that receive the Channel Select to begin a poll test, and signifies to all other distribution cards that the host system interface is busy. This busy indication is needed by non-connected distribution cards to resolve distribution card priority in handling DLP requests for connection in situation where there is a multiple distribution card base environment. 
     The distribution card (DC), which is selected by the Channel Select and the Address Select signal, acknowledges by returning a DLP strobe signal back to the host system 10. If another distribution card is actively engaged on the base module backplane, or a distribution card of higher priority is also taking a poll test connection, then a &#34;port busy&#34; indication accompanies the acknowledge signal. If the vertical parity carried on the address is incorrect, a parity error indication accompanies the acknowledged signal. If the base module is available for connection and the address parity is correct, then the host system receives only the acknowledge signal. The distribution card addresses and connects to the addressed DLP. 
     The distribution card connects with the selected DLP by decoding the DLP address and driving one of eight address lines. The DLP specified by the address line has the address receiver jumpered to the correct address line. The DLP receiving this address sends a DLP connected signal (LCPCON) back to the host system to indicate its presence. 
     Upon receiving an &#34;acknowledge&#34; signal, and no indication of a port busy or parity error signal, the host system drops its Channel Select signal. This makes all lines available for connected communication. However, if an exception condition is reported, the host system must disconnect from the DLP. 
     When the distribution card detects the absence of a Channel Select, it sends another acknowledge (strobe signal). If the DLP addressed, has responded with LCPCON, the distribution card &#34;enables&#34; the DLP&#39;s status signal to the host system with this second &#34;acknowledge&#34;. With the second acknowledge, the DLP and host system are connected via the distribution card. If the selected DLP did not respond (because it was off-line or not installed) the host system receives (together with the acknowledge signal) a DLP status signal of &#34;zero&#34; indicating that the DLP is not available. 
     If the DLP is unable to handle a poll test at this time (because it is busy) the DLP status indicates this condition. It is the function of the host system to detect and to respond to this condition by &#34;disconnecting&#34; or converting to a poll request. The connection established with this &#34;poll test&#34; remins intact as long as &#34;address select&#34; remains true. 
     The DLP participation in the poll test-host system connection routine is minimal. The DLP only receives its unique address line and returns LCPCON in acknowledgement and enables the status line. If there are multiple distribution cards installed in a base module, then access to the backplane of the base module is achieved by a request granted mechanism in the path selection module (PSM). The PSM also provides for distribution card priority resolution. POLL TEST TO POLL REQUEST CONVERSION: If the host system 10 is performing a poll test, and the initial DLP status indicates that the DLP desires access to the host system for information exchange because of a previous I/O descriptor, the host system has three options: 
     (a) disconnect 
     (b) selectively clear a DLP 
     (c) handle and service the DLP 
     If the option (c) is selected, the host system converts to a &#34;poll request&#34;. A &#34;poll request&#34; is defined as the procedure used by a data link processor to establish communication with a host system. A &#34;poll request&#34; is the opposite of a &#34;poll test&#34; because the origination of the communication is reversed, that is, with the DLP being the transmitter and the host system being the receiver and responder. 
     In conjunction with any initial status in the &#34;read&#34; direction (information flow toward the host system) the distribution card sends the DLP data toward the host system. If the initial status is &#34;Send descriptor link&#34;, this data is the first word of the descriptor link and contains the host return field. The host system must check this field to be certain that it can handle the DLP information transfer. If the host system cannot handle it, the DLP must &#34;disconnect&#34; before sending any host system strobes. DLP INITIATED CONNECTION (POLL REQUEST): When access is required to the host system and a disconnection has occurred, the DLP re-establishes connection by initiating a &#34;poll request&#34; sequence. Because all DLP&#39;s may request connection at the same time, a decision is made on a priority basis. Priority is partitioned into two types--(a) global priority, and (b) base module priority. The global priority of a DLP is based on the type of peripheral device which it services. There are 6 standard levels of global priority for DLPs. The global priority of a particular DLP is assigned with regard to the host system access requirements of the peripheral device, such as speed, stream mode, etc. 
     An additional and higher level of global priority is provided to designate an emergency request. This is designated as global priority equal to 7. An emergency request is defined as a condition requiring immediate access to the host system in order to avoid difficulty in error recovery or operator action. Global priority is used by the main host system to determine the priority of each requesting distribution card. Base module priority is the priority of each DLP within the base. Base module priority is used by the distribution card to determine the priority of each requesting DLP. 
     Base module priority is determined by the DLP number of each DLP in the base module. This number corresponds to the DLP address jumpered on each DLP, for example, the DLP address 7 is equal to DLP number 7 which is equal to the base module priority equal to 7. The established priority levels are arranged such that the highest base module priority is 7 while the lowest is 0. Only one DLP within each base module is assigned to each priority number. Thus, a base module has been organized to contain a maximum of up to 8 DLPs. 
     To begin a poll request, the DLP raises the &#34;request&#34; level jumpered to one of eight DLP request lines in the DLP, depending upon and equal to tha address of the DLP. Upon detecting a DLP request, if the host system is idle, the distribution card raises an interrupt request to the host system. When the host system detects &#34;Interrupt Request&#34; it sends &#34;Access Granted&#34; to all the base modules and begins a poll request sequence. Access Granted allows all distribution cards that have requests on-line to begin poll requesting. Access Granted also signifies that the message level interface (MLI) is busy to all distribution cards that are not requesting. 
     CONNECTED STATE: In the connected state, the distribution card (DC) provides the communication path between the host system and the selected data link processor. All communication between the host system and the base module is asynchronous. This requires that send/acknowledge on the message level interface be pulses rather than levels. The host system send/acknowledge (AG+SIO) and the DLP send/acknowledge (LCPST/) are pulses on the message level interface (MLI). 
     The base module of the I/O subsystem can support up to eight data link processors which are mounted on slide-in cards which connect to the backplane of the base module. In each base module facilities are provided for slide-in cards which can be installed and which work with the data link processors and the main host system. These cards may be summarized as follows: 
     Distribution Card (DC): 
     The distribution card provides an electrical and functional interface between a given DLP base module and a host system. The circuitry of this card provides host system connection for 1-8 data link processors (DLPs). The routines for establishing a connection between a given DLP and the host system are implemented by logic circuitry contained on the distribution card. A connection can be initiated by either the host system or by a data link processor. When the host system initiates the connections, this is designated as &#34;poll test&#34;; when the data link processor initiates connections, these are designated as &#34;poll requests&#34;. 
     All communications between a host system and a base module are asynchronous. The distribution card synchronizes this communication. The distribution card provides for the capabilities of up to 6 transfer paths into a base module from one or more host systems. Each separate path into a base module requires a separate distribution card, if a multiple configuration is used, then a card known as the PSM or path selection module is required. Once a distribution card has provided a connection between the host system and a selected data link processor, the distribution card assumes a &#34;connected&#34; state and then becomes practically transparent to communication transfers between the host system and the selected data link processor. 
     PATH SELECTION MODULE (PSM): The path selection module is a card inserted into the base module and is required when the base module contains two or more distribution cards. The path selection module governs access to the base backplane, and it selects and routes the DLP request, in addition to also handling Master Clear of all base modules and in addition to Selective Clear of selected base modules. 
     BCML/TTL CONVERTER (BURROUGHS CURRENT MODE LOGIC/TRANSISTOR TYPE LOGIC) CONVERTER: This optional converter card is used to interface a main system host dependent port (HDP) that is using Burroughs current mode logic architecture with the standard transistor type logic format of the data link processor subsystem. 
     TERMINATION CARDS: Each DLP base standardly requires two termination cards (TC). These cards are designated as numbers 1 and 2 and contain the required transistors and capacitors in order to properly balance and load the base module backplane lines. 
     MAINTENANCE CARD: Since a single backplane is provided in common for all of the data link processors of a given base module, this permits a large proportion of the DLP maintenance to be centralized. This centralized maintenance circuitry is located on the maintenance card that is installed at one end of the DLP base module (opposite from the position of the distribution card). The maintenance card contains the clock generation circuitry for the entire base module, in addition to other maintenance circuitry, such as circuitry for diagnostic procedures. These diagnostic procedures include the ability to stimulate the peripheral interface, to control the clock to the data link processor, and to access the storage elements of the data link processor for comparison with standardly expected results. 
     As stated, the clock generation logic circuitry of the entire base module is located on the maintenance card. Since every card in a given base module receives this signal, the actual signal distribution has been divided into two lines in order to halve the driver requirements. The actual clock is an 8-megahertz, 50% duty cycle positive pulse on the DLP backplane. 
     DATA LINK PROCESSOR (DLP): A data link processor provides the control logic and the host system interface function for the associated peripheral units which it services. The DLP logic that controls the peripheral device is device-dependent; therefore, some DLPs contain different hardware and/or different micro-codes. Communication with the main host system is via the message level interface by way of the distribution card. Requests for host system access originate from a data link processor. The DLP disconnects from the host system while transferring information to or from the peripheral device it services. A DLP consists of two or more cards (generally 10 by 13 inches each). Each card can contain a maximum of 96 TTL chips arranged in a 6×16 array. The cards plug into adjoining slots in the base module backplane. A data link processor is made up of a common front end card (CFE) and one or more peripheral dependent boards (PDB). The CFE and first PDB are connected together by three foreplane connectors, containing 50-pins each (FIG. 3). If more than one PDB is required for a CFE, then the PDB cards are connected to each other by a fourth 50-pin foreplane connector. The CFE card contains the necessary logic to store and execute the micro-code that is dictated by the particular peripheral dependent board being used. Random access memory (RAM) chips and programmable read-only memory (PROM) chips are located on the common front end card. The unique logic required by a particular peripheral device is located on the PDB 80 and can vary according to the particular type of peripheral device being serviced. On the other hand, the CFE 10 c  is a standardized card which provides functions common to all types of peripheral dependent boards and peripheral devices serviced. 
     The function of the DLP is to interface a particular peripheral device to the main host system. The required communication between the host system and the DLP is standardized in method and content. Every DLP communicates to the main host system using this standard discipline. The DLP masks the peripheral idiosyncrasies from the host system. Thus, the DLP is independent of the host system because of this. The data link processor performs its peripheral-controlling functions in response to information received over its interface to the host system. 
     Data link processor communication with the host system is accomplished by use of standard DLP &#34;status&#34; states. These status states indicate to the host system the requirements of the DLP (at any given unit of time). Once a connection is established between a data link processor and a host system, all information transfers are performed as indicated by the DLP &#34;status&#34; signal. Thus, the entire communication process is oriented about the DLP and is &#34;DLP status driven&#34;. The &#34;status state signals&#34; are common for every type of data link processor. This has been described in detail in U.S. Pat. No. 4,162,520. 
     Every data link processor contains a message buffer of a size sufficiently applicable to its associated type of peripheral device. Data transmissions between the DLP and the host system operate to and from this message buffer. This eliminates access errors for peripherals having fixed record lengths such as card readers, printers or card punches. Stream mode devices such as tapes and disks contain and utilize two or more buffers to accomplish variable length message transfers. 
     DLP STATUS FLOW SIGNALS: The DLP status flow is designed to allow orderly message transfers. When it is said that the host system is &#34;DLP status driven&#34;, this means that the DLP indicates its requirements to the host system by sending its status state signal. These status states are transmitted continuously by the DLP once it is connected to the host system. The host system examines these status lines at DLP &#34;send/acknowledge&#34; time. Every DLP implements this common signal flow in a manner designed to provide as much commonality as possible so that each DLP will have a standardized routine. Status states are generated for host system usage only; they are not used logically by the DLP. They are used to make the host system aware of the internal operation routine of the DLP. Thus, this allows the host system to anticipate somewhat the requirements of the DLP. 
     STATUS STATE SIGNALS FOR THE DATA LINK PROCESSOR: As was previously described in U.S. Pat. No. 4,162,520 on column 70 and in connection with FIG. 6A and Table VII, each condition or situation of the operating routine of the DLP will have a Mnemonic and a &#34;status state count number&#34;. In the present system which uses the data link processor and a host dependent port, HDP, rather than the input/output translator (IOT) of the above mentioned patent, the following Table O-SS outlines the description of each of the status states of the data link processor in a generalized sense. It will be seen that each of the states follow the same routine pattern of that shown for the earlier line control processors. Slight variations in meaning occur for each type of DLP according to the type of peripheral it services. A specific table for the Train Printer DLP will be presented hereinafter. 
     
                                           TABLE O-SS                              
__________________________________________________________________________
(Generalized Pattern)                                                     
Status -    Status State                                                  
Mnemonic    Count Usage or Meaning                                        
__________________________________________________________________________
Cleared     STC=0 Entered by the DLP when it is cleared. This             
                  status is also shown if the DLP is not present,         
                  it has a PROM parity error, or it has been              
                  taken off-line by the Maintenance Card.                 
Disconnect  STC=1 Used by the DLP to indicate that no more                
                  transfers are possible during the connection, or        
                  to indicate that the DLP is unable to accept a          
                  new I/O descriptor.                                     
Reserved    STC=2 Reserved for expansion. Detection of this status        
                  results in an error.                                    
Idle        STC=3 Indicates that the DLP can accept a new I/O             
                  descriptor, or that a DLP receiving this new            
                  descriptor requires additional descriptor words.        
Read        STC=4 Indicates that data is being transferred to the         
                  host system by the DLP.                                 
Send Descriptor Link                                                      
            STC=5 Indicates that the descriptor link is being sent,       
                  to the host system.                                     
Receive Descriptor Link                                                   
            STC=6 Indicates that the DLP needs to receive, or is          
                  receiving the descriptor link.                          
Result Descriptor                                                         
            STC=7 Indicates that the result descriptor is being sent      
                  to the host system.                                     
Write       STC=8 Indicates that the DLP needs data from the              
                  host system.                                            
Encoded Status                                                            
            STC=9 Indicates that the DLP is sending special status        
                  information on the data lines.                          
Port Busy   STC=10                                                        
                  The host is waiting on port busy, but the LEM           
                  has a request from another DLP.                         
I/O Descriptor LPW                                                        
            STC=11                                                        
                  Indicates that the DLP requires the I/O                 
                  descriptor LPW.                                         
Break       STC=12                                                        
                  Indicates the end of a data message, and the            
                  DLP now wants an LPW.                                   
Break Enable                                                              
            STC=13                                                        
                  Indicates the desire by the DLP to transmit             
                  another message to the host system. The host            
                  system may accept or refuse this request.               
Character Transfer                                                        
            STC=14                                                        
                  Used by certain DLPs to resolve the contents            
                  of the last data word that has been received            
                  from the host system.                                   
Result Descriptor LPW                                                     
            STC=15                                                        
                  Indicates that the final word of the result             
                  descriptor is being sent to the host system and         
                  is followed by the appropriate LPW.                     
__________________________________________________________________________
 
    
     INFORMATION TRANSFERS: All information transfers between the DLP I/O subsystem and the host system are asynchronous and accomplished by means of control strobes. On the other hand, transfers between distribution card and the DLP are synchronous. The various types of information transfers are briefly described hereinunder. 
     SYSTEM TRANSMIT: When information is ready to be sent to that I/O subsystem, the host system emits a pulse (SIO) to the DLP I/O subsystem. When the subsystem has received data it then emits a pulse (LCPST) over to the host system. At this point the next transfer takes place beginning with a SIO signal. Resynchronizing of the asynchronous signals occurs in the distribution card (DC). Upon receipt of the host system strobe, the distribution card synchronously sets the STIOL level to the DLP. STIOL is synchronously reset when LCPSTL is &#34;true&#34; from the DLP. The answer to the host system occurs as soon as LCPSTL is &#34;true&#34; from the DLP. If this is the case, STIOL is &#34;true&#34; for only one clock period and the host system strobes are immediately answered. Data from this system is latched in the distribution card on the trailing edge of the host system strobe. 
     SYSTEM RECEIVE: When the host system can accept another word of data from the DLP I/O subsystem, it emits a pulse (SIO) to the subsystem. In turn, when the subsystem is capable of sending a new word it emits a pulse (LCPSTL) to the host system. Upon receipt of the host system strobe, the distribution card synchronously sets the STIOL &#34;level&#34; to indicate that a new cycle can begin. The new cycle is completed and the host system is strobed when LCPSTL is &#34;true&#34; and STIOL is synchronously reset. LCPSTL can be &#34;true&#34; before the SIO is received. If this is the case, STIOL is &#34;true&#34; for only one clock period and the host system&#39;s strobe pulses are immediately followed by subsystem strobe pulses with new data. Data to the host system is latched in the distribution card on the leading edge on the subsystem strobe pulse to the host system. 
     LINE TURN: In the course of a message transfer, it is sometimes necessary to change the information direction. The host system and the DLP coordinate in this turnaround of bi-directional lines. The DLP controls the data direction in the base module with a backplane line called I/O send (IOSND/). I/O send, when low, directs the data lines toward flow into the host system. The DLP initiates the &#34;line turn&#34; with a status transition that requires a change in information direction. Two situations occur here: 
     1. Host System Transmit to Host System Receive: If the host system detects a status change when it receives an acknowledge (on the information transfer that requires it to receive information) the host system sends another strobe to acknowledge the &#34;status change&#34;. The DLP detecting the host systems &#34;acknowledge&#34; raises the I/O send and begins transmission to the host system. 
     2. Host System Receive to Host System Transmit: If the host system acknowledges an information transfer in conjunction with a status change that requires a line turnaround, the DLP inactivates the I/O send and sends another DLP strobe to the host system. When the host system receives the &#34;acknowledge&#34; (that the base lines have been turned), then the host system begins to transmit to the DLP. 
     DLP BASE ADDRESS: As the DLP base backplane is made up of common lines running the length of the base, the printed circuit cards, which form the DLP, can function in almost any set of slide-in card locations. The base address chosen for a DLP is &#34;jumpered&#34; on the DLP card (FIG. 3). The base address serves only to make the DLP unique within the base. The DLP&#39;s global priority is not affected by its base address. This priority is selected on the distribution card (DC). 
     There are two backplane line functions that are linear: the DLP request and the DLP address. They are allocated eight lines each (O-7). The request and address jumpers must correspond. 
     DLP ID (IDENTIFICATION): Upon receiving a TEST/ID OP code, the DLP returns a two-word result descriptor (R/D). The second word contains DLP ID information. Digits A and B of the ID word are a predetermined bit pattern specifying the type of DLP. Digits C and D of the ID word are a bit pattern specified by field-installed jumpers, and are used to uniquely identify the DLP. The ID word for the DLP is formatted as follows:______________________________________ ##STR1##______________________________________ 
     I/O DESCRIPTORS: The I/O descriptor is transmitted in 17 parallel bits including odd parity. The DLP OP codes are limited to the following four types: 
     1. Read 
     2. Write 
     3. Test 
     4. Echo 
     Operations that do not transfer data are considered &#34;test&#34;. Thus, a test is defined as an operation that results in the host system receiving a result descriptor only. Echo is a maintenance operation that causes the DLP to accept a buffer load of data from the host system, and then return it back to the host system. This allows a quick confidence check of the I/O data path as far as the DLP buffer is concerned. Also, the various translation logics can be checked by an echo operation. 
     DLPS that require further information associated with the basic operations obtain that information in the form of variants. The first I/O descriptor transfer contains the four OP code bits and up to 12 variant code bits. Further variants are transferred in 16 parallel bit increments and are not limited to any size. 
     RESULT DESCRIPTORS: A result descriptor is transmitted to the host system along 17 parallel bits including odd parity. The first four bits (one digit) of the first word of the DLP result descriptor are common for every DLP. These first four bits are shown as follows: 
     
         ______________________________________                                    
Bit          Significance                                                 
______________________________________                                    
A8           Not ready peripheral                                         
A4           I/O Descriptor error                                         
A2           MLI Vertical parity error                                    
A1           MLI longitudinal parity error                                
______________________________________                                    
 
    
     The B, C and D digits of the &#34;DLP result descriptor&#34; will vary according to the type of data link processor involved. 
     FIG. 4A shows a block diagram of the common front end, 10 c , (which is sometimes designated as a &#34;Common I/O&#34; unit). The maintenance control bus 30, coming from the maintenance card 20 om  provides input to a receiver 15 which has one output to the address multiplexor 12 and another output on bus 37 to the peripheral dependent board (PDB) 80. 
     The distribution card data bus 32 provides input to receiver 16 while line 31 provides another input (RCV/) to receiver 16 from the peripheral dependent board 80. One output of receiver 16 is fed to the address multiplexor 12; while the other output forms bus 36 as a data bus to the PDB 80. 
     Receiver 17 has one input from the maintenance card data bus 34 and another control, SIMRCV/ (simulate receive) input line 33 from the peripheral dependent board PDB 80. Receiver 17 provides an output to the address multiplexor 12 and an output to the data bus 38. 
     Bus 35 from PDB 80 provides another input to the address multiplexor 12, while the low order address bit (AO) from the PDB 80 on line 36 provides an input to the PROM 13. PROM 13 provides a bus to the PROM register 14, which register also has an input from AND gate 24 which has two inputs; one of which constitutes the PROMCLK/ line and the other input is from the parity check circuit 18 to signal whether or not a parity error has been detected. 
     PROM register 14 has an output on bus 40 for maintenance display signals and connects to the common backplane of the base module. Another output of the PROM register 14 connects to the Request Status Latches circuit 19 which feeds its output to drivers 20 to provide signals designated IOSF (I/O Send flip-flop) Status, REQ (request) and EMREQ (emergency request) which are interrupts to the host. This bus also connects to the common backplane. 
     The PROM register 14 outputs include: the control lines of bus 43, the PDB usage bus 44 and the multiplexor enable bus 45, all of which connect to the peripheral dependent board, PDB 80. 
     A random access memory or RAM buffer storage 22 will be seen having four sets of inputs from the peripheral dependent board 80. These inputs are: the chip select line 50, the write enable line 51, the RAM data in-bus 52 and the RAM address bus 53. The output bus of RAM 22 is designated as the RAM data-out bus which connects to the peripheral dependent board 80. 
     COMMON FRONT END-CARD (CFE) 
     Referring to FIG. 4A which shows a block diagram of the common front end (CFE) 10 c , the central operative element of the Common Front-End is the PROM controller and storage unit 13. The PROM storage 13 consists of 13 separate PROM chips which may have a total storage capacity of 1024 52-bit words. This includes odd parity. 
     As seen in FIG. 4A the common front end also contains the receivers 15, 16 and 17, for the data link processor (DLP) interfaces with the distribution card on data bus 32 and the maintenance card on control bus 30. The &#34;enable&#34; signals for these buses are driven by the peripheral dependent board (PDB) 80. 
     The data link processor RAM storage buffer 22 has a capacity of 1027 17-bit words which includes odd parity. The RAM storage unit 22 is controlled entirely by the peripheral dependent board 80 (FIG. 5A). The following Table IA lists a glossary of various signals and terms used in the common front end. The Table IB is a glossary of terms used in the peripheral dependent board PDB 80, of the train printer-data link processor. 
     
                                           TABLE IA                                
__________________________________________________________________________
 COMMON FRONT END CARD GLOSSARY OF TERMS                                  
__________________________________________________________________________
A0                       BROP                                             
 PROM address bit 0.      When high, a 16-way PROM address branch is se-  
                          lected.                                         
A1                       BRST                                             
 PROM address bit 1.      When high, use stack register for PROM          
                         address.                                         
A2                       BUFFEND/                                         
 PROM address bit 2.      Active low, from PDB, used to stop burst mode.  
A3                       CLK8                                             
 PROM address bit 3.      8-megahertz clock.                              
A4                       CLK8/                                            
 PROM address bit 4.      8-megahertz clock not.                          
A5                       CLKEN                                            
 PROM address bit 5.      High active, clock conditioning level.          
A6                       CLKEN/                                           
 PROM address bit 6.      Low active, used to enable clock on PDB and     
                          CFE.                                            
A7                       CLKST                                            
 PROM address bit 7.      When high, PROM clock is disabled.              
A8                       CLOCK . . . 0                                    
 PROM address bit 8.      8-megahertz backplane clock from MC.            
A9                       CLOCK/                                           
 PROM address bit 9.      CFE control logic clock.                        
ADLOC/                   CLR/                                             
 When high, DLP is MC addressed or DLP address                            
                          Active low, logic clear term.                   
 is not valid.                                                            
ADRVLD/0                 CLRD                                             
 When low, LOCnn/.0 is valid.                                             
                          Active high, logic control term.                
AF                       CLRLAT                                           
 When high, strobe I/O has been received.                                 
                          Active high, logic term used to control SCLR.   
AF/                      CONECT/                                          
 When low, strobe I/O is sent to the PDB.                                 
                          Active low, DC is connected to DLP.             
BASLCL/0                 CS/                                              
 When low, the base is in local.                                          
                          Active low, RAM chip select level.              
BR6                      LCLCLR/0                                         
 A branch line from PDB for PROM address selec-                           
                          Active low, clear level from MC.                
 tion.                                                                    
DATAA8/0-DATAPR/0        LCPAD                                            
 17-bit data bus from DC.                                                 
                          Active high, DLP is addressed by DC or MC.      
DBUSA8-PARITY/0          LCPAD/                                           
 17-bit data bus.         Active low, DLP is addressed by DC or MC.       
DBUSn                    LCPADF                                           
 9-bit bus used as input to PROM address MPXs.                            
                          Active high, DLP is addressed by DC or MC.      
DIOSND/0                 LCPCON/0                                         
 Active low, I/O SEND level to MC.                                        
                          Active low, DLP is connected to DC.             
DLCPST/0                 LCPRQn/0                                         
 Active low, DLP strobe to MC.                                            
                          Active low, DLP request levels to DC.           
DPLY01/0                 LCPSTL                                           
 Ten display lines to MC.                                                 
                          Active high, DLP strobe level.                  
DSEL1/-DSEL4/            LCPSTL/0                                         
 Multiplexor address lines for data selection to dis-                     
                          Active low, DLP strobe level to DC.             
 play lines.                                                              
DSEL8/                   LCSTU1/0-LCSTU8/0                                
 Active low, multiplexor chip select line for input                       
                          Four DLP status lines to DC.                    
 to display lines.                                                        
DSIMA8/0-PARSIM/0        LOCAL/                                           
 17-bit data bus from MC.                                                 
                          Active high, DLP is not MC addressed, or        
                         address                                          
                          is not valid.                                   
DSIMn                    LOCAL/.1                                         
 9 bits of DSIM lines, used to become DBUSn.                              
                          Active low, DLP is MC addressed.                
DSTAT1/0-DSTAT8/0        LOCnn/.0                                         
 Four DLP status lines to MC.                                             
                          Local address lines from MC.                    
EMREQ                    MLCPAD/0                                         
 Active high, DLP emergency request.                                      
                          Active low, DLP is addressed by MC.             
EMRREQ/0                 MSTCLR/0                                         
 Active low, DLP emergency request to DC.                                 
                          Active low, base power-up clear from MC.        
GPRIF/                   MSTIOL/0                                         
 Peripheral bus control term to PDB.                                      
                          Active low, maintenance strobe I/O from MC.     
GPRIF/.0                 MTERM/.0                                         
 Active low, from MC, disconnects PDB peripheral                          
                          Active low, maintenance terminate from MC.      
 cable.                                                                   
GRP0/                    OFFLN                                            
 Active low, controls 16-line display to MC.                              
                          Active high, MC has localized the CFE.          
INRAMA8-INRAMPR          OFFLN/                                           
 17 RAM input data lines.                                                 
                          Active low, the DLP is in local.                
IOSF                     OFFLNE/0                                         
 I/O SEND flip-flop to PDB.                                               
                          Active low, off-line control level from MC.     
IOSND/.0                 START/.0                                         
 I/O SEND to DC.          Active low, from MC, allows clock in            
                         signal-pulse                                     
                          mode.                                           
OPDECI                   STCKA8-STCKA0                                    
 PROM address A0 bit from PDB when 16-way                                 
                          PROM address lines used during stack            
                         branching.                                       
 branching.                                                               
OPDECX                   STCLKEN                                          
 PROM address bits A1-A3 from PDB, when                                   
                          Active when going high, counts up the stack     
                         regis-                                           
 16-way branching.        ter +1.                                         
PARSIM/0                 STIOL/.0                                         
 Data simulate parity line from MC.                                       
                          Active low, strobe I/O from DC.                 
PER                      STOPB                                            
 Active high, PROM output parity is even (an er-                          
                          Active high, used to stop burst mode.           
 ror).                                                                    
PERF                     STOPB/                                           
 Active high, PROM parity error is present.                               
                          Active low, stop burst mode to PDB.             
PERF/                    STOPF                                            
 Active low, Prom parity error (disables clocks).                         
                          A 2-way PROM branching bit.                     
PROMCLK/                 SW1/                                             
 PROM clock.              Active low, used to do PROM maintenance read.   
RAM                      SWH.1/.0                                         
 17 bits of RAM addressed by RAMAD0                                       
                          Active low, SW1/ from MC.                       
 RAMAD9.                                                                  
RAMA8-RAMPR              TERMF                                            
 17 bits of RAM output information.                                       
                          A 2-way PROM branching bit.                     
RCV/                     TERMF/                                           
  Active low, enables data bus from DC.                                   
                          Active low, used to terminate burst mode.       
REQ                      TERM/..0                                         
 Active high, DLP request for DC attention.                               
                          Active low, terminate level from DC.            
SCLR                     TEST5 and TEST6                                  
 Active high, synchronized clear, sets PROM ad-                           
                          2-way PROM branching bits from PDB.             
 dress = 0.                                                               
SEL2/-SEL6/              TEST8-TEST14                                     
 Active lows, to PDB, for MPX enables.                                    
                          2-way PROM branching bits from PDB.             
SELCLR/0                 WE/                                              
 Active low, clear line from DC.                                          
                          Active low, RAM write enable level.             
SEMREQ/                  #BRANCH1-#BRANCH5                                
 Active low, an emergency request is present in the                       
                          PROM branching control lines.                   
 base.                                                                    
SIMRCV/                  #CONST0-#CONST7                                  
 Active low, enables MC DSIM lines, from PDB.                             
                          Multipurpose PROM outputs, PDB-dependent.       
SP/...0                  #G3-#L4                                          
 Active low, single-pulse mode from MC.                                   
                          PDB dependent PROM outputs (see PDB Glossary    
                          of Terms).                                      
ST1-ST8                  #LCPSTL/                                         
 DLP status lines internal to CFE.                                        
                          Active low, DLP strobe level, from PROM con-    
                          troller to DC.                                  
#LDINT/                  #NEXT0-#NEXT8                                    
 Active low, (load interface) PROM MLI bus con-                           
                          PROM address bits.                              
 trol level.                                                              
#LDSTK/                  #PARITY                                          
 Active low, allows stack register load of current                        
                          PROM parity bit (odd).                          
 PROM address. This level is held high during all                         
 microcode subroutines.                                                   
                         +5V                                              
                          VCC from power supply.                          
__________________________________________________________________________
 
    
     
                                           TABLE IB                                
__________________________________________________________________________
Train Printer-Data Link Processor                                         
PDB GLOSSARY OF TERMS                                                     
__________________________________________________________________________
#BRANCH1-#BRANCH3        CIDL/                                            
Branch lines from microcode that select which sig-                        
                         Chain identification level from the printer. A   
                         6-bit                                            
nal is selected to generate A0.                                           
                         ID code of negative pulses that is generated     
                         from                                             
                         the notched gear teeth of the train module.      
#CONST7-#CONST0          CIDLFLAG                                         
Control status lines to controlling registers on                          
                         Synchronous result of CIDL/.                     
PDB, microcode outputs. Also contains literal in-                         
put for accumulators.                                                     
#J4                      CID8-DID1                                        
Write enable signal from microcode. Gated with                            
                         Identification jumpers.                          
8-MHz clock to generate WE/ (write enable) to                             
CFE card.                                                                 
ACC1                     CLEAR/                                           
Accumulator address bit 1 from microcode.                                 
                         Clear signal from the CFE card.                  
ACC2                     CLKEN/                                           
Accumulator address bit 2 from microcode.                                 
                         Clock enable from the CFE card. Disables PDB     
                         clock when high.                                 
ACC4                     CLKLPWRG                                         
Accumulator address bit 4 from microcode.                                 
                         Clock longitudinal parity word (LPW), microcode  
                         output. Normally high. LPW generator is clocked  
                         on the positive edge of this signal.             
ACC8                     CLOCK . . . 0                                    
Accumulator address bit 8 from microcode.                                 
                         8-MHz clock from backplane.                      
AD0-AD7                  COLDONE                                          
Accumulator data. The 8-bit output of the accumu-                         
                         Column done. The output of the column done       
lators.                  RAM. A high level if column has been serviced.   
                         Becomes TEST14 on the foreplane and is sent to   
                         the CFE card for 2-way branching.                
A0                       CONECT/                                          
PROM address bit 0. Manipulated by the micro-                             
                         When low, this signal indicates a connected      
                         condi-                                           
code to provide 2-way branching depending upon                            
                         tion with the host system.                       
the state of the signal being tested.                                     
BOTCF/                   CONTRAD1-CONTRAD6                                
Bottom character of DBUS equals HEX CF (deli-                             
                         Control address input to control register, from  
                         mi-                                              
meter character code). Low active                                         
                         crocode.                                         
BR6                      CS/                                              
Branch 6 signal, microcode output #14. Used with                          
                         Chip select/. When low, enables the RAM buffer   
other #BRANCH signals to select which signal                              
                         on the CFE card. This signal is grounded on the  
generates A0 (LSB of CFE microcode address).                              
                         PDB.                                             
CDPARGEN                 CSL/                                             
DBUS digits C and D have odd parity. High ac-                             
                         Chain sync level from the printer. A negative    
tive                     pulse, occurs once for each revolution of the    
                         train                                            
                         module. It is used by the TP-DLP to keep in      
                         sync                                             
                         with the train module. It is developed from the  
                         first chain sync pulse on the train. The next    
                         six                                              
                         chain sync pulses are gated to the CIDL line.    
CFE                                                                       
Common Front End.                                                         
CSLF                     DELFOUND                                         
Chain sync level flip-flop.                                               
                         Delimiter found. Sent to the CFE card as TEST8   
                         to be used for 2-way branching. Its state        
                         depends                                          
                         upon the output of the DBUS decoding PROMs.      
                         It is high only when a delimiter character is    
                         pres-                                            
                         ent on the DBUS×n lines.                   
CSLFLAG                  DIRAMCOL                                         
Synchronous result of CSL/.                                               
                         Data in RAM column done. Data input to column    
                         done RAM. High active.                           
DATAA8/0-PARITY/0        DIRAMINV                                         
Backplane common data bus. From DBUS via tri-                             
                         Data in RAM invalid. Data input to invalid RAM.  
state inverters, enabled by XMIT.                                         
                         High active.                                     
DBUS                     DPLY11/0-DPLY13/0                                
17-bit data bus.         Display line 11, 12, and 13. Sent out the        
                         backplane                                        
                         to the Maintenance Card and used to test         
                         internal                                         
                         nets on the Peripheral-Dependent Board.          
DBUSA8-DBUSPR            DSEL4/-DSEL1/                                    
17-bit data bus.         Display select lines from Maintenance Card. Se-  
                         lects which inputs to the 8-to-1 multiplexor     
                         chips                                            
                         are selected.                                    
DBUSA8                   DSEL8/                                           
Data bus A, digit 8 bit. MSB of DBUS, see                                 
                         Display select 8/ from Maintenance Card.         
                         Enables                                          
DBUSA8-DBUSPR. Also sent to CFE card as                                   
                         8-to-1 multiplexor chips to display circuit      
                         nets.                                            
TEST11 to test for paper advance information in                           
descriptor.                                                               
DBUSC4                   DSELn/                                           
Data bus C, digit 4 bit. One bit of DBUS, see                             
                         Display select lines.                            
DBUSA8-DBUSPR. Also sent to CFE card as                                   
TEST13 to test for 1100 LPM bit in descriptor.                            
DBUSC8                   DSIM                                             
Data bus C, digit 8 bit. One bit of DBUS, see                             
                         Data simulate lines. Maintenance data bus.       
DBUSA8-DBUSPR. Also sent to CFE card as                                   
TEST12 to test for delimiter mask in descriptor.                          
DC                       DSIMA8/0-PARSIM/0                                
Distribution Card.       Backplane data simulate bus. Same as             
                         DATA×n/0                                   
                         lines except that it is only used for            
                         maintenance                                      
                         purposes.                                        
DC1L/                    DT1L/                                            
Data control 1 level to the printer. Used in con-                         
                         Data transfer 1 level to the printer. The least  
                         sig-                                             
junction with DC2L/. Part of a 2-bit code which                           
                         nificant bit of the 4-bit format code. Also      
                         used                                             
controls the operation of the printer (idle, scan-                        
                         during the scan-print operation to transfer an   
                         equal                                            
print, or paper advance).                                                 
                         compare bit to the column storage latches in     
                         the                                              
                         printer.                                         
DC1LCTR                  DT1LCTRL                                         
Data control 1 level control register output. Gener-                      
                         Data transfer 1 level control register output.   
ates DC1L and is used for DTnL selection.                                 
                         Source for DT1L when in a print cycle.           
DC2L/                    DT2L/                                            
Data control 2 level to the printer. Refer to                             
                         Data transfer 2 level to the printer. Refer to   
DC1L/.                   DT8L/.                                           
DC2LCTR                  DT4L/                                            
Data control 2 level control register output. Gener-                      
                         Data transfer 4 level to the printer. Refer to   
ates DC2L and is used for DTnL selection.                                 
                         DT8L/.                                           
DCnL                                                                      
Data control levels.                                                      
DCnL/                                                                     
Data control levels.                                                      
DT8L/                    INRAMAD7-INRAMAD0                                
Data transfer 8 level to the printer. DT8L/ thru                          
                         AD register outputs (A and D digits of           
                         register).                                       
DT2L/ are used in conjunction with DT1L to form                           
                         Supplies the 8-bit input to the column done RAM  
a 4-bit code which transfers the type of paper                            
                         and is used with RAMAD7-RAMAD0 (BC regis-        
spacing (format control) to the printer. This occurs                      
                         ter) to supply the 16-bit register output to the 
                         data                                             
only when DC1L/ is high and DC2L/ is low.                                 
                         multiplexor.                                     
DTnL                     INRAMA8-INRAMPR                                  
Data transfer levels.    16 bits of data plus parity sent to the RAM      
                         buffer                                           
                         on the CFE card. Same as DBUSA8-DBUSPR.          
DTnL/                    INTERFLG                                         
Data transfer levels. Low active.                                         
                         Internal flag. Called STOP in microcode          
                         listing.                                         
                         Used by microcode to determine when to stop re-  
                         ceiving buffer data from the host system.        
EDPL/                    INVALID                                          
End of page level from the printer. Goes low dur-                         
                         Data output of invalid RAM. High active.         
ing a single or double space paper advance, when                          
a format tape channel 12 punch is sensed. It is re-                       
set to a high level at the next paper advance cycle.                      
ENDATMPX                 IOSF/                                            
Enable data multiplexor, microcode output. En-                            
                         I/O SEND flip-flop. When low, the TP-DLP is      
ables output of data multiplexor onto the DBUS                            
                         driving the MLI. When high, the DLP can receive  
via tristate buffers.    MLI information.                                 
ENDLFLAG                 LATPRDAT -Synchronous result of EDPL/. Latch     
                         print data into compare register. Load input     
                         to compare register. Low active.                 
ENDTX1/                  LAT0-LAT7                                        
Enable DTX1. Enables DT1LCTRL to printer                                  
                         Latched outputs of compare register.             
DT1L/ line via a tristate inverter when low.                              
ENFOR/                   DLP                                              
Enable format information to printer when low.                            
                         Data Link Processor DL-2.                        
ENMDTX1/                 LDREGCAD                                         
Enable maintenance DTX1. Enables DT1LCTRL                                 
                         Load register column address. Load AD register   
via a tristate inverter to PRIF07/0 when low.                             
                         with the column address. Load signal to the A    
                         and                                              
                         D digits of the general purpose register.        
                         Active                                           
                         when low. Column address is only one of the      
                         uses                                             
                         of this register.                                
ENMFOR/                  LDREGAD                                          
Enable maintenance format. Enables format infor-                          
                         Load register RAM address. Load RAM address      
mation to PRIF04/0-PRIF07/0 (maintenance pe-                              
                         into the BC register. Load B and C digits of     
                         the                                              
ripheral lines).         general purpose register. Low active, from       
                         micro-                                           
                         code. RAM address is only one of the uses of     
                         this                                             
                         register.                                        
ENMPX/                   LOADCONT                                         
Enable multiplexor outputs to the DBUS.                                   
                         Load control register, microcode output. Loads   
                         control register when low.                       
FORERROR                 LOCAL/                                           
Format error. Sent to the CFE card as TEST10 to                           
                         When low, this signal indicates the Maintenance  
be used for 2-way branching. Its state depends up-                        
                         Card has access to the TP-DLP.                   
on the output of the DBUS decoding PROMs (high                            
= error).                                                                 
FREECLK/                 LOCDIS/                                          
CLOCK/. Always running.  Local display. Activates the DSIM×n (data  
                         simulate) lines to the Maintenance Card.         
GPRIF/                                                                    
Gate maintenance PRIF (peripheral) lines. When                            
low, backplane PRIF lines are selected instead of                         
train printer interface cable.                                            
LPWA8-LPWD1              MPR.L/                                           
Longitudinal parity word generator outputs.                               
                         Maintenance Card generated PR2L/.                
LPWCD                    MPXA8-MPXD1                                      
Longitudinal parity word in digits C and D of                             
                         Data multiplexor outputs.                        
DBUS is OK. High active.                                                  
LPWERROR                 MPXDATAA                                         
Longitudinal parity word error. Sent to the CFE                           
                         Data multiplexor A select. Refer to MPXDATAB.    
card as TEST6 to be used for 2-way branching. Its                         
state depends upon the output of the DBUS de-                             
coding PROMs (high = error).                                              
MAXCOUNT                 MPXDATAB                                         
Maximum count. High when BC register equals                               
                         Data multiplexor B select. Used in conjunction   
HEX FF.                  with MPXDATAA to select one of four inputs to    
                         the data multiplexor. Microcode output.          
MC                       MPXPAR                                           
Maintenance Card.        Data multiplexor parity bit output.              
MCIDL/                   MPXSELAD                                         
Maintenance Card generated CIDL/.                                         
                         Multiplexor select adder. Selects input to the   
                         ac-                                              
                         cumulators from #CONST lines when low, and       
                         from adder when high.                            
MCSL/                    OFFLINE/                                         
Maintenance Card generated CSL/.                                          
                         Off-line signal from Maintenance Card. Also      
                         gen-                                             
                         erates the signal TIBLOAD/ on the PDB to ensure  
                         that the train image buffer is reloaded after    
                         the                                              
                         TP-DLP has been Maintenance Card selected.       
MEDPL/                   OPDECSEL                                         
Maintenance Card generated EDPL/.                                         
                         Operation decode select. When high, the          
                         decoding                                         
                         PROMs decode the OP code; when low, the de-      
                         coding PROMs are used to decode LPW, vertical    
                         parity, delimiter characters, and format         
                         errors.                                          
MICROCODE                OPDEC1                                           
A name given to the program information con-                              
                         OP decode line 1 from operation decode PROM on   
tained in the 1K-by-52 bit PROMs on the CFE                               
                         PDB. Used by CFE card to perform a 16-way        
card.                    branch when OPDECSEL is high. When OPDEC-        
                         SEL is low, this signal is used to detect format 
                         er-                                              
                         rors. During this time, OPDEC1 is high if digit  
                         bits                                             
                         4, 2, and 1 equal zero.                          
MLI                      OPDEC2                                           
Message Level Interface. OP decode line 2 from operation decode PROM on   
                         PDB. Used by CFE card to perform a 16-way        
                         branch when OPDECSEL is high. When OPDEC-        
                         SEL is low, this signal is used to detect        
                         delimiters.                                      
                         During this time, this signal is low only if the 
                         A                                                
                         and B digits of DBUS contain a delimiter         
                         charac-                                          
                         ter (HEX CF). This signal is also sent to the    
                         CFE                                              
                         card as TEST9 to be used for 2-way branching.    
MOST/                    OPDEC4                                           
Motor start command to the printer. A low pulse                           
                         OP decode line 4 from operation decode PROM on   
causes the train module motor circuit in the print-                       
                         PDB. Used by CFE card to perform a 16-way        
er to enable the motor.  branch when OPDECSEL is high. When OPDEC-        
                         SEL is low, this signal is used for LPW          
                         checking.                                        
                         During this time, this signal is high only if    
                         the A                                            
                         and B digits of DBUS indicate a good LPW (A      
                         and B digits equal zero).                        
MOSTCTR                                                                   
Motor start control register output.                                      
MPAML/                                                                    
Maintenance Card generated PAML/.                                         
MPCSL/                                                                    
Maintenance Card generated PCSL/.                                         
MPFCL/                                                                    
Maintenance Card generated PFCL/.                                         
MPRSL/                                                                    
Maintenance Card genarated PRSL/.                                         
MPRIL/                                                                    
Maintenance Card generated PRIL/.                                         
OPDEC8                   PFCL/                                            
OP decode line 8 from operation decode PROM on                            
                         Printer final column level from the printer.     
                         Goes                                             
PDB. Used by CFE card to perform a 16-way                                 
                         low when the printer logic has reached the last  
branch when OPDECSEL is high. when OPDEC-                                 
                         column during a scan operation. Not used by the  
SEL is low, this signal is used for vertical parity                       
                         TP-DLP.                                          
checking. During this time, this signal is high only                      
if the A and B digits of DBUS have odd vertical                           
parity.                                                                   
PAML/                    PFCLFLAG                                         
Paper motion level from the printer. High while                           
                         Synchronous result of PFCL/.                     
paper is advancing.                                                       
PAMLFLAG                 PRIFnn/0                                         
Synchronous result of PAML/.                                              
                         Simulated peripheral interface lines.            
PARERROR                 PRSL/                                            
Parity error. Sent to the CFE card as TEST5 to                            
                         Printer speed level from the printer. Not used   
                         by                                               
be used for 2-way branching. Its state depends up-                        
                         TP-DLP.                                          
on the output of the DBUS decoding PROMs and                              
DBUSPR (high = error).                                                    
PARGEN                   PRSLFLAG                                         
Parity generate. The generated parity bit for data                        
                         Printer speed level. See PRSL/.                  
on the DBUS. High active (odd parity).                                    
PCSL/                    PR1L/                                            
Printer column scan level from the printer. A neg-                        
                         Printer ready 1 level from the printer. High     
                         when                                             
ative pulse used to duplicate the rotation of the                         
                         the printer is ready (on-line). Indicates that   
                         the                                              
train module and to identify the allowable scan                           
                         printer is ready to receive commands.            
time for the up-coming print position of the train                        
module.                                                                   
PCSLFLAG                 PR1LFLAG                                         
Printer column scan level flag. Used by the micro-                        
                         Synchronous result of PR1L/.                     
code to test for PCSL leading edge.                                       
PCSLT                    PR2L/                                            
Synchronous result of PCSL/.                                              
                         Printer ready 2 level from the printer. Low      
                         when                                             
                         the train module motor circuit is enabled (the   
                         mo-                                              
                         tor is running).                                 
PCTP/                    PR2LFLAG                                         
Printer column timing pulse to the printer. A                             
                         Synchronous result of PR2L/.                     
square wave clock.                                                        
PCTPFLAG                 RAMAD8                                           
Printer column timing pulse flag. A positive pulse                        
                         RAM buffer address bit 8. Generated by control   
occurring at the trailing edge of each PCTP pulse.                        
                         register.                                        
PCTP1-PCTP8              RAMAD9-RAMAD0                                    
Printer column timing pulse jumpers.                                      
                         RAM buffer address lines from the BC register.   
                         RAMAD7-RAMAD0 are also used to supply the B      
                         and C digits of the register to the data         
                         multiplexor                                      
                         along with the INRAMAD7-INRAMAD0 lines.          
                         RAMAD9 is grounded and not used. RAMAD8 is       
                         generated by the control register.               
PDB                      RAMAD8-RAMD1                                     
Peripheral-Dependent Board.                                               
                         16-bit RAM buffer output from CFE card.          
PERF/                    RAMPAR                                           
Parity error flip-flop. From CFE card. Kill signal                        
                         RAM parity. Parity bit from RAM buffer on CFE    
to the printer.          card.                                            
                         RCV/                                             
                         Receive/. When low, this signal turns on the     
                         DA-                                              
                         TA×n receivers to receive data from the    
                         Distribu-                                        
                         tion Card (DC). Generated by the line turn       
                         logic                                            
                         on the PDB.                                      
REGISTER FILES           THRAMCLD                                         
Register file chips. R440s. Six chips used as                             
                         Top half of RAM column done select. When high,   
twelve 8-bit accumulators.                                                
                         indicates upper half of PIB.                     
RESTLPW                  TIB=PIB                                          
Reset LPW. Microcode output. Presets the LPW                              
                         Train image buffer equals print image buffer.    
                         Data                                             
generator to all ones.   in compare register is equal to the data         
                         selected by                                      
                         the compare multiplexor. High active.            
RSETPCSL                 TIBLOAD/                                         
Reset PCSL flag.         Train image buffer load. When low, indicates     
                         that                                             
                         the train image buffer needs to be loaded.       
RSETTIB/                 TP-DLP                                           
Reset TIB load flip-flops.                                                
                         Train Printer Data Link Processor DL-2.          
SELTHBH                  TRAIN ID                                         
Select top half/bottom half. Microcode output. Se-                        
                         Train identification. The 6-bit identification   
                         num-                                             
lects RAM buffer output digits A and B, when                              
                         ber of the train installed on the printer.       
low, and selects digits C and D when high (for the                        
RAM output multiplexor chips).                                            
SEL0-SEL7                WECOLDN                                          
Selected 8 bits of data from compare multiplexor.                         
                         Write enable column done. Write enable signal    
                         to                                               
                         column done RAM. Low active.                     
SEL2/-SEL4/              WERAMBUF                                         
Select 2, 3, and 4. Generated from the #BRANCH                            
                         Write enable RAM buffer. Write enable signal to  
lines and BRANCH6 on the CFE card. Used to                                
                         RAM buffer on CFE card. Generated by micro-      
select which 8-to-1 multiplexor chip is used for A0                       
                         code output #J4 and 8CLK1. Low active.           
generation.                                                               
SEL5/                    WERAMINV                                         
Select 5/. When low, indicates that PROM address                          
                         Write enable RAM invalid. Write enable input to  
bit 0 is derived from the RAM address lines.                              
                         invalid RAM. Low active.                         
SELn                     WEREGFIL                                         
Select lines.            Register file write enable signal from           
                         microcode,                                       
                         supplies write enable signals to addressed       
                         4-by-4                                           
                         register file chips (accumulators).              
SETCSLF                  XMIT/                                            
Set chain sync level flop. Preset input to sync flip-                     
                         Transmit/. When low, DATA ×n drivers are   
                         en-                                              
flop. Low active.        abled.                                           
SIMRCV/                  1100LPMJ                                         
Simulate receive/. Generated by the line turn logic                       
                         1100 line-per-minute printer jumper. Field-      
on the PDB. Used during maintenance testing to                            
                         installed. Low active.                           
simulate the DATA×n lines.                                          
SPARE                    120COLIP                                         
Spare jumper. Not used.  120-column jumper. Low active. Field-installed   
                         jumper.                                          
SW1/                     132COLIP                                         
Switch one/. From Maintenance Card.                                       
                         132-column jumper. Low active. Field-installed   
                         jumper.                                          
SYNCFLAG                                                                  
Sync flag. Output of chain sync flip-flop.                                
TERMF/                                                                    
Terminate signal from CFE card. Used on the                               
PDB to generate the signal INTERFLG (STOP in                              
microcode listing).                                                       
__________________________________________________________________________
 
    
     DISTRIBUTION CARD INTERFACE: As previously indicated in FIG. 2, data link processors are housed in a base module unit. Each data link processor, consisting of two cards, slides into the base module housing having a common backplane to which the printed circuit boards of a data link processor are connected. 
     All communications between the common front end 10 c  and the distribution cards such as 20 0d , 20 1d  are performed through the data link processor base module backplane as 20 0B  of FIG. 3. The backplane is common to all cards that are installed in a base module. 
     Table II shows a list of all the backplane signals that occur on the distribution card interface to the common front end. The 17-bit wide data portion of the bus 32, FIG. 4A, is received from distribution card (DC) on the common front end (CFE). This same 17-bit bus is driven in the opposite direction (by drivers on the PDB) when the data link processor is sending data back to the distribution card. The enabling levels that control the direction of this bus are generated on the peripheral dependent board. However the Train Printer Data Link Processor, working with an output-only type peripheral, (Train Printer) does not send data back but rather receives data for printing. 
     
                       TABLE II                                                    
______________________________________                                    
Distribution Card/CFE Interface                                           
Level    Pin #    Direction                                               
                           Definition                                     
______________________________________                                    
LCSTU8/0 123      to DC    DLP status bit 8                               
LCSTU4/0 023      to DC    DLP status bit 4                               
LCSTU2/0 126      to DC    DLP status bit 2                               
LCSTU1/0 026      to DC    DLP status bit 1                               
LCPCON/0 074      to DC    DLP is connected                               
IOSND/.0 124      to DC    DLP is sending                                 
EMRREQ/0 191      to DC    DLP has emergency request                      
LCPRQ7/0 163      to DC    DLP #7 has request                             
LCPRQ6/0 063      to DC    DLP #6 has request                             
LCPRQ5/0 164      to DC    DLP #5 has request                             
LCPRQ4/0 064      to DC    DLP #4 has request                             
LCPRQ3/0 165      to DC    DLP #3 has request                             
LCPRQ2/0 065      to DC    DLP #2 has request                             
LCPRQ1/0 166      to DC    DLP #1 has request                             
LCPRQ0/0 066      to DC    DLP #0 has request                             
LCPSTL/0 024      to DC    DLP strobe                                     
STIOL/.0 127      to CFE   Strobe I/O                                     
TERM/..0 072      to CFE   Terminate                                      
SELCLR/0 167      to CFE   Selective clear                                
LCPAD7/0 176      to CFE   DLP #7 is addressed                            
LCPAD6/0 076      to CFE   DLP #6 is addressed                            
LCPAD5/0 177      to CFE   DLP #5 is addressed                            
LCPAD4/0 077      to CFE   DLP #4 is addressed                            
LCPAD3/0 178      to CFE   DLP #3 is addressed                            
LCPAD2/0 078      to CFE   DLP #2 is addressed                            
LCPAD1/0 179      to CFE   DLP #1 is addressed                            
LCPAD0/0 079      to CFE   DLP #0 is addressed                            
DATAA8/0 106      Bi-Dir   Data bit                                       
DATAA4/0 006      Bi-Dir   Data bit                                       
DATAA2/0 108      Bi-Dir   Data bit                                       
DATAA1/0 008      Bi-Dir   Data bit                                       
DATAB8/0 110      Bi-Dir   Data bit                                       
DATAB4/0 010      Bi-Dir   Data bit                                       
DATAB2/0 117      Bi-Dir   Data bit                                       
DATAB1/0 017      Bi-Dir   Data bit                                       
DATAC8/0 119      Bi-Dir   Data bit                                       
DATAC4/0 019      Bi-Dir   Data bit                                       
DATAC2/0 121      Bi-Dir   Data bit                                       
DATAC1/0 021      Bi-Dir   Data bit                                       
DATAD8/0 132      Bi-Dir   Data bit                                       
DATAD4/0 032      Bi-Dir   Data bit                                       
DATAD2/0 134      Bi-Dir   Data bit                                       
DATAD1/0 034      Bi-Dir   Data bit                                       
PARITY/0 136      Bi-Dir   Data parity bit (odd)                          
______________________________________                                    
 
    
     MAINTENANCE CARD INTERFACE: All communications between the common front end and the maintenance card (as 20 0m ) take place on the data link processor&#39;s base module backplane. Table III shows a list of all the backplane signals that occur between the common front end and the maintenance card. 
     
                       TABLE III                                                   
______________________________________                                    
Maintenance Card/CFE Interface                                            
Level    Pin #   Direction                                                
                          Definition                                      
______________________________________                                    
DSTAT8/0 044     to MC    DLP status bit 8                                
DSTAT4/0 145     to MC    DLP status bit 4                                
DSTAT2/0 045     to MC    DLP status bit 2                                
DSTAT1/0 146     to MC    DLP status bit 1                                
DLCPST/0 140     to MC    DLP strobe                                      
DIOSND/0 144     to MC    DLP is sending                                  
MSTIOL/0 040     to CFE   Strobe I/O                                      
MTERM/.0 184     to CFE   Terminate                                       
MLCPAD/0 154     to CFE   DLP is addressed                                
BASLCL/0 161     to CFE   Base is in local                                
MSTCLR/0 172     to CFE   Base clear                                      
LCLCLR/0 174     to CFE   Local clear                                     
SWH.1/.0 058     to CFE   Maintenance switch #1                           
SP/....0 147     to CFE   Single-pulse mode                               
START/.0 149     to CFE   Allow clock(s)                                  
CLOCK..0 048     to CFE   8 Mhz clock                                     
ADRVLD/0 043     to CFE   Local address is valid                          
LOC16/.0 187     to CFE   Local address bit 16                            
LOC08/.0 087     to CFE   Local address bit 8                             
LOC04/.0 188     to CFE   Local address bit 4                             
LOC02/.0 088     to CFE   Local address bit 2                             
LOC01/.0 189     to CFE   Local address bit 1                             
OFFLNE/0 073     to CFE   DLP is off line                                 
GPRIF/.0 061     to CFE   Gate off peripheral interface                   
DSEL8/.0 060     to CFE   Display select bit                              
DSEL4/.0 085     to CFE   Display select bit 4                            
DSEL2/.0 186     to CFE   Display select bit 2                            
DSEL1/.0 086     to CFE   Display select bit 1                            
DPLY01/0 054     to MC    Display line                                    
DPLY02/0 155     to MC    Display line                                    
DPLY03/0 055     to MC    Display line                                    
DPLY04/0 156     to MC    Display line                                    
DPLY05/0 056     to MC    Display line                                    
DPLY06/0 157     to MC    Display line                                    
DPLY07/0 057     to MC    Display line                                    
DPLY08/0 067     to MC    Display line                                    
DPLY09/0 168     to MC    Display line                                    
DPLY10/0 068     to MC    Display line                                    
DSIMA8/0 112     Bi-Dir   Data simulate line A8                           
DSIMA4/0 012     Bi-Dir   Data simulate line A4                           
DSIMA2/0 113     Bi-Dir   Data simulate line A2                           
DSIMA1/0 013     Bi-Dir   Data simulate line A1                           
DSIMB8/0 114     Bi-Dir   Data simulate line B8                           
DSIMB4/0 014     Bi-Dir   Data simulate line B4                           
DSIMB2/0 115     Bi-Dir   Data simulate line B2                           
DSIMB1/0 015     Bi-Dir   Data simulate line B1                           
DSIMC8/0 027     Bi-Dir   Data simulate line C8                           
DSIMC4/0 128     Bi-Dir   Data simulate line C4                           
DSIMC2/0 028     Bi-Dir   Data simulate line C2                           
DSIMC1/0 129     Bi-Dir   Data simulate line C1                           
DSIMD8/0 029     Bi-Dir   Data simulate line D8                           
DSIMD4/0 130     Bi-Dir   Data simulate line D4                           
DSIMD2/0 030     Bi-Dir   Data simulate line D2                           
DSIMD1/0 131     Bi-Dir   Data simulate line D1                           
PARSIM/0 137     Bi-Dir   Data simulate parity line (odd)                 
______________________________________                                    
 
    
     MAINTENANCE FACILITIES: In FIG. 2 certain maintenance facilities are provided. These consist of a console 50 c  (which may include a cathode ray tube CRT, a mini-disk, etc.) and the maintenance card, as 20 0m , plus other software packages and hardware which are included in the DLP. Under programmatic control, the console 50 c  can be used to manipulate the DLP in such a manner as to determine its internal state for given operations and to compare it to a known correct state by use of the maintenance card 20 0m . Thus diagnosis of a failing DLP can be made. 
     The console is the interface between the host system and the host system operator, as well as being the maintenance interface to the I/O subsystem. Maintenance of the Train Printer-Data Link Processor originates at the console, when in the off-line mode; and from the host system, when in the on-line mode of operation. A data base can be supplied on flexible diskettes or magnetic tape to perform diagnosis of the data link processor (DLP). The selection of the test and type of module can be done at the host system by a field engineer or a host system operator. 
     DIAGNOSTIC TESTING MODES: There are two modes of diagnostic testing, (a) off-line and (b) on-line. In either mode the units undergoing tests are not available to the host system as a resource and must be placed off-line prior to running diagnostics. Diagnostic programs use the console-to-maintenance card interface in the diagnosis of the subsystem modules. These programs can perform a card test on the addressed unit by means of a maintenance data base that is stored on flexible diskettes or is resident in the host system. 
     OFF-LINE MODE: This mode implies the following: 
     1. The host systems resources are not available 
     2. The test data base is console-diskette-resident 
     3. The operator must supply control information. 
     ON-LINE MODE: This mode implies the following: 
     1. The host systems resources are available 
     2. The test data base is host-system-resident 
     3. A host resident program performs the diagnosis. 
     Confidence test programs can be developed which use the message level interface (MLI) to ascertain a confidence level of the I/O subsystem unit or units under test. These tests may be used to isolate a failing unit so that maintenance can be invoked to determine if the cause of the failure is in an I/O subsystem module or in the peripheral device. 
     PERIPHERAL-DEPENDENT BOARD INTERFACE: As seen in FIG. 3 the peripheral dependent board (PDB) and the common front end card are provided with foreplane connectors 80 a , 80 b , 80 c , 80 d , and 80 a1 , 80 b1 , 80 c1 . 
     The interface between the common front end and the peripheral dependent board is made of three 50-pin foreplane connectors 80 a , 80 b  and 80 c . Table IV lists the connectors and shows the pin numbers together with the logic names of the signals involved specifically for the Train Printer Data Link Processor. 
     
                       TABLE IV                                                    
______________________________________                                    
CFE FOREPLANE CONNECTORS                                                  
FOR TRAIN PRINTER-DLP                                                     
           Pin       #-Side      $-Side                                   
Connector  Number    Signal      Signal                                   
______________________________________                                    
element 80 a1                                                             
1          00        RAMA4       RAMPAR                                   
1          01        RAMA1       RAMA8                                    
1          02        RAMB4       RAMA2                                    
1          03        RAMB1       RAMB8                                    
1          04        GND         RAMB2                                    
1          05        RAMC4       RAMC8                                    
1          06        RAMC1       RAMC2                                    
1          07        RAMD4       RAMD8                                    
1          08        RAMD1       RAMD2                                    
1          09        INRAMB8     INRAMA8                                  
1          10        INRAMB4     INRAMA4                                  
1          11        INRAMB2     INRAMA2                                  
1          12        INRAMB1     INRAMA1                                  
1          13        INRAMC2     INRAMC8                                  
1          14        INRAMC1     INRAMC4                                  
1          15        SIMRCV/     RCV/                                     
1          16        DBUSA4      DBUSA8                                   
1          17        DBUSA1      DBUSA2                                   
1          18        DBUSB4      DBUSB8                                   
1          19        DBUSB1      DBUSB2                                   
1          20        GND         DBUSC8                                   
1          21        DBUSC4      DBUSC2                                   
1          22        DBUSC1      DBUSD8                                   
1          23        DBUSD4      DBUSD2                                   
1          24        DBUSD1      DBUSPR                                   
2          25        WE/         GND                                      
2          26        RAMAD9      CS/                                      
2          27        RAMAD7      RAMAD8                                   
2          28        RAMAD5      RAMAD6                                   
2          29        GND         RAMAD4                                   
2          30        RAMAD3      RAMAD2                                   
2          31        RAMAD1      RAMAD0                                   
2          32        GPRIF/      PERF/                                    
2          33        IOSF        DSEL8/                                   
2          34        DSEL4/      DSEL2/                                   
element 80 b1                                                             
2          35        DSEL1/      SW1/                                     
2          36        GND         CONECT/                                  
2          37        GND         LOCAL/                                   
2          38        GND         CLR/                                     
2          39        GND         SEMREQ/                                  
2          40        INRAMD2     INRAMD8                                  
2          41        INRAMD1     INRAMD4                                  
2          42        TEST14      INRAMPR                                  
2          43        TEST12      TEST13                                   
2          44        TEST10      TEST11                                   
2          45        GND         TEST9                                    
2          46        TEST8       TEST6                                    
2          47        TEST5       OFFLN/                                   
2          48        AF/         BUFFEND/                                 
2          49        GND         CLKEN/                                   
element 80 c1                                                             
3          50        OPDEC4      OPDEC8                                   
3          51        OPDEC1      OPDEC2                                   
3          52        SEL6/       TERMF/                                   
3          53        SEL4/       SEL5/                                    
3          54        GND         BR6                                      
3          55        #L3         #L4                                      
3          56        #L1         #L2                                      
3          57        #K3         #K4                                      
3          58        #K1         #K2                                      
3          59        #J3         #J4                                      
3          60        #J1         #J2                                      
3          61        #I3         #I4                                      
3          62        #I1         #I2                                      
3          63        #H3         #H4                                      
3          64        #H1         #H2                                      
3          65        STOPB/      #G4                                      
3          66        #CONST6     #G3                                      
3          67        #CONST4     #CONST7                                  
3          68        #CONST2     #CONST5                                  
3          69        #CONST0     #CONST3                                  
3          70        GND         #CONST1                                  
3          71        #BRANCH3    #BRANCH2                                 
3          72        #BRANCH1    SEL3/                                    
2          73        SEL2/       GND                                      
3          74        GND         A0                                       
______________________________________                                    
 
    
     The following Table V is a listing of the signal lines connecting the Peripheral (Train Printer) to the peripheral-connector on the PDB card 80. 
     
                       TABLE V                                                     
______________________________________                                    
Signal Lines Between Train Printer and Peripheral                         
Dependent Board (PDB 80).                                                 
Peripheral     20                                                         
Connector      Conductor    Signal                                        
On PDB         Coax Cable   Name                                          
______________________________________                                    
$75                                                                       
$76                                                                       
$77            $C           PCTP/                                         
$78            $D           DC1L/                                         
$79            $E           DC2L/                                         
$80            $F           DT1L/                                         
$81            $G           DT2L/                                         
$82            $H           DT4L/                                         
$83            $I           DT8L/                                         
$84            $J           MOST/                                         
$85            $K           PR1L/                                         
$86            $L           CSL/                                          
$87            $M           PCSL/                                         
$88            $N           PFCL/                                         
$89            $P           PAML/                                         
$90            $Q           EDPL/                                         
$91            $R           PRSL/                                         
$92            $S                                                         
$93            $T                                                         
$94            $U                                                         
$95            $V           CIDL/                                         
$96            $W           PR2L/                                         
$97                                                                       
$98                                                                       
$99                                                                       
______________________________________                                    
 
    
     THE PROM CONTROL-STORE: The PROM control-store 13 shown in FIG. 4A is made up of 13 PROM chips located on the CFE card 10 c . These chips are combined to make up a 52-bit micro-code instruction word. The address lines of the 13 PROM chips are bused together so that all of the individual address lines are common to every single chip. The chip select on each PROM chip is always enabled (grounded). 
     The data output of the 13-chip PROM matrix forms the 52-bit word. This word is read out of the address that is present on the common input address line A0-A9 (FIGS. 4A, 5A). The PROMs herein are basically unclocked devices so that means is required to synchronize their outputs with an eight megahertz clock which feeds through gate 24. This is done by making use of register chips 14, The register chips contain eight flip-flops each. Thus, seven chips are used to synchronize and latch the 52-bit micro-code word. This latched micro-code instruction word is used to control the operation of the entire data link processor. Every eight-megahertz clock pulse latches the next subsequent word into the register chip 14. 
     Different types of data link processors require their own unique micro-code. Thus, all common front end cards will contain identical hardware except for the 13 PROM chips. Although the PROM word physically contains 52-bits, only 49 bits are used by the micro-code program. The remaining three bits are not party checked. Tables IVa and VIb indicate the 49-bit micro-code word, by bit position and mane. All PROM output signal names are preceded by a &#34;pound sign&#34; (#) so that they are easily recognizable. Bit 32 of the micro-code word is the odd parity bit. The CFE card is made to continually check for odd parity and it halts the micro-code program if a parity error (even parity) occurs on any 49-bit PROM micro-code word. 
     
                       TABLE VIa                                                   
______________________________________                                    
PROM Output Signals                                                       
Bit       Name       Bit         Name                                     
______________________________________                                    
48        #NEXT9                                                          
47        #NEXT7     23          #LDINT/                                  
46        #NEXT6     22          #LDSTK/                                  
45        #NEXT5     21          #G3                                      
44        #NEXT4     20          #G4                                      
43        #NEXT3     19          #H1                                      
42        #NEXT2     18          #H2                                      
41        #NEXT1     17          #H3                                      
40        #NEXT0     16          #H4                                      
39        #NEXT8     15          #I1                                      
38        #BRANCH1   14          #I2                                      
37        #BRANCH2   13          #I3                                      
36        #BRANCH3   12          #I4                                      
35        #BRANCH4   11          #J1                                      
34        #BRANCH5   10          #J2                                      
33        #LCPSTL/   09          #J3                                      
32        #PARITY    08          #J4                                      
31        #CONST7    07          #K1                                      
30        #CONST6    06          #K2                                      
29        #CONST5    05          #K3                                      
28        #CONST4    04          #K4                                      
27        #CONST3    03          #L1                                      
26        #CONST2    02          #L2                                      
25        #CONST1    01          #L3                                      
24        #CONST0    00          #L4                                      
______________________________________                                    
 
    
     
                       TABLE VIb                                                   
______________________________________                                    
TP-DLP                                                                    
CFE/PDB PROM Name                                                         
Cross Reference                                                           
Table                                                                     
         PROM                                                             
CFE      Word            PDB                                              
Name     Bit             Name                                             
______________________________________                                    
#G3      21              LOADCONT                                         
#G4      20              ENDATMPX                                         
#H1      19              LDREGRAD                                         
#H2      18              CONTRAD1                                         
#H3      17              CONTRAD2                                         
#H4      16              CONTRAD3                                         
#I1      15              CONTRAD4                                         
#I2      14              CONTRAD5                                         
#I3      13              CONTRAD6                                         
#I4      12              &#34;BRANCH 6&#34;                                       
#J1      11              CLKLPWRG                                         
#J2      10              MPXDATAB                                         
#J3      09              MPXDATAA                                         
#J4      08              (not named)                                      
#K1      07              SELTHBH                                          
#K2      06              THRAMCLD                                         
#K3      05              ACC8                                             
#K4      04              ACC4                                             
#L1      03              ACC2                                             
#L2      02              ACC1                                             
#L3      01              WEREGFIL                                         
#L4      00              RESTLPW                                          
______________________________________                                    
 
    
     MAINTENANCE CONTROL: As seen in FIG. 4A, the common front end contains receivers 15, 16, 17 which include decoding logic. Receiver 17 is used for operation of the maintenance control lines 33, 34. Table VII shows the addressing signals for maintenance mode of the data link processor. Thus, this Table lists all possible maintenance to common front end addressing codes with the response of any given data link processor. The Maintenance Card (MC) has the ability to address any one of eight DLPs in the base module. 
     
                       TABLE VII                                                   
______________________________________                                    
ADDRESS CODES: DLP Maintenance Mode Addressing                            
(Maintenance Card to CFE)                                                 
Base     Address     Address                                              
Local    Valid       Equal      Action                                    
(BASLCL/0)                                                                
         (ADRYLD/0)  (LOCnn/.0) Produced                                  
______________________________________                                    
1        1           X          Normal on-line                            
                                mode                                      
1        0           1                                                    
1        0           0          Standard local                            
                                mode, all main-                           
                                tenance available                         
0        0           0          Local mode with                           
                                base in local                             
0        0           1          Base in local and                         
                                the DLP clocks                            
                                are disabled                              
0        1           X          Base single-pulse                         
______________________________________                                    
 1 = high.                                                                
 0 = low.                                                                 
 X = don&#39;t care.                                                          
 
    
     The high order address line from the maintenance card (LOC 16/.0) must be &#34;high&#34; in order to address a DLP. It will be seen that all backplane signals are low active. The other four address lines provide encoding for DLP selection. The CFE does not decode the maintenance address until the maintenance card indicates the address is valid and stabilized by driving ADRVLD/0 low. 
     The maintenance card drives four lines which are used to enable unique maintenance features in any given DLP or connection module. 
     When in local mode, the CFE uses one of these lines (SWH.1/.0) for allowing the maintenance card to drive the address lines of the CFE PROM 13. When this line (SWH.1/.0) is low, the peripheral dependent board drives RCV/high and drives SIMRCV/low, as well as not driving the 17-line data bus 32. 
     Table VIII shows a list of the maintenance card lines which drive the PROM address lines in this local mode. This feature of the maintenance operation is used to verify PROM controller 13 integrity. 
     
                       TABLE VIII                                                  
______________________________________                                    
Maintenance Lines Driving PROM Address                                    
PROM                                                                      
Address     Maintenance                                                   
Line        Line       (Data Simulation)                                  
______________________________________                                    
A9          DSIMC8                                                        
A8          DSIMC4                                                        
A7          DSIMA8                                                        
A6          DSIMA4                                                        
A5          DSIMA2                                                        
A4          DSIMA1                                                        
A3          DSIMB8                                                        
A2          DSIMB4                                                        
A1          DSIMB2                                                        
A0          DSIMB1                                                        
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     RAM BUFFER: The common front end 10 c  of FIG. 4A contains a random access memory buffer (RAM) 22. This buffer memory 22 has a specifically designed organization for use in control of the Train Printer mechanism as seen in FIGS. 5B and 5C. This RAM buffer (data RAM) consist of 1,024 17-bit words. All inputs and outputs to this RAM are received by or driven by the peripheral dependent board PDB 80. The designation for the open collector line (60 nanosecond read access RAM) is RWON. This storage area is used to store the data, the OP code, the descriptor links, the descriptor links longitudinal parity word (LPW) and the various flags that are required to properly control the operation of the data link processor. 
     DLP ADDRESS AND REQUEST JUMPERS: There are eight backplane lines which are used by the distribution card(s) to address any one given data link processor (DLP). Likewise, eight backplane lines are used by the data link processors (DLPs) to indicate a service request to the distribution cards. The 16 lines are unique in that only one data link processor (DLP) can use a given request line. Further, the request lines are graded by priority. Once the priority of a data link processor is determined, that priority request line is &#34;jumpered&#34; for use on the common front end card (FIG. 3). The request and the address lines are identically numbered and operate as pairs. Therefore, once a request priority level is determined and jumpered, the corresponding address line is jumpered on the common front end card. 
     DLP LOCAL ADDRESS JUMPERS: The common front end card requires a minimum of two and a maximum of three jumpers (FIG. 3) to implement its local maintenance address. This address is used by a maintenance card, as 20 0m , to address a data link processor as 20 00 . The data link processor&#39;s local address must always correspond to its on-line DLP address. 
     STACK REGISTER: The stack register 11 consists of three binary counter chips. This register contains the value of the current PROM address, or the value of the address to be used when returning from a subroutine by way of a stack-branch operation. 
     The Train Printer Data Link Processor will be seen to be made of two slide-in printed circuit cards. These are the common front end (CFE) card and the peripheral dependent board (PDB). Each of these two boards are totally different in function and structure; however, when they are used together they make up the totality of the Train Printer DLP. 
     The primary function of the CFE 10 is to provide a device in which to store and execute the applicable micro-code. The micro-code is used to sequence the operation of the data link processor (DLP). The random access memory (RAM 22 ) is housed on the common front end card and provides internal data link processor storage for various uses such as data storage, translation storage, etc. 
     The peripheral dependent board (PDB), on the other hand, contains the necessary logic to interface the peripheral device (as, for example a Train Printer) to the host system through the message level interface (MLI). (The message level interface 15 i  will also be seen in U.S. Pat. No. 4,162,520 in FIGS. 1A, 2 and 3). The logical elements of the PDB are controlled by using the micro-code coming from the common front end (CFE). However, also, many signals are passed from the peripheral dependent board to the common front end to allow logical decision-making by the micro-code. 
     As seen in FIG. 4A there are interfaces to the front plane (double arrow) and to the backplane (single arrow) which involve the maintenance card. These MC lines will be seen as lines 30, 34 and 40. 
     MAINTENANCE CARD CONNECTION: There is a maintenance card common front end interface (MC/CFE) which is used when the maintenance card (MC) requires connection to a data link processor in order to perform normal maintenance functions. The maintenance card tries to connect to a DLP by driving the appropriate local maintenance address line (LOCnn/.0) and the signal ADRVLD/0 (address valid) low. This action causes the addressed CFE 10 c  to recognize the MC connection and to drive the logic LOCAL/.1 low. The LOCAL/.1 term is used to enable a backplane receiver chip which allows the MC to take the DLP off-line (OFFLNE/0 signal coming from the MC goes low). 
     If the MC takes a DLP off-line, it is unavailable to the host system. If the DLP is off-line it does not see any DC (Distribution Control Card) attempts at connection, and all DLP requests to the DC are inhibited. 
     The maintenance card always takes the data link processor off-line when maintenance routines are called for. This is done so that maintenance functions (such as single-pulse, and PROM verification) do not interfere with normal distribution control card operations to the on-line data link processors. The logic term LOCAL/.1, when low, is also used to enable the following functions: 
     1. It allows GPRIF/.0 (from the maintenance card) to turn off the data link processors peripheral interface. 
     2. It becomes LOCAL/ which is used by the peripheral dependent board PDB for bus interface directional control logic. 
     3. It is used to allow the maintenance card (MC) local clear and PROM verification (SW1/) function. 
     4. It is used by the common front end card (CFE) for the development of the clock-enable term CLKEN. 
     DISTRIBUTION CARD CONNECTION: The following discussion involves the connection mechanism as used by the distribution control card/common front end card interface (DC/CFE) when the distribution control card requires connection to a data link processor. This connection will be initiated by a distribution control card (DC) poll test or by a data link processor (DLP) initiated poll request. The connection logic is shown in FIG. 4D. 
     The CONST lines 4-7 are fed as input to a 4-bit binary counter J3-C. The output of this counter is fed to a tri-state inverter C4-C to provide, as output, the &#34;status&#34; lines LCPSTU--. The data link processor address LCPADn/0 and the off-line signal OFFLN provide inputs to buffer chip M5-C whose output is fed through inverter P4-C to form the LCPAD signal to NAND gate M3-C. NAND gate M3-C also has signal inputs for parity error (PERF/) and off-line (OFFLN/). The output of NAND M3-C is the signal CONECT/ which connects to the PDB bus directional control PROM and also provides inputs to inverters C4-C, C4-C1, E4-C and also to the buffer F4-C. The CONECT/ also is formed as input to NAND B3-C having an output which is fed to NOR gate A4-C. The NOR gate A4-C is fed to inverter B4-C to form the logic control signal CLRD. The outputs of the inverter C4-C1 form the signal IOSND/.0 (input-output send) and the signal LCPCON/0 (data link processor connected). The buffer F4-C provides a distribution card strobe output and a distribution card terminate signal. The inverter E4-C provides a strobe level signal LCPSTL/0 (data link processor strobe level). 
     The distribution control card (DC) will attempt to connect to a data line processor (DLP) by driving the appropriate DLP address line (LCPADn/0) low. This backplane signal is applied to a buffer chip (M5-C) which is enabled when the data link processor (DLP) is on-line. The output of the buffer chip is then fed through an inverter (P4-C) and applied to a 3-legged NAND gate (M3-C) to become the term-CONFECT/. This CONFECT/ term is used to enable the following set of signals on the DC/CFE interface: 
     1. LCPCON/0. This line goes low when the DLP is connected. 
     2. LCPSTUn/0. These are the four data link processors status lines. 
     3. IOSND/.0. This is the state of the I/O-send flip-flop. 
     4. LCPSTL/0. This is the data link processor strobe signal. 
     5. STIOL/.0. This is the host system strobe signal. 
     6. TERM/.0. This is the host system terminate signal. 
     7. SELCLR/0. This is the distribution card (DC) to the data link processor (DLP) selective clear signal. 
     8. DATAxn/0. This is the 17-bit data bus. 
     All these backplane signals are low active and are enabled only when (a) the CFE is properly addressed by the distribution control card (DC) and (b) a connection is possible, that is, when the data link processor is on-line, and there is no PROM parity error. The actual state of the enabled lines is dependent upon the logical condition of the data link processor and the distribution control card (DC) at the time of the connection. 
     DATA LINK PROCESSOR REQUESTS: A DLP request is the method by which a data link processor can notify the distribution control card that it needs host system attention. A request can be considered to be a DLP interrupt (REQ) to the distribution card (DC). 
     A DLP request is made to the distribution control card when the DLP drives its jumpered request line (LCPRQn/0) low, or when the DLP drives its jumpered request line and EMRREQ/0 (emergency request) low. Both types of these requests can be generated on the common front end card (CFE) by the PROM stored micro-code program. When the micro-code-program drives the PROM output line #LDINT/ (load interface) low, the signals #CONST1 (for emergency request), and #CONST2 (for request) are loaded into a register called the request (FIG. 4A) latch 19 (FIG. 4A) at CLK8/time. Combinational type logic on the common front end allows &#34;emergency request&#34; to be enabled on the backplane anytime it occurs, if the data link processor is &#34;on-line&#34;. If a DLP issues an emergency request, it also issues a non-emergency request. This is done so that the distribution control card can determine which DLP is doing the emergency-requesting. 
     The backplane line EMRREQ/0 is common to all data link processors in the base module, and the LCPRQn/0 lines are each unique. If a data link processor issues a non-emergency request, it is &#34;enabled&#34; on the backplane only if the data link processor is on-line and no other data link processors in the base module are doing any emergency-requesting. 
     DATA LINK PROCESSOR DATA TRANSFER RATE: Two possible modes of operation govern the data transfer rate between the data link processor DLP and the main host system 10. These modes are called (a) demand mode and (b) burst mode. 
     Demand mode causes data transfers to occur at a rate of less than four-megahertz. Burst mode permits data word (16-bit) transfers to occur at a four-megahertz rate, that is, 64 megabits per second. 
     The data link processor (DLP) and the distribution control card (DC) send &#34;strobe acknowledge&#34; signals back and forth to each other when they present or accept data. The DLP sends the term LCPSTL/0 (DLP strobe level) to the distribution control card, and the distribution control card sends STIOL/.0 to the data link processor. These strobing signals are exchanged during both the demand mode and the burst mode operations. The data transfer rate is determined by the speed at which these strobe signals are exchanged. FIG. 4E show a data transfer timing diagram from the distribution control card (DC) to the data link processor. The common front end card (CFE) receives a STIOL/.0 signal from the distribution control card, and synchronizes it with the 8-megahertz clock by the use of a Schottky J-K flip-flop whose outputs are named AF (asynchronous flop) and AF/. The synchronized strobe level AF is used on the common front end card for PROM address bit AO micro-code testing. The AF/ signal is available for use on the peripheral dependent board (PDB) by way of a foreplane connector pin. 
     DEMAND MODE: During buffer loading operations, the distribution card (DC) informs the data link processor that data is available by driving STIOL/.0 low. The data link processor micro-code program informs the distribution card that it has accepted the data that is on the distribution card interface line by driving the PROM logic term #LCPSTL/ low. 
     #LCPSTL/ becomes LCPSTL/0 to the distribution card and causes the distribution card (DC) to drive the STIOL/.0 high until the next new word of data is available. 
     During buffer read operations, the distribution card informs the data link processor that it is ready to accept new data by driving STIOL/.0 low. The data link processor micro-code instructions inform the distribution card that new data is available on the distribution card interface line by driving the PROM logic term #LCPSTL/ low. #LCPSTL/ becomes LCPSTL/0 to the distribution control card. The distribution card informs the data link processor that it has accepted the data by driving the STIOL/.0 high. 
     The direction of the data flow is controlled by the PROM output term #CONST3. The term #CONST3 is clocked into the request latch 19 at PROM signal #LDINT/ (Load system interface) time and becomes IOSF (I/O send flop). The state of IOSF is sent to the distribution control card as IOSND/.0, and is used to inform the distribution control card of the interface bus line direction. The state of IOSF is also sent to the peripheral dependent board (PDB) 80. The PDB 80 uses IOSF to assist in developing the interface bus line directional control logic. This logic determines which data link processor bus is active and what drivers or receivers must be used. 
     BURST MODE: While certain Data Link Processors can operate in either Demand Mode or in Burst Mode, the Train Printer Data Link Processor operates only in Demand Mode. 
     CLEAR FUNCTIONS: The CFE card 10 c  makes use of combinational type logic to implement its clearing functions. A description of clearing functions applicable to all data link processors using the common front end card follows: 
     Power-Up Clear: A power-up clear signal is provided to the base module from either the power supply of the cabinet or from some external source as determined by the host system 10. This signal is attached via coaxial cable to a pin of the maintenance card backplane connector. The maintenance card uses this signal to create the master clear, MSTCLR/0. The signal MSTCLR/0 clears the data link processor if it is on-line. 
     Base Clear: The base clear function is provided by a pushbutton switch located on the maintenance card foreplane connector. The switch is ORed with the power-up clear and performs the same function as a power-up clear. 
     maintenance local clear: If the maintenance card is connected to a data link processor, the maintenance card can clear the data link processor by driving LCLCLR/0 low (clear level from maintenance card). 
     Host System Master Clear: The host system 10 can issue a master clear signal across its message level interface 15 i  (MLI). The distribution control card (DC) contains a host system jumper option which, if installed, causes the master clear (MSTCLR/0) to go &#34;low&#34;. This signal clears all the data link processors that are not addressed by the maintenance card. If the distribution control card host system option jumper is not installed, then the host master clear signal is not passed through the distribution control card to the backplane. 
     Host System Selective Clear: The host system 10 can clear a single data link processor during a standard poll test operation by driving the message level interface (MLI) line TRM+ MC/1 low. This action causes the distribution card to drive SELCLR/0 low. The SELCLR/0 being &#34;low&#34; causes the connected data link processor to clear. 
     FIG. 3 shows in schematic form the physical structure of the common front end (CFE) card 10 c  while FIG. 4A shows the basic circuit block diagram of the common front end card. Included in the CFE is the clear circuitry shown in FIG. 4B. It will be seen that NOR gate 113 has inputs from NAND gates 110 and 111 in addition to an input from inverter 114 which is fed by buffer 112. The output of gate 113 is fed to buffer 115 and also to inverter 116. Inverter 116 feeds its signal to a Schottky data register chip 117 which provides an output to the PROM address multiplexor 12 of FIG. 4A. The output of data register 117 is also used to provide one of the inputs to NAND gate 118. 
     A &#34;low&#34; signal which results on the output of the NOR gate 113 occurs if any one of the following conditions is met: 
     1. MSTCLR/0 and OFFLN are both &#34;low&#34;. 
     2. SELCLR/0 and CONECT/ are both low. 
     3. LCLCLR/0 and LOCAL/ are both low. 
     The &#34;low&#34; output of NOR gate 113 is used to perform the following functions: 
     1. The output is fed through a buffer chip 115 to become signal CLR/. CLR/ clears the PROM parity error flip-flop (and flip-flops SOTB, AF, and TERMF) on the CFE 10 c . 
     2. CLR/ is sent to the peripheral dependent board (PDB) 80 to clear the specific peripheral-dependent logic. 
     3. The low output signal of NOR gate 113 is sent through an inverter 116 and is applied to one input of the data register chip 117. It is sent as the logic term CLRD. The signal CLRD is double synchronized with CLOCK/ and becomes the signal SCLR (synchronized clear). 
     4. The signal SCLR from register 117 is used to force the PROM clock-disabling term CLKST (clock stop) &#34;low&#34;. This is done in the event that a PROM parity error may have caused CLKST to go high. 
     5. The SCLR signal is used to disable the PROM address multiplexor chips 12. This forces the PROM address lines to all zeros. Address zero is the starting address of all data link processor micro-code programs. 
     CFE CLOCK CONTROL: The clock control logic in the CFE 10 c  uses combinational type logic (NAND gates, NOR gates, inverters, buffers, and a Schottky data register chip), in order to enable or disable the always-present 8-megahertz backplane clock (CLOCK . . . 0). The circuitry for the CFE clock control is shown in FIG. 4C. The CFE clock control logic constantly monitors the state of the maintenance bus 30 to determine how it should supply clock signals to the data link processor. In Table VII there was shown the available clock control options and the maintenance bus line states that are necessary to cause the various options to be active. Table VIII showed the maintenance lines for driving the PROM address lines A0-A9. 
     In the CFE clock control circuitry shown in FIG. 4C, it will be seen that the NAND gate A3 has three inputs and the NAND gate 13-1 has four inputs. The first input to NAND gate A3 is from the data register C3 along the SCLR line. The second input to gate A3 is the line PERF/. The PERF signal is the PROM parity flip-flop signal. When &#34;high&#34;, it indicates that an error has been detected on the PROM output register 14, thus stopping the DLP clock. The PERF/ signal is the complement of PERF. The third input to gate A3 is from the PROMCLK/ output of NOR H4-1. 
     The first input to gate 13-1 is the SCLR line. The second input to gate 13-1 is the PERF/ line. The third input to gate 13-1 is from the CLKEN line and the fourth input to gate 13-1 is the output of the buffer N5-1. 
     The NAND gate M3 has one input from inverter P4 (ADLOC) and another input coming from the inverter B4-1 (BASLCL/0). 
     The input for the data register C3 comes from NOR gate A3-2 which has one input from inverter B4, one input coming from NOR gate A3-1, and one input coming from the line START/.0. The NOR gate A3-1 has two inputs, one of which is the line BASLCL/0 and the other input is the line LOCAL/.1. 
     The output of gate A3 is fed to the input of NOR gate H4-1. The output of NAND gate 13-1 is fed to the input of NAND gate H4-13. The output of inverter H5 (CLOCK . . . 0) is fed to both the input of gate H4-2 and gate H4-13. 
     The output of gate A3 is fed to the input of NOR gate H4-1 to form the signal line PROMCLK/. The output of NAND gate H4-13 is fed to buffer 14-1 and to buffer G4. NORMAL ON-LINE MODE: When certain of the following conditions are met, then all data link processor clocks are active at the 8-megahertz rate. These conditions are: 
     1. BASLCL/0 is high (the base module is not in local). 
     2. LOCAL/.1 is high (the DLP is not Maintenance Card (MC) addressed). 
     3. PERF/ is high (no PROM parity error). The signals BASLCL/0 and LOCAL.1, both being &#34;high&#34;, causes a &#34;high&#34; on to one input of the Schottky data register C3. This register chip is clocked by the always-present signal CLOCK/. This signal is derived from the backplane clock signal CLOCK . . . 0 after a triple inversion. One output of the register chip C3 becomes the logic term clock enable (CLKEN). The signal clock enable is NANDed on two gates (A3-4 and 13-1) using the signals PERF/, SW1/, and the output of NAND gate M3. Gate M3 has inputs which are: 
     a. The signal ADLOC/ (after inversion). The ADLOC/ is low when the DLP is not addressed by the MC, or when ADRVLD/0 is high. 
     b. The backplane signal BASLCL/0 (after inversion). 
     The output of NAND gate A3-4 becomes the signal logic term B (P-CLKEN) and this is NANDed with the signal CLOCK . . . 0 (after inversion by inverter H5 which is fed as one input to gate H4-2). The output of gate H4-2 is fed through a NOR gate H4-1 to form the signal PROMCLK/. This signal PROMCLK/ is used to clock the PROM data registers 14 and the stack register 11 of FIG. 4A. 
     The output of NAND gate 13-1 becomes the signal CLKEN/. This signal CLKEN/, when &#34;low&#34;, enables the 8-megahertz backplane clocks on the peripheral dependent board, PDB 80. 
     The CLKEN/ signal is NANDed with the signal CLOCK . . . 0 (after inversion) at the gate H4-13. The output of gate H4-13 is fed through one inverter buffer 14-1 and also through one non-inverting buffer G4 to become the signals respectively CLK8/ and CLK8 (8-megahertz clock. These clock signals are used in the German Front End Card 10 c . 
     STANDARD LOCAL MODE: The standard local mode of &#34;maintenance&#34; is operative under the following conditions: (Also see Table VII). 
     1. BASLCL/0 is high (base is not in local). 
     2. ADRVLD/0 (address valid) is low; and the LOCnn/.0 lines on the maintenance bus are valid. 
     3. The LOCnn/.0 lines equal the local address jumpers on the CFE 10 c . 
     This equal comparison together with the ADRVLD/0 being low, forces LOCAL/.1 into low. LOCAL/.1 low means that the DLP is &#34;maintenance card addressed&#34;. 
     In this mode all the maintenance functions are available; for example, the maintenance card can now: 
     1. select the single-pulse mode. 
     2. perform PROM micro-code verification. 
     3. set up known conditions by manipulating the PROM address during single-pulse operations, and test for known predicted results by sampling the maintenance bus display (DPLY) and the data simulate (DSIM) lines. SINGLE-PULSE MODE: After the maintenance card has localized the data link processor (DLP), it can place the DLP in a single-pulse mode by driving the signal SP/ . . . 0 into &#34;low&#34;. This cation forces the logic term CLKEN into &#34;low&#34; because the NOR gate A3-2 of FIG. 4C is disabled by the following logic: 
     1. Signal SP/ . . . 0 is inverted into &#34;high&#34; and disables the top input line of gate A3-2. 
     2. The signal LOCAL/.1 is low because the DLP is not maintenance card addressed. This action disables the middle input line of gate A3-2 because of the high signal output of gate A3-1. The signal START/.0 is high and disables the third input of gate A3-2. When gate A3-2 is disabled, a low signal is set into the data register C3 at CLOCK/time. The output signal CLKEN, when low, turns off NAND gates A3-4 and 13-1, and causes their outputs to go high and to disable the NAND gates H4-2 and H4-13. When these two gates are disabled, the DLP clocks are turned off. 
     Once the DLP is placed into a single-pulse mode, the maintenance card can issue from 1 to 4,096 clocks by driving the START/.0 low. START/.0 is a pulse which effectively is a window around a desired number of clocks that the maintenance card wants issued. During the time frame when START/.0 is low, the high signal output of gate A3-2 is clocked into the data register C3 by the always-present clock signal CLOCK/. The CLKEN now becomes a pulse rather than a level, and the DLP clocks are enabled only during the low active time span of START/.0. 
     After the data link processor (DLP) has been placed into the single-pulse mode, the maintenance card can take the DLP out of it by driving SP/ . . . 0 high. The signal SP/ . . . 0, when high, forces a high level input into register C3 as follows (FIG. 4C): The signal SP/ . . . 0 is inverted low by the inverter B4. This low signal is fed into NOR gate A3-2 and causes its output to go high. This high level is clocked into data register C3 with CLOCK/ and becomes CLKEN. The signal CLKEN, when high, enables all the data link processor&#39;s clocks. 
     PROM MICRO-CODE VERIFICATION: If the maintenance card has localized and placed the data link processor into the single-pulse mode, it can read any common front end PROM micro-code word by driving the signal SWH.1/.0 low. The signal SWH.1/.0 is routed through the buffer N5-1 (FIG. 4C) to become the logic term SW1/. This signal SW1/ is sent to the peripheral dependent board PDB 80 and is used in the bus interface directional control logic in order to develop the logic term SIMRCV/, (Simulate Receive, FIG. 4A, line 33). When SIMRCV/ is low, the maintenance bus lines 10 dm  DSIMnn/0 of FIG. 5B are gated into the inputs of PROM address multiplexor chips 12 of FIG. 4A. The SW1/ also forces the PROM address multiplexors 12 to select the DSIMnn/0 data as the actual PROM address. This way the maintenance card controls the current PROM address. SW1/ is also tied to one of the inputs of NAND gate 13-1, FIG. 4C, and is used to keep the clock signals CLK8/, CLK8, and the clock-enable signal CLKEN/ (to the peripheral dependent board 80) turned off. 
     When the maintenance card issues a single pulse clock, the PROMCLK/ latches the addressed PROM data into the PROM registers 14 (FIG. 4A) because SW1/ is not tied to the NAND gate A3. The maintenance card now has the current PROM word available for sampling. The maintenance card can now read the PROM data (8-bits at a time) by driving the maintenance bus lines DSELn/.0. The DSELn/.0 lines 34, FIG. 4A, are used on the CFE 10 c  to select which 8 data bits will be enabled onto the maintenance bus lines DPLYnn/0, at 40 of FIG. 4A. Seven reads are required to sample one entire micro-code address. 
     LOCAL MODE WITH BASE IN LOCAL: Under certain conditions the maintenance mode is operatively in effect; these conditions are: 
     1. BASLCL/0 is low (the base is in local). 
     2. ADRVLD/0 is low (that is, the LOCnn/.0 lines on the maintenance bus are valid). 
     3. The LOCnn/.0 lines equal the local address jumpers on the CFE 10 c . 
     This equal comparison, along with ADRVLD/0 low, forces LOCAL/.1 low also. LOCAL/.1, when low, means that the data link processor DLP is being maintenance card addressed. This mode of operation causes the addressed data link processor to function in exactly the same manner as the standard local mode. All unaddressed DLPs disable their own clocks. BASE MODULE IN LOCAL MODE AND DLP CLOCKS DISABLED: This is a mode of operation which takes place when the following conditions are satisfied: 
     1. BASLCL/0 is &#34;low&#34; (the base is in local). 
     2. ADRVLD/0 is low (the LOCnn/.0 lines are valid). 
     3. The LOCnn/.0 lines do not equal the CFE local address jumpers. 
     This &#34;unequaled&#34; comparison along with ADRVLD/0 being low, forces LOCAL/.1 high and ADLOC/ low. The LOCAL/.1 when high, means that the DLP is not maintenance card addressed. 
     In this mode all the DLP clocks are disabled because the BASLCL/0 low level is inverted by the inverter B4-1 (FIG. 4C). This level &#34;high&#34; is NANDed by gate M3 with the inverted ADLOC/ signal (inverted by P4). When both inputs to gate M3 are &#34;high&#34;, the output goes low and disables the NAND gate A3-4 and 13-1. The outputs of these gates both go high and disable the gate H4-1 and H4-13. When these two gates are disabled, all the DLP clocks are inhibited. BASE SINGLE-PULSE: The single-pulse maintenance mode is in effect when the following conditions occur: 
     1. BASLCL/0 is &#34;low&#34; (the base is in local). 
     2. ADRVLD/0 is high (that is, the LOCnn/.0 lines on the maintenance bus are invalid). 
     In this mode of operation the maintenance card can place the entire base in a single-pulse mode by driving the logic as follows: 
     1. SP/ . . . 0 low--this action disables the top input of the NOR gate A3-2. 
     2. BASLCL/0 is low. This action puts the base in local and causes the output of NOR gate A3-1 to go high. This high level disables the middle input line of NOR gate A3-2. 
     3. START/.0 high disables the bottom input of NOR gate A3-2. 
     When NOR gate A3-2 (FIG. 4C) is disabled, its output goes low. This low output is latched into the data register C3 and becomes the logic term CLKEN. This clock enable line, when low, disables NAND gates A3-4 and 13-1. The outputs of these NAND gates go high and disable the gates H4-1 and H4-13. When the gates H4-1 and H4-13 are disabled, then all DLP clocks are inhibited. 
     The &#34;base single-pulse&#34; maintenance mode of operation will make all data link processors respond to single-pulse because ADRVLD/0 is high and BASLCL/0 is low. The signal ADRVLD/0 when high, disables the tri-state 8-2-1 multiplexor chip (12, FIG. 4A) on the CFE that is used to generate the logic terms LOCAL/.1 and ADLOC/. When this multiplexor chip is disabled, both signals are pulled up high with 1100 ohm resistors to +5 volts. ADLOC/, when high, (after inverson to &#34;low&#34; by the inverter P4, FIG. 4C) disables the NAND gate M3. The high level output of gate M3 allows the single-pulse clock to be enabled when the maintenance card drives the signal START/.0 low. Further description of the single-pulse operation has been previously described in the &#34;standard local mode&#34; of operation. 
     STACK REGISTER OPERATION: The operation of the stack register 11 (FIG. 4A) is controlled by the state of a PROM 13 output signal line #LDSTCK/. The signal #LDSTCK/ is held low during non-subroutine micro-code instructions. This low level is applied to the low-enable inputs of the 3 stack register chips 11. This action causes the stack register 11 to load and hold the current PROM micro-code address. 
     All micro-code programs have the ability to enter one level of subroutine by driving and holding the signal #LDSTCK/ high. With #LDSTCK/ going high the following actions will occur: 
     1. The stack register load function is disabled. 
     2. The stack register count function is enabled. 
     The signal #LDSTCK/ of FIG. 4A is also routed through a Schottky data register chip to become the logic term STCLKEN/. This signal is then inverted and used to count up the stack register plus one. This address is held in the register until the subroutine is completed. This updated address is used when the subroutine does a stack branch to return to the main body of the micro-code instructions. The non-subroutine code is then re-entered and the signal #LDSTCK/ goes low. 
     MAINTENANCE DISPLAY LINES OF COMMON FRONT END (CFE) The CFE 10 c  contains standard items such as inverting tri-state buffers and eight-to-one multiplexor chips that are used to enable the maintenance display lines 40 of FIG. 4A (DPLY01/0-DPLY10/0). The lines 40 are shown with a /10 to indicate the 10 individual lines which compose this group of lines. Table IX lists the various individual display lines (DPLY01/0-DPLY10/0). 
     
                                           TABLE IX                                
__________________________________________________________________________
CFE Signals Displayed With Given                                          
Display Select Codes                                                      
Maintenance                                                               
       DSEL8/,DSEL4/,DSEL2/,DSEL1/Value*                                  
Display                                                                   
       (GRPO/=0)                                                          
Lines***                                                                  
       1111** 0111 0110   0101                                            
                             0100   0011  0010  0001                      
                                                   0000                   
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DPLY01/0                                                                  
       STCKA9 *    *      *  *      *     *     *  *                      
DPLY02/0                                                                  
       STCKA8 *    *      *  *      *     *     *  *                      
DPLY03/0                                                                  
       STCKA7 AF   #LDINT/                                                
                          #K1                                             
                             #NEXT8 PER   #CONST7                         
                                                #I1                       
                                                   #NEXT7                 
DPLY04/0                                                                  
       STCKA6 TERMF                                                       
                   #LDSTCK/                                               
                          #K2                                             
                             #BRANCH1                                     
                                    PERF  #CONST6                         
                                                #I2                       
                                                   #NEXT6                 
DPLY05/0                                                                  
       STCKA5 REQ  #G3    #K3                                             
                             #BRANCH2                                     
                                    SCLR  #CONST5                         
                                                #I3                       
                                                   #NEXT5                 
DPLY06/0                                                                  
       STCKA4 EMREQ                                                       
                   #G4    #K4                                             
                             #BRANCH3                                     
                                    CLKEN #CONST4                         
                                                #I4                       
                                                   #NEXT4                 
DPLY07/0                                                                  
       STCKA3 A3   #H1    #L1                                             
                             #BRANCH4                                     
                                    NEXT9 #CONST3                         
                                                #J1                       
                                                   #NEXT3                 
DPLY08/0                                                                  
       STCKA2 A2   #H2    #L2                                             
                             #BRANCH5                                     
                                    LCPADF                                
                                          #CONST2                         
                                                #J2                       
                                                   #NEXT2                 
DPLY09/0                                                                  
       STCKA1 A1   #H3    #L3                                             
                             #LCPSTL/                                     
                                    STOPB #CONST1                         
                                                #J3                       
                                                   #NEXT1                 
DPLY10/0                                                                  
       STCKA0 A0   #H4    #L4                                             
                             # PARITY                                     
                                    CLKST #CONST0                         
                                                #J4                       
                                                   #NEXT0                 
__________________________________________________________________________
 NOTES:                                                                   
 1 = HIGH                                                                 
 0 = LOW                                                                  
 *Maintenance display line not driven by CFE with this display select code
 **With this display select code, the CFE also drives DSTAT8/0-DSTAT1/0,  
 DLCPST/0, and DIOSND/0 (MC Group 24).                                    
 ***Signals are low true on backplane.                                    
 
    
     The common front end also has the ability to display signals on the lines DSTAT8/0-DSTAT1/0 (Glossary Table I), which are the data link processor &#34;status lines&#34; to the maintenance card. The CFE also can display DLCPST/0 (the data link processor strobe to the maintenance card) and also the DIOSND/0 (which is the SEND level of I/O to the maintenance card). All of these lines are used by the maintenance card (as 20 0m ) to test and verify the operation of the CFE card. The DSEL8/0 lines (multiplexor chip select line for input to the display lines)--DSEL1/0 (multiplexor address line for data selection to display lines) are sent to the common front end from the maintenance card and are used to select the CFE internal signals that are enabled on to the display lines after the data link processor has been addressed by the maintenance card. The Table IX shows a list of the displayable CFE signals and also the DSELn/0 line codes that are required to enable them on to the maintenance display bus. 
     PROM ADDRESS SELECTION: Referring to Tables Xa and Xb, the selection of micro-code addresses is implemented on the common front end card 10 c  which uses circuitry that employs standard logic gates, multiplexor chips, register chips and a decoder chip. The multiplexors 12 drive PROM address lines A0-A9 (FIG. 4A). This causes the micro-code data to be latched into the registers 14 with every 8-megahertz clock pulse. 
     
                       TABLE Xa                                                    
______________________________________                                    
CFE PROM Address Selection                                                
PROM Output                                                               
          PROM Input                                                      
Branch Lines                                                              
          Address Lines                                                   
(#BRANCHn)                                                                
          (An)                                                            
                                          See                             
BR6   54321   9-4         3-1     0       Note                            
______________________________________                                    
0     00000   #NEXTn      #NEXTn  #NEXT0  1                               
0     00001   #NEXTn      #NEXTn  AF      2                               
0     00010   #NEXTn      #NEXTn  TERMF   2                               
0     00011   #NEXTn      #NEXTn  LCPADF  2                               
0     00100   #NEXTn      #NEXTn  STOPF   2                               
0     00101   #NEXTn      #NEXTn  TEST5   2                               
0     00110   #NEXTn      #NEXTn  TEST6   2                               
0     00111   #NEXTn      OPDECn  OPDEC1  3                               
0     01000   #NEXTn      #NEXTn  TEST8   2                               
0     01001   #NEXTn      #NEXTn  TEST9   2                               
0     01010   #NEXTn      #NEXTn  TEST10  2                               
0     01011   #NEXTn      #NEXTn  TEST11  2                               
0     01100   #NEXTn      #NEXTn  TEST12  2                               
0     01101   #NEXTn      #NEXTn  TEST12  2                               
0     01110   #NEXTn      #NEXTn  TEST14  2                               
0     01111   STCKAn      STCKAn  STCKA0  4                               
--    1--     #NEXTn      #NEXTn  PDB MX  5                               
1     --      #NEXTn      #NEXTn  PDB MX  5                               
SW1/ = 0  Maintenance Data Bus    6                                       
CLEAR     0           0         0       7                                 
______________________________________                                    
 NOTES                                                                    
 1 = Unconditional branch.                                                
 2 = 2way branch (TEST5-TEST14 from PDB).                                 
 3 = 16way branch (OPDEC from PDB).                                       
 4 = Stack branch.                                                        
 5 = 2way branch (A0 is driven directly from the PDB).                    
 6 = Maintenance Card PROM read mode.                                     
 7 = Clear forces zeros onto the PROM address lines.                      
 1 = high. 0 = low. -- = don&#39;t care.                                      
 
    
     
                       TABLE Xb                                                    
______________________________________                                    
Enable Lines For Expanded A0 Selection                                    
PROM output                                                               
branch lines   Enable line to Peripheral                                  
(#BRANCHn)     Dependent board for expanded                               
BR6    5         4     selection of A0                                    
______________________________________                                    
0      0         0 . . . A0 is driven by multiplexors                     
                    on CFE card                                           
0      0         1                                                        
0      1         0     SEL2                                               
0      1         1     SEL3                                               
1      0         0     SEL4                                               
1      0         1     SEL5                                               
1      1         0     SEL6                                               
______________________________________                                    
 
    
     With reference to Table Xa showing the CFE signals, the primary multiplexor input-select lines are the current latched PROM outputs of #BRANCH1-#BRANCH5, Table IX and the signal term BR6 (Branch Line from PDB 80 for PROM address selection). 
     In order to accomplish Address Bit AO Selection (at line 36, FIG. 4A): #BRANCH1-#BRANCH3 will select the input lines to two separate multiplexor chips. #BRANCH4, #BRANCH5 and BR6 is peripheral-type dependent and may be driven either by the PDB logic or by one of the latched PROM outputs. If a PROM output line is used to drive BR6 (Table Xb), it is sent over to the peripheral dependent board PDB 80 and then is returned as the BR6 line. There are 22 such lines, #G3-#L4, Table IX, which are available to the peripheral dependent board, PDB 80. These lines are primarily used for controlling the peripheral dependent logic on the PDB 80. 
     The #BRANCHn bits and BR6 are also used for the generation of PROM address lines A0-A9 (FIG. 4A). This is accomplished by the use of the term BROP (PROM address branch) and BRST (use stack register for PROM address). BROP is &#34;high&#34; whenever the #BRANCHn bits and the BR6 select a branch to one of 16 addresses (16-way branch). BRST is high when the #BRANCHn bits and BR6 select a branch that uses the stack register 11 as the address. If neither a 16-way or a stack register branch is selected, then either an unconditional or a two-way branch is performed. 
     PROM address selection can also be achieved by the use of the maintenance term SW1/, (FIG. 4C). 
     When the CFE 10 c  is under the control of the maintenance card and the MC drives SW1/ &#34;low&#34;, the MC can address and read any micro-code address. This feature is utilized to read and verify the contents of the PROM matrix and to test the integrity of its associated registers. 
     The #BRANCHn bits and the BR6 are overridden whenever a master, selective, or local clear is issued to the CFE 10 c . Any clear condition forces the PROM address bus (A0-A9, FIG. 4A) to equal 0. The address bus A0-A9 is defined in Glossary Table I. Address 0 is the starting point of all micro-code programs. 
     As will be seen in Table Xa, there is a listing of all the possible branching conditions that can be used for the generation of the next PROM address. Table Xb shows a list of five CFE generated enable lines that can be used for an expanded selection of PROM address bit A0. These enable lines are activated when the PDB 80 requires additional two-way branching capabilities. 
     PROM PARITY CHECKING: The CFE card 10 c  performs an odd parity check on every 49-bit PROM micro-code word. The total word length is actually 52-bits but the three most significant bits are not used by the micro-code program. These three bits are not parity checked. 
     The currently addressed micro-code word is latched into the register chips 14 and from these register chips is fed into cascaded parity checking chips 18 of FIG. 4A. Thus, this circuitry allows the CFE 10 c  to examine the entire 49-bit word at one time. If the sum total of the word equals &#34;even&#34; parity, then the logic term PER (parity error) goes high. PER going &#34;high&#34; causes the &#34;parity error&#34; flip-flop of 18 to be set on the next 8-megahertz clock pulse. The setting of the parity error flip-flop causes the logic term PERF/ to go &#34;low&#34;. PROM PARITY ERROR: If &#34;even&#34; PROM parity is detected by the CFE card, the logic term PER goes &#34;high&#34; and causes the PROM parity error flip-flop to set. At the same time that PER goes high, then PER/ goes low. PER/ going low causes the logic term CLKST (clock stop) to go high. CLKST going &#34;high&#34; disables the clock that controls the latching of the PROM register chips (PROMCLK/). The disabling of PROMCLK/ causes the data (that caused the parity error) to remain in the registers 14. The disabling of PROMCLK/ also locks the stack register 11 to its current value. If the parity error flip-flop gets set, then the logic term PERF/ goes low on line 41 of FIG. 4A. The PERF/ going &#34;low&#34; causes the following actions to occur. 
     1. PERF/ is routed to the peripheral dependent board PDB 80 to turn off the peripheral device. 
     2. The request latch 19 on the CFE card is cleared. This action causes the I/O SEND flip-flop, REQ (request), and EMRREQ (emergency request) signals from Drivers 20 to turn &#34;off&#34;. 
     3. The logic term CONECT/ is forced high. This action prevents any distribution card (DC) connection and severs any DC connection that may have been in progress at the time of the error. 
     4. The peripheral dependent board 8-megahertz clock is turned off. PERF/ going low forces the logic term CLKEN/ high. CLKEN/ is used by the PDB 80 to permit 8-megahertz clocks. 
     5. The CFE 8-megahertz clocks (CLK8 and CLK8/) are turned off. 
     If a PROM parity error disables (locks up) a data link processor, the error condition can be removed through the use of a base power-up clear, a maintenance card base clear, a dis Master Clear or a maintenance card initiated DLP local clear. 
     TRAIN PRINTER DATA LINK PROCESSOR: Description 
     The Data Link Processor used in Burroughs I/O subsystems, whereby the Data Link Processors act as peripheral-controller interfaces between specified peripheral terminal units and a main host computer, have been designed so as to be assembled from two slide-in printed circuit cards. One of these cards is the CFE or common front end card which provides the common standardized functions of all types of data link processors, and the second card is the PDB, or peripheral dependent board, which supplies the particular functions required for a given type of peripheral terminal unit, in this case, the terminal unit being a Train Printer. Such Train Printers, which may also be called a train-type printer mechanism, come with generally 132 print positions and have unique 18, 48, 72 or 96 character sets. These line printers provide high quality, high speed alphanumeric output at operating speeds of 675 LPM (lines per minute), or up to 1100 LPM, or even up to 1800 LPM with an 18-character set. For example, a basic Train Printer is capable of printing at a rate of 1100 LPM when printing with a 48-character set and with single line spacing. 
     Train Printers of the above-mentioned categories are described in printed publications of the Burroughs Corporation, Detroit, Michigan 48232, such as Form #1094802 Copyright 1976, or Form #1066328 Copyright 1972, 1973, 1974. 
     The peripheral dependent board of the Train Printer Data Link Processor (TP-PDB) contains the necessary logic circuitry which typically interface 1100 and 1500 lines per minute Train Printers into the Burroughs I/O subsystems which use data link processors (DLP). 
     The PDB 80 is controlled by the micro-code words contained in the common front end card and its PROMs. As previously mentioned, the CFE and TP-PDB, in combination with the proper micro-code, make up the Train Printer Data Link Processor (TP-DLP). 
     The Train Printer-Data Link Processor accepts print information from a main host system, stores this information in a buffer, and sends hammer actuating signals to the Train Printer as each character in the buffer is matched by a train character coming into the proper position over a paper sheet to be printed. 
     The Train Printer-Data Link Processor contains the following elements and features: 
     (a) a print buffer which holds one full line of print. 
     (b) a soft-loaded universal train image buffer storage. 
     (c) print position compatibility recognition unit for 80, 120 or 132 characters. 
     (d) variable definition of graphic character sets. 
     (e) train and train image ID verification, when required. 
     (f) line delimiter capability. 
     The &#34;train&#34; or print-train is a rotating unit of blocks each individually connected to the next one like a string of box cars in a railroad system, and where each of the boxes or units carries a single character or several characters of the character sets to be used in printing on the paper which runs juxtaposed to the line of box cars of character sets. Graphics on the train or &#34;string of box cars&#34; are numbered sequentially beginning with the number &#34;0&#34; for the first character of each set on the train. Overprinting is permitted via the software. 
     I/O Descriptors for the Train Printer DLP 
     The TP-DLP I/O descriptors contain 1 digit of OP code and three digits of variant which describe the type of operation. This descriptor is received by the Train Printer Data Link Processor in one transmission from the host system. 
     There are three valid types of OP codes for the Train Printer Data Link Processor. These are codes for: 
     WRITE: This is any operation where data is transferred from the host system to the Train Printer-DLP buffer storage. 
     TEST: This is any operation where no data transfer takes place between the Train Printer-DLP and the host system, but operates by sending a result-descriptor to the host system. 
     ECHO: This is an operation that results in receiving a message block from the host system and then the transmission of the same information back to the host system. 
     It will be noted that there is no &#34;Read&#34; operation used in this system. At no time is a &#34;Read&#34; operation accepted as &#34;valid&#34; by the Train Printer-DLP. Thus the &#34;Read&#34; operation bit, A8, must always be &#34;0&#34;. If a &#34;Read&#34; operation should be received by the Train Printer-DLP, a &#34;descriptor-error&#34; is flagged and a result descriptor is returned to the host system. 
     The I/O descriptors, sometimes called Command Words, are obtained in the form of variants which accompany the OP codes. The various permissible operations used are shown in the following Table 11. 
     
                       TABLE 11                                                    
______________________________________                                    
Train Printer-DLP OP codes and Variants                                   
            OP Code  Variant  Variant                                     
                                     Variant                              
Operation              1        2      3                                  
Digit:      A A A A  B B B B  C C C C                                     
                                     D D D D                              
Bit:        8421     8421     8421   8421                                 
______________________________________                                    
Write       0100     0000     0000   0000                                 
Write/halt  0100     0001     0000   0000                                 
Write line  0100     0100     0000   N N N N                              
Writeline/halt                                                            
            0100     0101     0000   N N N N                              
Write/load TIB                                                            
            0100     1000     JJFF   FFFF                                 
Move/write line                                                           
            0100     0110     0000   N N N N                              
Move write  0100     0111     0000   N N N N                              
line/halt                                                                 
Test        0010     0000     0000   0000                                 
Test/wait ready                                                           
            0010     0001     0000   0000                                 
Test/wait not ready                                                       
            0010     0010     0000   0000                                 
Test/skip   0010     0100     0000   N N N N                              
Test/conditional                                                          
            0010     1000     0000   0000                                 
cancel                                                                    
Test/ID     0010     1100     0000   0000                                 
Echo        0001     0000     0000   0000                                 
______________________________________                                    
 NOTES                                                                    
 1. N designates the bits which are used to encode the paper motion format
 See table 12 for permissible formatting operations.                      
 2. JJ designates the printer speed; that is, 00 = 750 LPM, 01 = 1100 LPM,
 10 = 1500 LPM, and 11 is reserved.                                       
 3. F designates the bits which are used to encode the train identifier.  
 4. D designates the bit which, when true, inhibits the delimiter check   
 capability of the TPDLP.                                                 
 5. TIB refers to &#34;Train Image Buffer&#34; which is a portion of RAM buffer 22
 shown in FIGS. 4A, 5A, 5B, 5C.                                           
 
    
     DESCRIPTION: In reference to Table 11 
     WRITE: The &#34;Write&#34; operation prints and also moves paper, or else moves paper and also prints, according to the format information received in the first word of the data stream for each line to be printed. It continues printing lines until a &#34;termination condition&#34; is reached. If an invalid bit pattern is detected on a print buffer (FIGS. 5B, 5C) load operation, the operation is terminated after printing the line, and a result-descriptor (R/D) is returned to the host computer with the &#34;print-check/invalid-found&#34; bit set and also the &#34;print/error-line-printed&#34; bit set. 
     WRITE/HALT: The &#34;write/halt&#34; operation performs the same actions as the Write operation except that if an &#34;invalid&#34; bit pattern is detected, the loading of the print buffer (FIG. 5B) is completed, but that line is not printed. 
     WRITE/LINE: The &#34;write/line&#34; operation prints one line with the paper motion as defined in the OP code. If an invalid bit pattern is detected, the line is printed and a result descriptor (R/D) (indicating the error condition) is returned to the host system. The paper motion format code is shown in the following Table 12: 
     
                       TABLE 12                                                    
______________________________________                                    
Train Printer-DLP Paper Motion Format                                     
Codes                                                                     
Code    Description      Printer Format Tape                              
______________________________________                                    
0 0 0 0                                                                   
      No paper motion    (2 and 12 channel tape)                          
0 0 0 1                                                                   
      Advance to heading (2 and 12 channel tape)                          
0 0 1 0                                                                   
      Advance to ch. 2   (2 and 12 channel tape)                          
0 0 1 1                                                                   
      Advance to ch. 3   (2 and 12 channel tape)                          
0 1 0 0                                                                   
      Advance to ch. 4   (2 and 12 channel tape)                          
0 1 0 1                                                                   
      Advance to ch. 5   (2 and 12 channel tape)                          
0 1 1 0                                                                   
      Advance to ch. 6   (2 and 12 channel tape)                          
0 1 1 1                                                                   
      Advance to ch. 7   (2 and 12 channel tape)                          
1 0 0 0                                                                   
      Advance to ch. 8   (2 and 12 channel tape)                          
1 0 0 1                                                                   
      Advance to ch. 9   (2 and 12 channel tape)                          
1 0 1 0                                                                   
      Advance to ch. 10  (2 and 12 channel tape)                          
1 0 1 1                                                                   
      Advance to ch. 11  (2 and 12 channel tape)                          
1 1 0 0                                                                   
      Advance to end of page                                              
                         (2 and 12 channel tape)                          
1 1 0 1                                                                   
      Advance to EOP or next ch.                                          
                         (2 channel tape only)                            
1 1 1 0                                                                   
      Single space       (2 and 12 channel tape)                          
1 1 1 1                                                                   
      Double space       (2 and 12 channel tape)                          
______________________________________                                    
 NOTES-                                                                   
 1. With the exception of write and write/halt, the above format          
 information must appear in variant digit V3 as defined by NNNN in table  
 11.                                                                      
 2. For write and write/halt operations, the format information must appea
 in the first word transmitted with each line of print. The composition of
 the format word is as follows:                                           
 
    
     
         Digit:   A        B          C      D                                     
Code:    S R R R  N N N N    R R R R                                      
                                    R R R R                               
 Where:-                                                                  
 S = The bit which, when true, causes paper motion to occur prior to      
 printing the line.                                                       
 N = The bits which define format as in table 12.                         
 R = The bits which are reserved and must be zero.                        
 
    
     WRITE LINE/HALT: The write line/halt operation performs the same actions as the &#34;write-line&#34; except that if an invalid bit pattern is detected, the print buffer loading is completed, but that line is not printed. 
     MOVE/WRITE LINE: The move/write line operation performs the same operation as the write-line operation except that paper motion occurs prior to the printing of the line. 
     MOVE/WRITE LINE/HALT: The move/write-line/halt operation performs the same action as the write-line/halt operation except paper motion occurs prior to the printing of the line. 
     WRITE/LOAD TIB (TRAIN IMAGE BUFFER): The write/load TIB operation serves to load the train image buffer (FIG. 5B) which is described later. The variant digits Table 11, V2 and V3 define the train identification (ID). Bits 8 and 4 of V2 are encoded to define the printer&#39;s speed (00=750 LPM; 01=1100 LPM; 10=1500 LPM; 11 is reserved). 
     TEST: The test operation returns a test result descriptor (R/D) to the main host system. These test result descriptors will be discussed further hereinafter. 
     TEST/WAIT READY: The test/wait ready OP code causes the DLP to be placed in an &#34;enable&#34; state. It remains enabled until the Train Printer (50 p  FIG. 2) is made ready by the operator or until the test/wait operation is cancelled. If the DLP is in a &#34;ready&#34; state on receipt of this operation, the DLP branches and writes an &#34;operation-complete&#34; result descriptor. If the operator makes the Train Printer (peripheral) &#34;ready&#34; when the DLP is in a test/wait ready state, the DLP will report &#34;operation complete&#34; in its result descriptor (R/D). 
     If the DLP receives a &#34;conditional cancel&#34; OP code, the &#34;test&#34; OP code is cancelled and the conditional-cancel-complete bit is set in the result descriptor. If the Test/Wait operation is in progress and an operation is received which is not a valid &#34;conditional cancel&#34;, the test operation is cancelled and both the descriptor error and the conditional cancel-completed-bits are set in the result descriptor. 
     TEST/WAIT-NOT-READY: The test/wait-not-ready operation performs similarly to the test/wait-ready operation except that the DLP waits for a &#34;not ready&#34; condition and then reports a &#34;not ready&#34; result descriptor. 
     TEST/SKIP: The test/skip operation is used to perform paper motion without doing any printing. The paper motion control is encoded in variant #3, as described in Table 12. 
     TEST/CONDITIONAL-CANCEL: This test/conditional-cancel operation is used to cancel a test/wait-ready operation or a test/wait-not-ready operation. If a valid conditional-cancel OP code is received while a test/wait operation is in progress, the test operation is terminated and the conditional-cancel-completed-bit is &#34;set&#34; in the result descriptor. If a test/wait operation is in progress and an operation is received which is not a valid &#34;conditional-cancel&#34;, the test operation is cancelled and both the descriptor error and the conditional-cancel-completed-bit are set in a result/descriptor. 
     TEST ID: This operation results in the data link processor (DLP), sending a 2-word result/descriptor to the main host system. The first word is described below in discussion under the title of &#34;Train Printer DLP Result/Descriptors&#34;. The second word of the result/descriptor is the ID word which establishes a unique identification for that DLP to the main host system. This second word of the result descriptor is composed of 4 digits: the first two digits containing the identifier for a Train Printer-DLP and the next two digits which are field-jumpered to identify the different TP-DLPs in a given subsystem. This ID word should be differentiated from the Train ID which is a 6-bit number for the &#34;train block&#34; of characters which is presently installed on the Train Printer mechanism 50 p . The ID or &#34;second word&#34; of the result/descriptor is formatted as follows:______________________________________A        B             C       D______________________________________8 4 2 1  8 4 2 1       8 4 2 1 8 4 2 10 0 0 0  0 0 1 0       x x x x x x x xfixed byte         field-installed                   jumpersSecond Word (ID) of Result Descriptor______________________________________ 
     The variants A and B, above, are preset to always emit a HEX 02, thus specifying to the main host system that the peripheral involved is a &#34;Train Printer&#34;. The variants C and D specify, to the main host system, additional identification which provides the host system software with a method of differentiating between the multiple TP-DLPs within a data link processor subsystem. The digits C and D are field-jumperable in order to allow flexibility of identification. 
     ECHO: The echo operation causes the data link processor (DLP) to accept a full buffer of data from the main host system and return the data back again to the main host system. This is to permit a confidence check of the data paths. 
     TRAIN PRINTER-DLP RESULT/DESCRIPTORS: The Train Printer-DLP has the capability of returning different result-descriptors, depending upon the current operation. The A digit is the same for all result/descriptors. The following Table 13 is a description of the information conveyed by the bits in this A digit. 
     
                       TABLE 13                                                    
______________________________________                                    
A8       Not ready. The train printer is in a                             
         condition other than power on and ready                          
         to print.                                                        
A4       Descriptor error. A parity error was                             
         detected on the I/O descriptor or                                
         descriptor link. The OP code was not                             
         valid at this time (conditional cancel), or                      
         an invalid OP code was received.                                 
A2       Vertical parity error (host system                               
         interface). The vertical parity detected on                      
         the host system interface was even.                              
A1       Longitudinal parity error (host system                           
         interface). The longitudinal check word                          
         was incorrect.                                                   
______________________________________                                    
 
    
     WRITE RESULT/DESCRIPTORS: The following Table 14 provides the meanings for the bits in the B, C and D result/descriptor digits returned on any &#34;Write&#34; operation: 
     
                       TABLE 14                                                    
______________________________________                                    
B8       Train image buffer not loaded. The                               
         TP-DLP has detected a condition which                            
         invalidates the data which was formerly                          
         stored in the TIB.                                               
B4       Incorrect train on printer. The train                            
         identifier of the train on the printer does                      
         not match the train ID stored with the                           
         TIB.                                                             
B2       End of page. The printer is at the end of                        
         the present page of print. This condition                        
         occurs only when single or double spaced                         
         format is executed.                                              
B1       Reserved. Is always zero.                                        
C8       Reserved. Is always zero.                                        
C4       Reserved. Is always zero.                                        
C2       Print check/sync error. An out-of-sync                           
         condition between the TP-DLP and the                             
         printer train was detected.                                      
C1       Print check/invalid found. An invalid bit                        
         pattern was detected on print buffer load                        
         and the TP-DLP substituted the invalid                           
         character at that position in the print                          
         buffer.                                                          
D8       Print check/error line printed. The                              
         TP-DLP detected an error condition as                            
         reported by bit C1 but because of the                            
         type of write operation it printed the line.                     
D4       Print check/print cycle parity error. A                          
         parity error was detected on the print                           
         cycle                                                            
D2       Print check/hung, no match found. The                            
         TP-DLP did not find an &#34;equal compare&#34;                           
         on some bit pattern in the print buffer                          
         after one full revolution of the print train.                    
D1       Print check/formatting error. The TP-DLP                         
         detected an error condition in the format                        
         word on a write or write/halt operation.                         
______________________________________                                    
 
    
     TEST, TEST ID AND ECHO RESULT/DESCRIPTORS: The following Table 15 provides the meaning for the bits in the B, C and D result/descriptor digits returned on the Test, Test/ID and Echo operations: 
     
                       TABLE 15                                                    
______________________________________                                    
B8        Train image buffer not loaded. Refer to                         
          the description of bit B8 of the write                          
          result descriptor.                                              
B4        Incorrect train on printer. Refer to the                        
          description of bit B4 of the write result                       
          descriptor.                                                     
B2        Column width bit 2. Used in conjunction                         
          with bit B1 to specify the length of the                        
          print line..sup.1                                               
B1        Column width bit 1. Used in conjunction                         
          with bit B2 to specify the length of the                        
          print line..sup.1                                               
C8        LPM type bit 2. Used in conjunction                             
          with bit C4 to indicate the printer speed                       
          (condition of jumper)..sup.2                                    
C4        LPM type bit 1. Used in conjunction                             
          with bit C8 to indicate the printer speed                       
          (condition of jumper)..sup.2                                    
 C2-      Train ID. The 6-bit identification number                       
D1        for the train currently installed on the                        
          printer.                                                        
______________________________________                                    
 NOTES-                                                                   
 
    
     
         .sup.1 Column width                                                       
                 .sup.2 LPM type                                          
CWB2/*CWB1/ = 132                                                         
                 LTB2/*LTB1  = 1100                                       
CWB2/*CWB1  = 120                                                         
                 LTB2 *LTB1/ = 1500                                       
 
    
     TEST/WAIT AND TEST/CONDITIONAL CANCEL RESULT/DESCRIPTORS 
     The following Table 16 provides the meaning for the bits in the B, C and D Result/Descriptor digits returned on a Test/Wait-ready, Test/Wait-not ready and Test/conditional-cancel operation: 
     
                       TABLE 16                                                    
______________________________________                                    
B8        Train image buffer not loaded. Refer to                         
          the description of bit B8 of the write                          
          result descriptor.                                              
B4        Incorrect train on printer. Refer to the                        
          description of bit B4 of the write result                       
          descriptor.                                                     
B2        Reserved. Is always zero.                                       
B1        Cancel complete. The TP-DLP has                                 
          cancelled the current operation.                                
 C8-      Reserved. Are always zero.                                      
C4                                                                        
 C2-      Train ID. The 6-bit identification number                       
D1        for the train presently installed on the                        
          printer.                                                        
______________________________________                                    
 
    
     TEST/SKIP RESULT/DESCRIPTORS: The following Table 17 provides the respective meanings for the bits in the B, C and D result/descriptor digits returned on a Test/skip operation. 
     
                       TABLE 17                                                    
______________________________________                                    
B8        Train image buffer not loaded. Refer to                         
          the description of bit B8 of the write                          
          result descriptor.                                              
B4        Incorrect train on printer. Refer to the                        
          description of bit B4 of the write result                       
          descriptor.                                                     
B2        End of page. Refer to the description of                        
          bit B2 of the write result descriptor.                          
 B1-      Reserved. Are always zero.                                      
D1                                                                        
______________________________________                                    
 
    
     ERROR RECOVERY: The Train Printer data link processor indicates error condition information in the result/descriptor. The errors are logged in the host system maintenance log. The actual recovery techniques are the responsibility of the host system software. 
     MESSAGE GENERATION AND PARITY CHECKING: The I/O descriptor, the descriptor links (D/L) and the result descriptors (R/D) are transmitted (FIG. 1) between the Train Printer-data link processor 20 3  and the main host system 10 through the message level interface 15 i  (MLI) via the distribution card 20 od . The message level interface 15 i  is a 25 wire cable used to connect the data link processor&#39;s base module 20 O  with the main host system 10 as seen in FIG. 2. 
     Parity errors or invalid OP codes which are detected on descriptor transfers will cause the Train Printer-DLP to send a result/descriptor to the host system and suppress the data transfer. A parity error detected on a data transfer during print operation causes the Train Printer-DLP to complete the buffer loading and to send a result/descriptor to host 10 without printing the line. 
     A parity error detected on data transfer during the load-train-image-buffer operation causes the Train Printer-DLP to complete the operation and to send the result/descriptor to the host 10 indicating that the train-image-buffer is not loaded. Parity is stored in both the train image and the print image buffers (FIGS. 5B, 5C) and, during print operations, is checked when the Train-Printer-DLP is in a print cycle on to the printer mechanism. If an error is detected, the printing is terminated and a result/descriptor indicating the error is returned to the main host system 10. 
     TRAIN PRINTER-DLP CHARACTERISTICS 
     The following discussion will describe the major characteristics of the Train Printer-Data Link Processor, especially the train image buffer, the print image buffer, the delimiter character, format control and train identification in addition to other factors of significance. 
     TRAIN IMAGE BUFFER 
     In reference to RAM 22 of FIG. 5A and FIGS. 5B, 5C, the Train Printer-DLP contains an element designated as a train image buffer (TIB). The contents of this buffer are used to determine when to fire the print hammers of the printer mechanism 50 p . For valid printing to occur, this buffer must be loaded with a character set that matches the currently-used print train blocks in the printer mechanism. The train image buffer (TIB) has 288 addressable bytes corresponding to the 288 characters on the train module. Each byte is an 8-bit entry corresponding to a particular character on the train module. The Train Printer-Data Link Processor receives two 8-bit bytes on each memory access as the train image buffer (FIG. 5B) is being loaded. 
     A new train image must be loaded from the host system into this buffer whenever train modules are interchanged or whenever the TP-DLP has been either master-cleared or taken off-line for maintenance operation. 
     The first word of data transmitted to the Train Printer-DLP on the &#34;load TIB&#34; operation must contain a host system definition for the blank and the invalid code bit pattern (two extra characters are supplied for a &#34;load TIB&#34; operation and are stored separately from the TIB). These special characters are described later. 
     The &#34;blank code&#34; bit pattern is designed to appear in the A and B digits of this first word and the &#34;invalid code&#34; bit pattern appears in the C and D digits. 
     The definition of &#34;variable&#34; graphic character-sets on the same train module is permitted. Also partial sets, or more frequently-used characters can be created in order to maximize print speed. 
     Most printer trains contain multiple copies of each graphic item. For example, on a 48-graphic train, the graphic &#34;A&#34; occurs in positions 18, 66, 114, and 162. If the EBCDIC character &#34;A&#34;, (coded as HEX C1) is to be printed as a graphic A, then HEX C1 must be loaded into the train image buffer at location 17, 65, 113, and 161. The first location in the train image buffer is designated as &#34;O&#34;. 
     If there were other EBCDIC values, in addition to &#34;A&#34;, to be printed as the graphic &#34;A&#34;, then the 4 train image buffer locations would be distributed among the alternative values. For example, if the lower case EBCDIC character &#34;a&#34;, HEX 81, is to be printed upper case as well, the train image buffer is loaded as follows: TIB 17=C1; TIB 65=81; TIB 113=C1 and TIB 161=81. 
     During a train image &#34;load&#34;, if a delimiter character (HEX CF, described hereinafter) is detected, the operation completes by returning a result/descriptor indicating the train image buffer is &#34;not loaded&#34;. 
     PRINT BUFFER: The Train Printer-data link processor contains a &#34;print buffer&#34; which is loaded once per print operation with data entries consisting of two 8-bit characters plus parity per word. FIGS. 5B and 5C show the allocations in RAM 22 which are dedicated for the print image buffer, PIB. The buffer capacity is sufficient for one line of print (132 characters maximum). The number of printer columns determines when the buffer is full. The result/descriptor is returned just prior to paper motion. The Train Printer-DLP is capable of accepting a new descriptor and/or loading the buffer while paper motion is occurring in response to the previous descriptor or line of print. 
     DELIMITER CHARACTER: The delimiter character, HEX CF, is recognized when it occurs in a data train, and the data transmission is discontinued and the printing of the remaining columns is inhibited. 
     The delimiter character is not printable and the 8-bit HEX CF code cannot be used to represent a graphic at any time. The check for a delimiter character can be inhibited by setting bit C8 of the variant digit V2 of the print OP code. If a delimiter character is received from the host system during a print image buffer load with bit C8 for the variant digit V2 &#34;set&#34; (inhibit delimiter character check), this character is then flagged as invalid. 
     FORMAT CONTROL: The skipping and spacing of paper is determined by descriptor variant digit V3 on single line print operations or by information received in the first 16-bit transmission of data for each line of print on multiple-line print operation. The note 2 on Table 12 gives a pictorial description of this format word. This word contains 11 reserved bits (R) which must always be &#34;0&#34;. On multiple line print operations, the TP-DLP examines these 11 bits as it is loading the format word. If it detects anything other than a &#34;0&#34; in anyone of these 11 positions, the operation is terminated and a result/descriptor is returned indicating a format error. 
     When accepting a line during a multiple line print operation, the Train Printer-DLP must receive at least one data character to be printed. If the host system terminates or transmits a delimiter immediately following the format word, the format error bit is set in the result/descriptor and the operation is terminated. The alignment of the paper in the train printer is determined by the use of printer format types which are inserted in the Train Printer mechanism 50 p . 
     TRAIN IDENTIFICATION: A sequence of 6-bits is received from each train module as an identifier. This information is reported on the result/descriptor returned in response to all tests and echo operations, except the test/skip operation. 
     Trains with no identifier are interpreted as having an identifier of &#34;0&#34;. The train ID bits of the &#34;load train image buffer&#34; (LTIB) descriptor must also be all &#34;O&#39;s&#34; or else printing is inhibited. 
     Whenever the printer changes from a &#34;not-ready&#34; to a &#34;ready&#34; state, compatibility between the train identifier (ID) and the train image buffer (TIB) is rechecked. Compatibility is also checked whenever the Train Printer-DLP has been off-line or master-cleared. If the two were unequal, the Train Printer Data Link Processor sets the &#34;incorrect-train-on-printer&#34; bit in the result/descriptor for all subsequent operations until the correct train image is loaded or the correct train is installed on the printer. 
     SPEED CAPABILITY: The Train Printer-Data Link Processor has 1100 LPM and 1500 LPM (lines per minute) capability provided that the proper hard wired jumper 82 w  is installed. The operation of the Train Printer-DLP is not changed except that: 
     (a) in response to a &#34;test&#34; operation, the result descriptor bits are &#34;set&#34; to indicate the jumpered speed. 
     (b) the Train Printer-DLP is designed to expect the variant digit V2 (bit position C4 of the load-train-image-buffer) descriptor (LTIB) to indicate the printer speed (that is, 1100 or 1500 lines per minute). If this bit does not compare with the installed (speed) jumper, the operation is terminated and the bit B4 (incorrect train) of the result/descriptor is set. 
     BLANK CHARACTER: On a train image buffer (TIB) &#34;load&#34;, the Train Printer-DLP is designed to receive the host system defined-bit configuration for the &#34;blank&#34; code. The characteristics for this &#34;blank character&#34; are as follows: 
     (a) if loaded into the print buffer on a print operation, the hammer associated with that column position on the paper, is not fired. 
     (b) if loaded into the train image buffer, the graphic character associated with that position on the print train will never be printed. 
     The &#34;blank character&#34; must be transmitted to the Train Printer-DLP in the A and B digits of the first word of the load-TIB data transmission. 
     INVALID CHARACTER: The Train Printer-DLP, on a &#34;load-TIB&#34; operation is designed to receive the host system defined-bit configuration for the invalid code. This configuration must be either the host system defined blank-code-bit pattern or any one of the validly defined characters in the train image buffer (TIB). 
     If it is any other configuration and it becomes necessary on a &#34;Write&#34; operation to substitute and print the invalid character because of an invalid bit pattern in the print buffer, the Train Printer-DLP attempts to print this character. However, since it never finds a &#34;compare-equal&#34; as defined under the print cycle, (hereinafter described) the Train Printer-DLP terminates the operation by returning the result/descriptor indicating a &#34;hung-no-match-found&#34;. 
     The invalid character will be transmitted to the Train Printer-Data Link Processor in the C and D digits of the first word of the &#34;TIB-load-data&#34; transmission. 
     PAPER WIDTH DETERMINATION: A jumper option is provided in the Train Printer-DLP for paper widths of 80, 120, 132 columns. The jumper must be correctly installed for the current printer which is in use. 
     PRINT CYCLE: The actual printing of characters is controlled by scanning a message to be printed and comparing it with the train image buffer of FIG. 5C. The character in the print buffer corresponds to the hammer position in the printer. The address of the train image buffer (TIB) corresponds to the character positioned over the hammer. If the 8-bit entry in the train image buffer is equal to the 8-bit code in the print buffer, a set level is sent to the printer and that hammer is fired on the next print cycle of the printer. 
     A pointer into the train image buffer TIB is kept synchronized with the train movement. During print scans, the print message is scanned and the address of the train image buffer is changed to correspond with the character hammer positions. This is continued until all the characters in the print buffer have been printed. 
     TRAIN-PRINTER-DLP: The elements of the hardware of the Train Printer-DLP are shown in FIGS. 4A and 5A. 
     The Train Printer-Data Link Processor consists of two 96-chip, multi-layered printed circuit cards, which plug into adjacent slots in the DLP Base Module backplane. These two printed circuit boards are designated as the Common Front End (CFE) card and the peripheral-dependent board (PDB). These two boards are connected together by the use of three 50-pin foreplane connectors. Table IV, previously shown, lists the connectors by number and shows the signal name associated with each pin. 
     As seen in FIGS. 4A and 5A, the elements of the CFE are shown in block form. The central heart of the CFE is the PROM controller and storage sections 13 and 14. The PROM storage consists of 13 separate PROM chips giving a total storage capacity of 1,024 52-bit words, and this includes odd parity. The CFE 10 c  contains the receivers 110 for the DLP interfaces with the Distribution and the Maintenance Cards. The &#34;enable&#34; signals for these buses are driven by the PDB 80. The DLP RAM storage buffer 22, which has a capacity of 1024 17-bit words (including odd parity), is also located in the CFE 10 c . However, the RAM storage 22 is controlled entirely by the PDB 80. The Table I-A contains a glossary of applicable CFE terms. 
     PROM CONTROLLER: The 13 PROM chips designated element 13 (FIGS. 4A, 5A) on the CFE card are combined to make up a 52-bit micro-code instruction word. The address lines to the 13 PROM chips are bused together so that all individual address lines are common to every chip. The data outputs of the 13 chip PROM matrix forms the 52-bit word. This word is &#34;read-out&#34; of the address that is present on the common input address lines. Since these PROMs are unclocked devices, a means is required to synchronize the outputs with the 8-megahertz clock. This is accomplished by the use of register chips 14. The register chips contain 8 flip-flops each, and thus 7 chips are used to synchronize and latch the 52-bit micro-code word. This latched micro-code instruction is used to control the operation of the entire data link processor. Every 8-megahertz clock pulse latches the next subsequent word into the register chip 14. 
     Different types of data link processors require their own unique micro-code; thus, all CFE cards contain the identical hardware except for the 13 PROM chips. Although the PROM word physically contains 52-bits only 49 bits are used by the micro-code program. 
     Tables VIa, VIb shows the 49-bit (0-48) micro-code word, by bit position and name. It will be seen that all PROM output signal names are preceded by a pound sign (#) so that they are easily recognizable in the drawings and tables and in hardware flows. Bit 32 of the micro-code word is the parity bit (odd). The CFE card continually checks for odd parity and halts the micro-code program if a parity error (even parity) occurs on any 49-bit PROM micro-code word. 
     RAM BUFFER: The common front end (CFE) 10 c  of FIG. 5A contains the random access memory buffer, RAM 22. The RAM buffer (sometimes called the Data RAM) consists of 1024 17-bit words. All inputs and outputs of this RAM memory are received or driven by the PDB 80. The designation used for this open collector (60 nanosecond) read-access RAM is &#34;RWON&#34;. This storage area is used to store the data, the OP code, the descriptor links, the descriptor links longitudinal parity word (LPW) and various flags that are required to properly control the operation of the Train Printer-DLP. 
     STACK REGISTER: The stack register 11 consists of 3 binary counter chips. This register can contain the value of the current PROM address or the value of the address to be used when returning from a subroutine by way of a stack branch operation. 
     PERIPHERAL-DEPENDENT BOARD (PDB): The peripheral-dependent board contains a general purpose register 230, of six 4×4 register files 212 used for accumulators, a column-done RAM 214, an invalid character circuit 216, a LPW (longitudinal parity word) generator 218, a peripheral interface 220, line control logic 222, a data path multiplexor 224 (using nine dual 4-1 data selector chips), compare logic 226, data bus drivers 28, plus error detection logic and 2-way branching logic. 
     The Table I-B contains a glossary of the PDB terms. A simplified description of the micro-code treatment of a &#34;load train image&#34; buffer command will be found described hereinafter. 
     The Train Printer-Data Link Processor (TP-DLP) is typically used to interface 1100 and 1500 lines per minute train printers to a host system via the message level interface 15 i  (MLI). As mentioned before, the TP-DLP consists of two cards which constitute: a common front end card (CFE) and peripheral dependent card which is commonly called a peripheral dependent board (PDB). 
     As previously discussed the common front end card provides 1024 52-bit words of PROM that is used to store micro-code, and 1024 17-bit words of RAM for data storage, maintenance logic, and part of the DLP interface. 
     The PDB 80 provides: 
     1. the controlling signals to the Train Printer, 50 P . 
     2. address logic for the RAM buffer or the CFE and the RAM access logic. 
     3. longitudinal parity word (LPW) and the vertical parity generation and checking logic. 
     4. identification jumpers. 
     5. twelve 8-bit accumulators. 
     6. a general purpose register. 
     7. compare logic. 
     8. other necessary flags and control logic. 
     FIG. 5A shows the basic block diagram of the Train Printer-DLP. 
     ACCUMULATOR: The accumulators 212 of FIG. 5A provide data storage for twelve 8-bit registers (designated accumulators 0-11). The accumulators are made up of six 4×4 register file chips. Only two of these chips are selected at any one time. There are 8 input information lines and 8 output information lines associated with these accumulators. The addressing of the chips is indicated by Table 18 hereinbelow: 
     
                       TABLE 18                                                    
______________________________________                                    
Accumulator Addressing and Usage                                          
______________________________________                                    
       Chips Selected                                                     
Accumulator                                                               
         MSD     LSD     ACC8  ACC4  ACC2  ACC1                           
______________________________________                                    
0        K2      I3      0     0     0     0                              
1        K2      I3      0     0     0     1                              
2        K2      I3      0     0     1     0                              
3        K2      I3      0     0     1     1                              
4        I2      K3      0     1     0     0                              
5        I2      K3      0     1     0     1                              
6        I2      K3      0     1     1     0                              
7        I2      K3      0     1     1     1                              
8        J2      J3      1     0     0     0                              
9        J2      J3      1     0     0     1                              
10       J2      J3      1     0     1     0                              
11       J2      J3      1     0     1     1                              
______________________________________                                    
Accumulator         Usage                                                 
______________________________________                                    
0          Train absolute address                                         
1          TIB subscan address                                            
2          TIB subscan address (restore)                                  
3          Print buffer subscan address                                   
4          Print flags                                                    
5          CSL count                                                      
6          Result descriptor A and D digits                               
7          Result descriptor B and C digits                               
8          Initiate LPW flag, columns printed                             
9          Prescan blank address                                          
10         Initiate flags, print data flags                               
11         Various addresses (working register)                           
______________________________________                                    
                    Accumulator 4                                         
Bit                 Description                                           
______________________________________                                    
0          Column bottom/top (0 = top).                                   
1          1 = PCSL 3 or 4.                                               
2          0 = PCSL counted from 4 back to 1.                             
3          Train absolute bottom/top (0 = top).                           
4          1 = Absolute train address overflow.                           
5          Not used.                                                      
6          Not used.                                                      
7          Train sub scan bottom/top (0 = top).                           
______________________________________                                    
                    Accumulator 5                                         
Bit                 Description                                           
______________________________________                                    
0          WAITAF flag. Tells the PCSL update                             
           procedure to exit to the WAITAF                                
           procedure.                                                     
1          PRINT flag. Tells various procedures to                        
           return to the WRITE procedure.                                 
2          ECHO flag. Tells various procedures to                         
           return to the ECHO procedure.                                  
3          Load train buffer flag. Tells various                          
           procedures to return to the LOAD TIB                           
           procedure.                                                     
4-7        CSL count. This value is incremented by 1                      
           each time CSL is anticipated.                                  
______________________________________                                    
                    Accumulator 10                                        
Bit                 Description                                           
______________________________________                                    
0          Wait flag (1 = wait operation pending).                        
1          Wait for ready (1 = wait for ready                             
           pending).                                                      
2          Test ID flag. Tells the result procedure to                    
           send the second ID result word.                                
3          Print halt flag. Tells the write procedure                     
           not to print the line if an invalid occurs.                    
4          Move 1st flag. Tells the write procedure to                    
           move paper first for a single line print.                      
5          Multiple line flag. Tells the write procedure                  
           that this is a multiple line print.                            
6          Terminate flag. Tells the write procedure to                   
           stop a multiple line print.                                    
7          Back up flag. Tells the write procedure to                     
           perform an MLI back-up on one character.                       
______________________________________                                    
 
    
     The register chip select is derived from signals ACC 8 and ACC 4 by means of decoder chips (not shown). A chip LO (not shown) provides &#34;write enable&#34; to the chips selected by the ACC 8 and ACC 4. Another chip KO (not shown) provides the &#34;read enable&#34; to the chips selected by the ACC 8 and ACC 4. 
     Locations within the 4×4 register file chips are addressed by signals ACC 1 and ACC 2 as seen in Table 18. 
     The input to the accumulators 212 is either the #CONST lines or the sum of the #CONST lines and the RAMAD lines (BC register), depending upon the state of the MPXSELAD control signal. 
     GENERAL PURPOSE REGISTER: The general purpose register 230 provides the only sink for the accumulators 212. The general purpose register has 8 input information lines and 16 output information lines, and the register is divided into four 4-bit portions, A, B, C and D. These 4-bit portions are grouped into two 8-bit portions: AD and BC. Information is loaded from the accumulators 212 into register portions AD or BC, or both depending upon the signals LDREGCAD (load register column address), LDREGRAD (load register RAM address), or both respectively. 
     The 8-bit BC register drives the address line for the RAM buffer on the CFE card 10 c . The 8-bit AD register addresses the column &#34;Column Done&#34; RAM chip 214. The 16-bits from the register (A, B, C and D digits) provide one of the four sources for the data multiplexor 224. 
     COLUMN DONE LOGIC: The purpose of this logic (214) is to determine if a column has been &#34;printed&#34; and hence is given the mnemonic COLDONE. This output signal, when high, indicates that the column has already been printed or is not intended to be printed (for example: a blank is to be left in a column). 
     Referring to the block diagram of FIG. 4D (DLP connection logic) and the schematic in FIG. 5G which is a general purpose register: The column-done logic consists of one 1K RAM chip L4. The address inputs to this chip L4 are controlled by the AD registers of register 230 and the signal THRAMCLD (top half RAM column done), which is a high active micro-code output. 
     The signal THRAMCLD designates--&#34;upper/lower column select&#34; (1=upper). The signal WECOLDN (write enable column done) is a low active signal generated by the micro-code which supplies &#34;write-enable&#34; to the chip. 
     The signal DIRAMCOL (data input to column done RAM) is a high active signal generated by the micro-code that supplies the input information to the RAM 22. 
     RAM CONTROL: The Random Access Memory Buffer 22 (RAM), as seen in FIGS. 5A, 5B and 5C, has a storage capacity of 1K×17 bits (16 data bits with one odd parity bit) and is located on the CFE card 10 c . All the input, output, address, write enable, and chip select lines are located on the peripheral dependent board 80. The above mentioned signals are connected to the PDB 80 through the top two foreplane connectors 80 a  and 80 b . 
     The RAMADn lines 53 are always driven by contents of the BC register in register 230. The signal WE/ (write enable/) is generated by a micro-code output which is gated with a clock (8 CLCK 1) for the proper timing. The signal CS/ (chip select/) is grounded on the PDB 80 thus continuously enabling the RAM 22. 
     When the signal WE/ is &#34;high&#34;, the RAM 22 is placed in the &#34;Read&#34; mode. The information at the location addressed by the RAMADn lines 53 (RAM address) is placed on the RAMxn (RAM output) lines 22 a  and is routed to the data multiplexor 83 m  and thence to the 2-1 multiplexor of the data comparison logic 226. 
     When the signal WE/ is &#34;low&#34;, the RAM 22 is placed in the &#34;Write&#34; mode. The lines DBUSxn are connected directly to the INRAMxn lines 100 on the peripheral dependent board 80. The information contained on the DBUSxn lines are written into the RAM 22 at the location addressed by the RAMADn lines 53. 
     The RAM buffer 22 is used to store the &#34;train image&#34; and also the print image, the OP codes, the descriptor links, the longitudinal parity word (LPW) and other information. This is shown in FIGS. 5B, 5C which show the mapping of the RAM buffer 22. 
     FIG. 5B shows the entire RAM mapped according to words 0 through 1023. The upper half of the RAM is not used (words 512-1023); thus RAMAD 9 (address bit 512) is grounded on the PDB 80. 
     The 512 locations of RAM buffer 22 that are used are referred to as the lower RAM (RAMAD 8=0) and the upper RAM (RAMAD 8=1) which is shown in FIGS. 5B and 5C. 
     FIG. 5B illustrates a map of the RAM buffer 22 where it can be seen that addresses 512 through 1023 are not used. The remainder of the buffer space is then divided into the bottom half of the RAM (RAMAD 8=0) which include addresses 0-255, and this space is used for the descriptors and control words (addresses 0-16) while addresses 190-255 are used for the print image buffer. 
     The top half of the RAM 22 (RAMAD 8=1) involves addresses 256-511, where addresses 256-367 are dedicated for control words, and addresses 368-511 are provided for the train image buffer. 
     The following Table 19 shows the layout of the lower and upper RAM buffer 22. 
     
                       TABLE 19                                                    
______________________________________                                    
Decimal                                                                   
Address    Stored Information                                             
______________________________________                                    
Lower RAM (RAMAD8 - 0)                                                    
0          Descriptor (temporary result descriptor)                       
1          Descriptor LPW                                                 
2          Descriptor link #1                                             
3          Descriptor link #2                                             
4          Descriptor link #3 (LPW)                                       
5          Multiple line format                                           
7          Train ID (from last load TIB operation)                        
9          Fetched ID                                                     
16         Back-up word (01)                                              
190        PIB starting address for 132-column                            
--         printer                                                        
196        PIB starting address for 120-column                            
           printer                                                        
216        PIB starting address for 80-column                             
           printer                                                        
255        Buffer end                                                     
Upper RAM (RAMAD8 - 1)                                                    
0          Descriptor during test/waits                                   
1          Descriptor LPW during test/waits                               
2          Descriptor link #1 during test/waits                           
3          Descriptor link #2 during test/waits                           
4          Descriptor link #3 (LPW) during test/                          
           waits                                                          
5          Cancel OP code for compare (2800)                              
111        Blank/invalid address                                          
112        TIB starting address                                           
255        TIB ending address                                             
______________________________________                                    
 
    
     INVALID CHARACTER DETECTION: To provide a means of determining whether a character to be printed is invalid or not invalid a 1K-by-1 bit RAM chip is used (RWOO) and this is referred to as the invalid RAM which is located on element 216 of FIG. 5A. The address lines of the invalid RAM are driven by the contents of two chips which are used as two 4-bit latches 215. 
     The DI (data input) connection to the 1K-RAM 216 is DIRAMINV (data in the RAM is invalid) which signal is micro-code generated. The WE/ connection to the invalid RAM 216 is WERMINV (write enable to invalid RAM) which is also micro-code generated. The invalid character detection is performed entirely by the micro-code. 
     DATA PATH MULTIPLEXOR: As seen in FIG. 5A the data multiplexor 224 is used to select which of four possible &#34;sources&#34; is to drive the DBUS and the INRAM bus 100. The data multiplexor 224 is made of nine dual 4-1 data selector chips which provide an output of 18-bits of which only 17 bits are used. The nine data selector chips are always enabled by the grounding of the strobe inputs (ST1/ and ST2/). The SLA inputs to the nine data selector chips are common and are driven by a micro-code term MPXDATAA (multiplexor data A). The SLB inputs are also common and are driven by the micro-code term MPXDATAB (multiplexor data B). 
     The data selector chips of data multiplexor 224 are fed by four sources which are: 
     1. 16 bits from the general purpose register 230 (IRAMAD 7--IRAMAD 0 and also RAMAD 7--RAMAD 0) which are tied to the A-1 and A-2 inputs of the data selector chips. 
     2. 16 bits which make up the second word of the test/ID result descriptor (ID word). This comes from the ID jumpers 82 and the hard wired circuitry 82 w . 
     As previously mentioned in the test/ID description heretofore, the 8 most significant bits are fixed. Bit B2, which is connected to the B2 input of chip D3, is pulled high via a 1100 ohm pull-up resistor while the remaining 7 bits are grounded producing a fixed byte of HEX 02. The least significant 8-bits are from the identification jumpers CID 8-DID 1. This is the second source of the data selector chips and this source connects to the B1, B2 inputs of the data selector chips. 
     3. 17-bits from the 1K by 17-bit RAM 22 (located on the CFE 10 c ) output lines on the RAMnn bus 22 a  (RAMA8-RAMD1 and RAMPAR) which are connected to the C1 and C2 inputs of the data selector chips. 
     4. 16-bits from the LPW generator 218 on the lines LPWnn which provide input to the data multiplexor 224. These bits are designated LPWA8--LPWD1; they are connected to the D1 and D2 inputs of the data selector chips of multiplexor 224. 
     Vertical parity (odd) is generated for each of the 16-bit sources and connected to a chip H4 (in 217, vertical parity circuit) having inputs A2, B2, and D2. This is named PARGEN (parity generation). The RAM 22 output is the only source that supplies its own parity bit designated RAMPAR. 
     The Table 20 listed below illustrates the charting for the selection of the source of input to the data selector multiplexor 224. 
     
                       TABLE 20                                                    
______________________________________                                    
Source Selection for Input to Data                                        
Selector Multiplexor                                                      
MPXdataB  MPXdataA    Data                                                
______________________________________                                    
0         0           General Purpose Register                            
0         1           ID word                                             
1         0           RAM output                                          
1         1           LPW                                                 
______________________________________                                    
 
    
     The outputs of multiplexor 224 provide a path MPXnn to the DBUSnn. This is done via 3 tri-state chips. These devices are enabled when the micro-code generated signal ENMPX/ (enable multiplexor/) is &#34;low&#34;. 
     LPW GENERATOR/CHECKER: The longitudinal parity word generator checker 218 is made up of 16 JK flip-flops. All of the clock inputs are driven by the micro-code terms CLKLPW RG (clock LPW register). All the preset inputs are driven by the micro-code term RESTLPW (re-set LPW) which is gated with a clock (8CLK2). The J and K inputs on each flip-flop are tied together by one of the 16-bits on the DBUS. The output lines of the LPW generator 218 are designated LPWA8-LPWD1 and these provide one of the four inputs to the data multiplexor 224. 
     The LPW generator 218 is initially reset to all 1&#39;s by the micro-code signal RESTLPW. The &#34;receive&#34; or &#34;transmit&#34; data is applied to the DBUS as the input to the LPW generator. The micro-code then applies a clock pulse to the LPW generator 218 (CLKLPWRG). If an individual data bit is &#34;high&#34;, the associated flip-flop toggles. If the data bit is &#34;low&#34;, the associated flip-flop remains in its previous state. Each data word received or transmitted is placed on the DBUS and clocked by CLKLPWRG. 
     For transmission &#34;from&#34; the TP-DLP, this data is transmitted to the main host system 10 as it is clocked into the LPW generator 218. After all the data has been transmitted, the 16-bit word left in the LPW generator is the longitudinal parity word and it is the next word transmitted to the main host system. 
     For transmission &#34;to&#34; the TP-DLP, the LPW generator 218 is preset by the micro-code signal RESTLPW. As each word is received, it is presented onto the DBUSnn. The micro-code generates CLKLPWRG in order to clock the LPW generator with this received data. Just like the data transmission operation of the LPW generator 218, the LPW generator contains a DLP generated longitudinal parity word. The last word received by the Train Printer-Data Link Processor, on a data transfer, is the LPW from the host system 10. This LPW is gated onto the DBUS and is also clocked by the CLKLPWRG signal into the LPW generator 218. The data link processor generated LPW must equal the host system supplied LPW. The LPW will equal &#34;0&#34; if no LPW error occurs. 
     IDENTIFICATION JUMPERS: There are 8 identification jumpers 82 (FIG. 3) located on the peripheral-dependent board (PDB) 80 and these are named CID8-DID1. These signals are tied to 1100 ohm pull-up resistors, which provide a logic &#34;high&#34; in the absence of a jumper. Individual signals are forced &#34;low&#34; with the installation of a jumper. These signals are fed to the data multiplexor 224. 
     COMPARE LOGIC: The compare logic 226 (FIG. 5A) uses two 4-bit comparator chips. These chips are configured in a tandem mode to provide comparison between two 8-bit sources. 
     Source A for the compare logic is the 8-bit compare latch 215 which is made of two 4-bit counter chips. 
     Source B is the 8-bit output of the two quad 2-1 multiplexor chips 83 m . The 16-bit input to these multiplexors comes from the outputs of the RAM buffer 22 on line 22 a . These 16-bits are divided into &#34;top&#34; and &#34;bottom&#34; 8-bit portions, according to their selection from the top half (TH) or bottom-half (BH) of RAM 22. 
     The SL inputs to the multiplexor chips 83 m  are tied together and controlled as to whether the &#34;top&#34; or the &#34;bottom&#34; 8-bit portion is selected. The SL inputs are tied to SELTHBH (&#34;high&#34; equals bottom half) which is a micro-code signal. The compare register has LD/ (load/) inputs which are tied to the low active term LATPRDAT (load the compare register with print data) which is micro-code generated. 
     Data from the multiplexor chips 83 m  is located into a compare register on the leading edge of the clock (8CLK/3) when the term LATPRDAT is low. Data from this compare register is also the data path to format control. The output of the compare logic 226 is the term &#34;TIB equals PIB&#34; which is fed into an 8-1 multiplexor chip 221 p . The micro-code may use the term TIB=PIB in order to generate the term A0 by bringing up the micro-code outputs #BRANCH 5 and #BRANCH 2. The term #BRANCH 5 effectively derives SEL2/ low which enables the 8-1 multiplexor chip 221 p  causing the signal A0 to reflect the state of TIB=PIB. 
     ERROR DETECTION: The major part of error detection in the TP-DLP is done in two 512 by 4 bit PROM chips. As seen in FIG. 5H these are designated PR 50 a  and PR 50 b . 
     The signal OP DECSEL (OP decode select) is applied to the A8 address line inputs to the PROM chips PR 50 a  and to PR 50 b . The OP decode select signal actually designates the function of the PROMS: 
     1. when OPDECSEL is &#34;high&#34;, the PROM&#39;s 50 a  and 50 b  are used to decode the OP code and detect invalid OP codes. 
     2. when OPDECSEL is &#34;low&#34;, the PROM&#39;s 50 a  and 50 b  are used for vertical parity generation/detection, LPW error detection, format error detection, and delimiter character detection. PROM 50 a  is used to monitor the digits A and B of the DBUS while the PROM 50 b  monitors the digits C and D of the DBUS. The information on the DBUS is used as addresses to the PROMS. The PROM 50 a  and 50 b  and the CSO/ enable line inputs are always enabled, that is, grounded. The 8 PROM outputs produce a function that is determined by the 16 inputs in the DBUS. The Table 21 below shows the function of the 8 PROM outputs of 50 a  and 50 b . 
     
                       TABLE 21                                                    
______________________________________                                    
Error Detection PROMS                                                     
PROM Output   OPDECSEL = 1 OPDECSEL = 0                                   
______________________________________                                    
(A0-D1) OPDEC1                                                            
              OP decode    A digit bits 4, 2,                             
              bit wt.1     and 1 =                                        
(A0-D2) OPDEC2                                                            
              OP decode    A-B digits =                                   
              bit wt.2     delimiter                                      
(A0-D3) OPDEC4                                                            
              OP decode    A-B digits = 0                                 
              bit wt.4                                                    
(A0-D4) OPDEC8                                                            
              OP decode    A-B digits are odd                             
              bit wt.8                                                    
(B0-D1) Not used                                                          
(B0-D2) BOTCF/                                                            
              Not used     C-D digits =                                   
                           delimiter                                      
(B0-D3) LPWCD Not used     C-D digits = 0                                 
(B0-D4) CDPARGEN                                                          
              Not used     C-D digits are odd                             
______________________________________                                    
 
    
     VERTICAL PARITY GENERATION/DETECTION: As seen in Table 22, and FIG. 5H, the two output signals from the PROMs (at 50 a  and 50 b ) are used for vertical parity generation and detection. These are OPDEC8 (ABPARGEN) and CDPARGEN. The OPDEC8 is &#34;high&#34; for odd parity on the A and B digits of DBUS, while CDPARGEN is &#34;high&#34; for odd parity on the C and D digits. 
     
                                           TABLE 22                                
__________________________________________________________________________
Vertical Parity Generation/Detection                                      
CDPARGEN                                                                  
        ABPARGEN                                                          
               PARGEN                                                     
                     DBUSPR                                               
                           PARERROR                                       
__________________________________________________________________________
0       0      1     0     1                                              
0       0      1     1     0                                              
0       1      0     0     0                                              
0       1      0     1     1                                              
1       0      0     0     0                                              
1       0      0     1     1                                              
1       1      1     0     1                                              
1       1      1     1     0                                              
__________________________________________________________________________
 
    
     For vertical parity generation it is necessary to generate a &#34;high&#34; level for even parity and a &#34;low&#34; level for odd parity. For vertical parity detection, it is necessary to generate a high level to indicate a parity error if either of the following two conditions exist: 
     1. The parity bit is high while the 16-bits of data have an odd number of bits &#34;ON&#34;. 
     2. The parity bit is low while the 16-bits of data have an even number of bits &#34;ON&#34;. 
     As seen in FIG. 5I these functions are generated by a dual 4-1 data selector chip S4. SLA and SLB are the select inputs to the data selector chip S4. 
     The signal PARGEN is a parity bit input to the data path multiplexor 224. The micro-code selection of the multiplexor inputs determine when this PARGEN bit is used. The signal PARERROR (parity error) is sent to the CFE 10 c  as TEST 5 via the foreplane to be tested by the micro-code. 
     LPW ERROR DETECTION: Two output signals from the PROMs 50 a , 50 b , are used for LPW error detection as will be seen in Table 21, previously shown hereinbefore and in FIG. 5H. 
     These two output signals are named OPDEC4 (LPWAB) and LPWCD. OPDEC4 (LPWAB) is &#34;high&#34; if the A and B digits of DBUS equal 0. LPWCD is &#34;high&#34; if the C and D digits of DBUS equal 0. 
     As seen in FIG. 5H the PROM outputs OPDEC4 and LPWCD are fed into the NAND gate 50 g  thus developing the output signal LPWERROR (high equals error). This signal is sent to the CFE 10 c  as TEST 6 via the foreplane to be tested by the micro-code. 
     FORMAT ERROR DETECTION: The two output signals of FIG. 5H, with reference to Table 21, from the PROMs 50 a  and 50 b  are used in checking for valid format commands on multiple line operations. These signals are named OPDEC1 (digit A equals 8 or A equals 0) and LPWCD. The signal OPDEC1 is &#34;high&#34; if digit A of DBUS equals 8 or equals 0. LPWCD is &#34;high&#34; if C and D digits equal 0. The previous Table 12, under note 2 shows the composition of the format word. 
     Referring to FIG. 5H, the PROM outputs OPDEC1 (instead of OPDEC4) and LPWCD are fed into the NAND gate 50 g  developing the output signal FORERROR instead of LPWERROR (high equals error). This signal is sent to the CFE 10 c  as TEST 10 via the foreplane and to be tested by the micro-code. 
     DELIMITER CHARACTER DETECTION: The two output signals from the PROMs 50 a  and 50 b  are also used for delimiter character detection as seen in Table 21 and FIG. 5J. These signals are named: OPDEC2 (TOPCF/) and BOTCF/ (which refer to top and bottom). 
     The signal OPDEC2 (TOPCF/) is &#34;low&#34; if A and B digits of DBUS equal HEX CF (which is the delimiter character). The signal BOTCF/ is &#34;low&#34; if the C and D digits of DBUS equal HEX CF as seen in FIG. 5J. 
     PROM outputs OPDEC2 and BOTCF/ are fed into the NAND gate 50 g , thus developing the output signal DELFOUND (delimiter found). This signal is sent to the CFE 10 c  as TEST 8 via the foreplane to be tested by the micro-code (&#34;high&#34; equals delimiter found). After the micro-code detects a delimiter, it tests to see if the delimiter was detected in the A and B digits or, in the C and D digits, by testing the signal TEST 9 (top delimiter) which is also sent to the CFE 10 c  via the foreplane (low equals top delimiter). 
     OPERATION DECODE (OPDEC): As seen in FIG. 5H, the operation decoding is performed by PROM 50 a  when the address A8 input (OPDECSEL) is &#34;high&#34;. This signal is controlled by micro-code and is forced &#34;high&#34; when OP decoding is desired. This PROM, while OPDECSEL is high, serves the function of monitoring A and B digits of DBUS and generates OPDEC lines corresponding to the operation to be performed, according to Table 23, which is shown below. 
     
                       TABLE 23                                                    
______________________________________                                    
OP Decode                                                                 
OPDEC                      DBUS                                           
Lines        Operation     AB digits                                      
______________________________________                                    
0          Test wait ready 21                                             
1          Test wait not ready                                            
                           22                                             
2          Test            20                                             
3          Test ID         2C                                             
4          Test skip       24                                             
5          Echo            10                                             
6          Load train image buffer                                        
                           48                                             
7          Invalid operation                                              
                            XX*                                           
8          Write           40                                             
9          Write halt      41                                             
A          Write line      44                                             
B          Write line halt 45                                             
C          Move write line 46                                             
D          Move write line halt                                           
                           47                                             
E          Not used                                                       
F          Not used                                                       
______________________________________                                    
 *= Undefined.                                                            
 
    
     Any invalid (undefined) bit code in A and B digits produces an output of HEX 7 on the OPDECn lines. No bit code in the A and B digits will produce an output of HEX E or F on the OPDECn lines (when OPDECSEL is high, the PROM 50 is not programmed to output this bit pattern). These OPDECn lines are sent to the CFE 10 c  via the foreplane to be used as inputs to the address multiplexor 12. This enables the micro-code program to perform a 16-way branch on the OP code. FIG. 5A shows the four OPDECODE lines from 83 d  which feed over to address multiplexor 12. 
     STOP BIT GENERATION: As seen in FIG. 5K there is shown a PROM which provides the &#34;stop bit&#34; generation. This PROM 50 s  produces a &#34;high&#34; out of line output D4 which is named INTERFLG (which means STOP bit in the micro-code). This is done for any one of the following conditions: 
     1. a delimiter was detected. 
     2. the BC register reached the maximum count (BC=FF). 
     3. a vertical parity error occurred. 
     4. a &#34;terminate&#34; was received from the main host system. 
     The micro-code may use the &#34;stop bit&#34; (INTERFLG) in order to generate the term AO by bringing up the proper micro-code #BRANCH lines. 
     CONTROL LOGIC: Referring to FIG. 5A and control logic 222, a 3-8 decoder is enabled during 8CLK1 time. The micro-code output signals CONTRAD1, -2 and -3, are used to make the decoder produce a low output: from RSETPCSL (reset PCSL); from LDREGCAD (load register column address or load AD register); from WECOLDN (write enable to invalid RAM); from RSETTIB/ (reset train image buffer load/); from SETCSLF (set CSLF); or none at all. Another 3-8 decoder is constantly enabled, thus producing a low output depending on the binary weight of its output signals CONTRAD 4, -5 and -6, which are micro-code signals. The outputs that may be driven &#34;low&#34; are: MPXSELAD (high=select adder output as the input to the Accumulator); the DIRAMCOL (data input to column done RAM); to DIRAMINV (data input to invalid RAM); to LATPRDAT (latch print data into compare register); the load input to the 4-bit binary counter in the compare logic; or no outputs at all. 
     Another 4-bit binary counter in logic block 222 is used as a 4-bit latch; it has inputs #CONST 4 through 7 (micro-code outputs) which are latched by LOADCONT (load control register--which is a micro-code output) being low at the leading edge of the clock 8 CLK/1. Its outputs are OPDECSEL (OP decode select), DC1LCTR (DC1L, control register output), DC2LCTR (DC2L, control register output) and DT1LCTRL (DT1L control register output). 
     The 4-bit counter in the compare logic is used as a 4-bit latch. It has inputs #CONST 0 through 3 (micro-code outputs) which are latched by the Y7/ output of the secone 3-8 decoder during the leading edge of clock 8 CLK/1. The 4-bit counter has outputs at 220 which are RAMAD 8 (RAM buffer address line 8) and MOSTCTR (the motor start signal for the printer). 
     AO ADDRESS GENERATION: The micro-code within the CFE 10 c  does bit tests on desired conditions by gating these conditions onto the AO line which is seen in FIGS. 4A and 5A. This line AO is the least significant bit of the micro-code PROM address. The desired test condition (bit) is selected by the #BRANCH lines and the #I4 from the micro-code. The #BRANCH 4, 5, and BR6 (#I4) lines develop the SELn/ lines. The following Table 24 indicates all of the test conditions on the PDB 80 which are used for AO line generation and the terms needed to select them. 
     
                                           TABLE 24                                
__________________________________________________________________________
AO Selection (PDB Only)                                                   
                                     Term Gated                           
SEL5/                                                                     
    SEL4/                                                                 
        SEL3/                                                             
            SEL2/                                                         
                #BRANCH3                                                  
                       #BRANCH2                                           
                              #BRANCH1                                    
                                     Onto A0                              
__________________________________________________________________________
1   1   1   0   0      0      0      PCTPFLAG                             
1   1   1   0   0      0      1      INVALID                              
1   1   1   0   0      1      0      TIB=PIB                              
1   1   1   0   0      1      1      PCSLFLAG                             
1   1   1   0   1      0      0      TIBLOAD/                             
1   1   1   0   1      0      1      CSLFLAG                              
1   1   1   0   1      1      0      PFCLFLAG                             
1   1   1   0   1      1      1      PAMLFLAG                             
1   1   0   1   0      0      0      ENDLFLAG                             
1   1   0   1   0      0      1      CIDLFLAG                             
1   1   0   1   0      1      0      PR1FLAG                              
1   1   0   1   0      1      1      PR2LFLAG                             
1   1   0   1   1      0      0      PRSLFLAG                             
1   1   0   1   1      0      1      (spare)                              
1   1   0   1   1      1      0      120COLIP                             
1   1   0   1   1      1      1      132COLIP                             
1   0   1   1   0      0      0      1100LPMJ                             
1   0   1   1   0      0      1      MAXCOUNT                             
1   0   1   1   0      1      0      INTERFLG                             
1   0   1   1   0      1      1      SYNCFLAG                             
1   0   1   1   1      0      0      PCSLT                                
1   0   1   1   1      0      1      (not used)                           
1   0   1   1   1      1      0      (not used)                           
1   0   1   1   1      1      1      (not used)                           
0   1   1   1   0      0      0      RAMAD0                               
0   1   1   1   0      0      1      RAMAD1                               
0   1   1   1   0      1      0      RAMAD2                               
0   1   1   1   0      1      1      RAMAD3                               
0   1   1   1   1      0      0      RAMAD4                               
0   1   1   1   1      0      1      RAMAD5                               
0   1   1   1   1      1      0      RAMAD6                               
0   1   1   1   1      1      1      RAMAD7                               
__________________________________________________________________________
 1 = high.                                                                
 0 = low.                                                                 
 
    
     OPERATING MICROCODE- General Description: Referring to FIGS. 5A, 5E and 5F, the TP-DLP is controlled by the micro-code contained in the PROM 13 on the CFE card. Micro-code simply means object code or program code. The PROM outputs from the PROM micro-code registers 14 on the CFE card change to produce the new micro-code word on the leading edge of PROMCLK/. 
     Referring to Table 25, the TP-DLP is normally in idle status (status=3). In this state it can accept an I/O descriptor from the host system. The TP-DLP receives an I/O descriptor while at status=3, goes to status=11, and receives the I/O descriptor LPW. The TP-DLP then goes to status=6 to receive the two descriptor links and their LPW. The DLP then goes to status=1 and performs a 16-way branch dependent upon the OP code which it receives as part of the I/O descriptor at status=3 time. 
     On the loading of the train image buffer as seen in FIG. 5F, description of the train image buffer load operation, by status count is shown in Table 25 herein: 
     
                       TABLE 25                                                    
______________________________________                                    
Status = 03                                                               
1.     Idle.                                                              
2.     If AF/ is low, then set status = 11.                               
3.     Go to step 1.                                                      
Status = 11                                                               
1.     Write I/O descriptor at lower RAM buffer ad-                       
       dress 0.                                                           
2.     Set status = 06.                                                   
Status = 06                                                               
1.     Write descriptor LPW at lower RAM buffer ad-                       
       dress 1.                                                           
2.     Write descriptor link #1 at lower RAM buffer                       
       address 2.                                                         
3.     Write descriptor link #2 at lower RAM buffer                       
       address 3.                                                         
4.     Write descriptor link LPW at lower RAM buf-                        
       fer address 4.                                                     
5.     Set status = 01.                                                   
Status = 01                                                               
1.     Read lower RAM buffer address 0 (location of                       
       I/O descriptor).                                                   
2.     Perform a 16-way branch using the OPDECn                           
       lines.                                                             
3.     Set status = 05.                                                   
Status = 05                                                               
1.     Send descriptor links back to the host system                      
       (descriptor link #1, descriptor link #2, and de-                   
       scriptor link LPW, located in lower RAM buf-                       
       fer address 2, 3, and 4 respectively).                             
2.     Set status = 08.                                                   
Status = 08                                                               
1.     Store HEX 6F (decimal value of 111) in accu-                       
       mulator 11 (address of blank and invalid char-                     
       acter storage).                                                    
2.     Receive the first word from the host system,                       
       (the A and B digits contain the code for a blank                   
       character while the C and D digits contain the                     
       code for an invalid character) and write it in                     
       upper RAM buffer, as addressed by accumula-                        
       tor 11.                                                            
3.     If a stop condition exists, set status = 14.                       
4.     Increment accumulator 11.                                          
5.     Receive the next word from the host system                         
       and write in into upper RAM buffer, as ad-                         
       dressed by accumulator 11.                                         
6.     Go to step #3.                                                     
Status = 14                                                               
1.     Receive a dummy word from the host system.                         
2.     Set status = 12.                                                   
Status = 12                                                               
1.     Receive the LPW from the host system.                              
2.     Set status = 01.                                                   
Status = 01                                                               
1.     Flush invalid RAM with invalids (write ones in                     
       all addresses of the invalid RAM).                                 
2.     Store HEX 6F (decimal value of 111) in accu-                       
       mulator 11 (address of blank and invalid char-                     
       acter storage).                                                    
3.     Move accumulator 11 to register BC (RAM                            
       buffer address).                                                   
4.     Latch top character of RAM buffer output into                      
       the compare register.                                              
5.     Write 0 in invalid RAM. The address of the in-                     
       valid RAM is the character that is latched into                    
       the compare register. This operation marks                         
       this character as a valid character code.                          
6.     Latch the bottom character of the RAM buffer                       
       output into the compare register.                                  
7.     Same as step #5.                                                   
8.     Increment accumulator 11.                                          
9.     Move accumulator 11 to register BC (RAM                            
       buffer address).                                                   
10.    If register BC = HEX FF (MAXCOUNT = 1).                            
       then reconnect to the host system and set stat-                    
       us = 05.                                                           
11.    Go to step #4.                                                     
Status = 05                                                               
1.     Send descriptor links back to the host system                      
       (descriptor link #1, descriptor link #2, and de-                   
       scriptor link LPW, located at lower RAM buf-                       
       fer address 2, 3, and 4 respectively).                             
2.     Set status = 07.                                                   
Status = 07                                                               
1.     Move accumulator 6 to AD register (A and D                         
       digits of the result descriptor).                                  
2.     Move accumulator 7 to BC register (B and C                         
       digits of the result descriptor).                                  
3.     Send this result descriptor to the host system.                    
4.     Set status = 15.                                                   
Status = 15                                                               
1.     Send result descriptor LPW to the host system.                     
2.     Set status = 03. This step restarts at the begin-                  
       ning (idle).                                                       
______________________________________                                    
 
    
     TP-DLP STATUS DEFINITIONS: DLP status information is the means by which the TP-DLP communicates it&#39;s conditions and requirements to the host system 10. Status states are transmitted by the TP-DLP when it is connected to the host system. The use of status states is designed to allow the message transfers between the TP-DLP and the host system to be conducted in an orderly manner. 
     The following Table 26 defines each individual status state operation used specifically in the Train Printer-Data Link Processor: 
     
                       TABLE 26                                                    
______________________________________                                    
Status States - TP-DLP                                                    
______________________________________                                    
Status = 0                                                                
Cleared. The DLP is in the cleared state. This                            
status is asynchronously entered when the DLP is                          
master cleared, local cleared, or selective cleared.                      
Status = 1                                                                
Disconnect. The DLP is busy and cannot accept a                           
new I/O descriptor.                                                       
Status = 3                                                                
Idle. The DLP can accept an I/O descriptor at this                        
time.                                                                     
Status = 4                                                                
Read data. This is the read buffer portion of an                          
echo operation.                                                           
Status = 5                                                                
Send descriptor link. The DLP is sending the de-                          
scriptor links and their longitudinal parity word                         
(LPW) to the host system.                                                 
Status = 6                                                                
Receive descriptor links. The DLP is receiving, or                        
is ready to receive, the descriptor links and their                       
LPW.                                                                      
Status = 7                                                                
Result descriptor. The DLP sends the result de-                           
scriptor to the host system with the next DLP                             
strobe                                                                    
Status = 8                                                                
Write data. The DLP can accept train image or                             
print image buffer data from the host system.                             
Status = 9                                                                
Encoded status (back-up). Indicates to the host                           
system that DLP address back-up information is                            
present on the data lines.                                                
Status = 11                                                               
Receive descriptor LPW. This status informs the                           
host system that the DLP expects to receive the                           
I/O descriptor LPW.                                                       
Status = 12                                                               
Break. The DLP requires no more data because it                           
has reached an end condition. The host system                             
sends the data LPW with the next strobe.                                  
Status = 14                                                               
Character transfer. Used by the DLP to handle                             
single character transfers from the host system or                        
to receive the last character or word from the host                       
system. This status acts like a request to the host                       
system in that it requests the host system to notify                      
it of whether or not the last word sent to it from                        
the host system was a single character (8-bits) or                        
a full word. The host system responds with the                            
next STIOL and a terminate, if the word sent was                          
a single character, and responds with an STIOL                            
and no terminate, if the last word was a full word.                       
Status = 15                                                               
Result descriptor LPW. The DLP is sending the                             
result descriptor. The LPW will follow with the                           
next DLP strobe.                                                          
______________________________________                                    
 
    
     PCTP GENERATION: This refers to the printer column timing pulse generator which generates the printer column timing pulse PCTP in FIG. 5D. The PCTP generator is made of a binary counter, a SJK flip-flop, 2 NOR gates and a tri-state inverter. In FIG. 5A, the PCTP generator is shown as the element 232. The binary counter is the heart of this timing network and the printer column timing pulse jumpers are installed in accordance with the speed rating of the printer. Maximum PCTP frequency is achieved when no jumpers are installed; then a slower PCTP frequency is produced as jumpers are installed. The binary counter increments on the leading edge of the clock (8CLK/). As the counter reaches the count of 16, a &#34;carry output&#34; is generated (125 nanoseconds positive pulse). This pulse is supplied to the J and K inputs to the flip-flop and causes it to toggle with the next trailing edge of the clock (8CLK). The carry output is also inverted by a NOR gate and fed to the LD/ (load/) input of the binary counter which causes the binary counter to be loaded with the value of the PCTP jumpers. The cycle is then repeated. 
     For 1100 lines-per-minute (LPM) operation, all the PCTP jumpers are installed, thus forcing the binary counter to count through all 16 steps before emitting a carry output. The signal (FIG. 5D) PCTP/ from the tri-state inverter is a 250-kilohertz square wave (2 micro-second pulse width). The signal PCTPFLAG is a 125 nanosecond positive pulse occurring at 4 micro-second intervals which flags the micro-code at the end of each PCTP pulse. 
     PERIPHERAL INTERFACE: The printer interface-control 220 is seen in FIG. 5A. The TP-DLP receives 9 signals from the printer 50 p  and feeds them into tri-state inverters. These asynchronous signals are converted to synchronous signals and then used as inputs for AO branching as seen in Table 24. The signal PRSL/, from the printer 50 p , is not fed through a chip because it is a level rather than a pulse and needs no synchronization. 
     The Train Printer-Data Link Processor sends out 8 signals to the printer 50 p . All these interface signals are driven by tri-state inverters, and all the tri-state interface drivers and receivers are (except the DTnL/ lines) enable when GPRIF (gate maintenance peripheral line signal) is &#34;low&#34;. The interface signals between the printer interface 220 and the printer 50 p  are shown below in Table 27. 
     
                       TABLE 27                                                    
______________________________________                                    
Printer Interface to DLP                                                  
______________________________________                                    
From Printer                                                              
       Foreplane                                                          
       Signal  Pin                                                        
______________________________________                                    
       PCSL/   S87                                                        
       CSL/    S86                                                        
       PFCL/   S88                                                        
       PAML/   S89                                                        
       EDPL/   S90                                                        
       CIDL/   S95                                                        
       PR1L/   S85                                                        
       PR2L/   S96                                                        
       PRSL/   S91                                                        
To Printer                                                                
       Foreplane                                                          
       Signal  Pin                                                        
______________________________________                                    
       DC1L/   S78                                                        
       DC2L/   S79                                                        
       MOST/   S84                                                        
       PCTP/   S77                                                        
       DT8L/   S83                                                        
       DT4L/   S82                                                        
       DT2L/   S81                                                        
       DT1L/   S80                                                        
______________________________________                                    
 
    
     PRINT AND FORMAT CONTROL: The Train Printer-Data Link Processor commands the printer 50 p  either to remain idle, to print data, or to advance paper by way of the DC1L and the DC2L signal lines which carry signals shown in FIG. 5D. Printer 50 p  decodes and responds to the signals as shown in Table 28. 
     
                       TABLE 28                                                    
______________________________________                                    
Data Control Level Signals                                                
Binary    DC2L    DC1L        Printer Response                            
______________________________________                                    
0         0       0           No action                                   
1         0       1           Print                                       
2         1       0           Format                                      
3         1       1           Invalid                                     
______________________________________                                    
 
    
     When DCnL=2, the DTnL lines, from interface 220, contain format information for the printer. However, while DCnL=1, then only DT1L of the DTnL lines is used and it is &#34;high&#34; for a data match. 
     The 3-8 line decoder in interface 220, FIG. 5A, is disabled only if a PROM parity error occurs. This 3-8 line decoder produces a low active signal at the output corresponding to the binary weight of the three input lines. The signal GPRIF originates on the maintenance card 20 om  and is &#34;high&#34; to effectively disconnect the PDB peripheral cable lines and replace them with peripheral simulation lines. 
     The mentioned 3-8 decoder, in interface 220, operates in two modes, when is to say--&#34;maintenance&#34; and &#34;normal&#34; depending upon the signal state of GPRIF/, the third input to the 3-8 decoder chip. The normal mode of operation is with GPRIF/ high thus enabling ENDTX1/ (enable DT1L), ENFOR/ (enable format/) or neither. The signal ENDTX1/, when low, enables a tri-state inverter which supplies the format information to the printer 50 p  from a 4-bit binary counter in interface 220. GPRIF, when low, also supplies the DCnL/ signals to the printer 50 p . 
     PERIPHERAL LINES FOR MAINTENANCE: The lines designated PRIFnn/O are used by the maintenance card 20 om  to verify and check the operation of the peripheral dependent board (PDB) of the Train Printer-Data Link Processor. These PRIFnn/O lines are enabled by tri-state chips located in the TP-DLP. These peripheral maintenance lines are enabled when the signal GPRIF/ is &#34;low&#34;. The signal GPRIF originates at the maintenance card MC 20 om . These lines permit simulation of the Train Printer by means of the maintenance card 20 om . The following Table 29 shows a listing of these simulated peripheral lines, PRIFnn/O, and also shows the corresponding inputs and outputs to the aforementioned tri-state chips. 
     
                       TABLE 29                                                    
______________________________________                                    
Maintenance Card Lines to/from PDB                                        
PRIF Lines     Outputs      Inputs                                        
______________________________________                                    
PRIF01/0       MPCTP/                                                     
PRIF02/0       MDC1L/                                                     
PRIF03/0       MDC2L/                                                     
PRIF04/0       MT8L/                                                      
PRIF05/0       MT4L/                                                      
PRIF06/0       MT2L/                                                      
PRIF07/0       MT1L/                                                      
PRIF08/0                    MPRSL/                                        
PRIF09/0                    MPCSL/                                        
PRIF10/0                    MCSL/                                         
PRIF11/0                    MPFCL/                                        
PRIF12/0                    MPAML/                                        
PRIF13/0                    MEDPL/                                        
PRIF14/0                    MC1DL/                                        
PRIF15/0                    MPR1L/                                        
PRIF16/0                    MPR2L/                                        
PRIF17/0       MOST/                                                      
______________________________________                                    
 
    
     MAINTENANCE DISPLAY LINES: The Train Printer-Data Link Processor contains three 8-1 multiplexor chips that are used to enable the PDB 80 at its maintenance display lines (DPLY11/0-DPLY13/0, as can be referred to in Table 21). These lines are used by the maintenance card 20 om  to verify and check the operation of the peripheral dependent board (PDB) of the Train Printer-Data Link Processor. The DSELn/ lines are used to select the internal signals that are enabled onto the backplane maintenance bus. The DSELn/ lines are sent to the PDB 80 from the Common Front End, CFE 10 c  and are enabled when the data link processor is &#34;maintenance card&#34; addressed. 
     DATA LINK PROCESSOR INTERFACE TO TRAIN PRINTER: Referring to FIG. 5A there is shown the printer interface control 220 having connections to the Train Printer 50 p  ; also referring to FIG. 5D which shows a timing diagram for the interface control signals to the Train Printer, the following discussion will describe this interface with respect to a typical Train Printer such as the 1100 line per minute Train Printer. 
     The printer column timing pulse (PCTP) is a 2.0 micro-second pulse operating at a frequency of 250 kilohertz. It is during the &#34;information transfer cycle&#34;, that the PCTP is used to gate bit information into column storage and also to advance the printer column counter. The printer column timing pulse signals are always transmitted when power is &#34;on&#34; in the printer. All of the interface signals start with the trailing edge of the PCTP, except the signals MOST (motor start) and the signal EDPL (end of page level). 
     There are two data control level signals designated as DC1L and DC2L (Table 28). The data control levels when &#34;true&#34; indicate that either print information or format control information is present on the data transfer lines (DTXL). The control levels will change state only on the trailing edge of the PCTP. A &#34;print cycle&#34; starts with the receipt of DC1L and DC2L being equal to binary 1 and terminates only upon the receipt of signals DC1L and DC2L being=to binary 2. The paper motion cycle starts with the receipt of signals DC1L and DC2L being=to binary 2 and terminates on the trailing edge of the signal PAML (paper motion level) going &#34;false&#34; at the same time that PCSL (printer column strobe level) is &#34;false&#34;. 
     The binary 1 state will go &#34;true&#34; at the trailing edge of the first PCTP after PCSL if there is bit information to be transferred into the printer column storage. Data is transferred into column storage, and the column counter is advanced at the rate of PCTP. Print columns are counted starting with column 1. A space suppress code (format=0) must be transmitted if a particular column is to be printed again on the same line. A binary 1 state must be followed by a binary 2 state if the printer is to be made &#34;not ready&#34;. 
     The binary 2 state may be transmitted any time the PAML level is &#34;false&#34;, and must be present a minimum of 1 PCTP period. Response to the binary 2 state will not occur until PCSL goes &#34;false&#34;. The binary 2 state will be transmitted at least once for each line printed. 
     DATA TRANSFER LINES: The data transfer lines are designated DT1L, DT2L, DT4L, DT8L. When the control levels DC1L and DC2L are=to binary 1, the data transfer line DT1L is used to transfer print information into the printer column storage. Data is always transferred from the data transfer lines to the printer at the trailing edge of the printer column timing pulse. 
     When the control levels DC1L and DC2L are=to binary 2, the data transfer lines (DT1L, DT2L, DT4L, DT8L) are used to transfer format information to the printer. 
     The data transfer lines are shown on the following Table. 
     
                       TABLE 30                                                    
______________________________________                                    
Data Transfer Lines to Printer                                            
DT1L, DT2L                                                                
DT4L, DT8L  DATA TRANSFER LINES                                           
______________________________________                                    
DURING TRANSFER OF FORMAT INFORMATION,                                    
THE BINARY SUM, DTXL, OF THE DATA TRANSFER                                
LINES RESULT IN THE FOLLOWING:                                            
DTXL = 0    NO ADVANCE (SPACE SUPPRESS).                                  
DTXL = 1    ADVANCE TO HEADING POSITION.                                  
DTXL = 2-11 ADVANCE TO DESIGNATED                                         
            FIELD POSITION.                                               
DTXL = 12   ADVANCE TO END OF                                             
            PAGE POSITION.                                                
DTXL = 13   INVALID CODE. CAUSES A                                        
            ONE SECOND PAPER SLEW AND                                     
            A PAPER SLEW INDICATION ON                                    
            THE FUNCTION INDICATOR                                        
            PANEL.                                                        
DTXL = 14   ADVANCE SINGLE SPACE                                          
DTXL = 15   ADVANCE DOUBLE SPACE                                          
______________________________________                                    
 
    
     The motor start level (MOST) when &#34;true&#34;, is used to start the train drive motor from a remote location whenever PR1L is &#34;true&#34; which indicates that the printer is in a &#34;ready&#34; state, and when PR2L is &#34;false&#34; indicating the train drive motor is off. 
     The data transfer lines involve the signals to the Train Printer from an external device such as the data link processor. This is also true of the motor start level signal MOST. 
     There are signals which the printer provides to the data link processor which are called the &#34;printer ready levels&#34; and which are designated PR1L and PR2L. 
     The printer ready level PR1L when &#34;true&#34; indicates that: the power is on; the paper is loaded; there is no slew alarm; the train is installed and locked in the print position; the ready switch is depressed. 
     The depression of the single print switch will override the paper-out switch and allow PR1L to go &#34;true&#34;. When the signal PR2L is also &#34;true&#34;, the train drive motor is &#34;on&#34;. 
     After the signal PR2L goes &#34;true&#34;, with also PR1L being true, the first signal DC1L or DC2L which is received from the interface control 220 initiates a print or paper motion cycle. 
     If the ready switch is depressed when the printer is in a print operation, the PR1L and the PR2L levels will be inhibited from changing state until a minimum of one PCTP clock period after the following PAML (paper motion level signal) has been initiated. The indicated paper motion operation is completed even though the printer is not ready. The printer cannot be made &#34;not ready&#34; if the last command was a print command (DC1L and DC2L being=to binary 1). The ready levels will change state only on the trailing edge of the PCTP and when the PCSL (printer column strobe level) is &#34;false&#34;. 
     
                       TABLE 31                                                    
______________________________________                                    
(Printer Ready Level Signals to DLP)                                      
PR2L    PR1L                                                              
______________________________________                                    
0       0         Not Ready (Binary 0)                                    
0       1         Ready &amp; Train Motor Off (Binary 1)                      
1       0         Invalid Code (Binary 2)                                 
1       1         Ready &amp; Train Motor On (Binary 3)                       
______________________________________                                    
 
    
     Referring again to the timing diagram of FIG. 5D, the &#34;scan cycle&#34; within the printer is monitored by the printer column strobe level or PCSL. The signal PCSL, when &#34;true&#34;, in conjunction with PR1L and PR2L being &#34;true&#34;, indicates the printer is prepared to accept information into column storage. The PCSL signal changes state only on the trailing edge of the PCTT (printer column timing pulse). 
     As seen in FIG. 5D, one &#34;scan set&#34; consists of four PCSL pulses. In the case of 1100 LPM printer, the minimum pulse length for the PCSL is 144 micro-seconds. Data for columns 1, 5, 9 and 13 is transferred during the first PCSL of a scan set. Then data for columns 2, 6, 10 and 14 is transferred during the second PCSL of the scan set. Then data for columns 3, 7, 11 and 15 is transferred during the third PCSL of a scan set. Then data for columns 4, 8, 12 and 16 is transferred during the fourth PCSL of a scan set. A minimum of four PCSL&#39;s (or one &#34;scan set&#34;) are required to transfer a full line of print. The second graphic on each train is the graphic in print position 1 during the first PCSL of a scan set following the chain sync level (CSL), also called the train sync level. 
     The train sync level is designated CSL (chain sync level) and is a signal which occurs once per train revolution. It defines the beginning of the train set. The signal CSL changes state only on the trailing edge of PCTP. The CSL is &#34;true&#34; for 6 PCTP&#39;s and it goes &#34;true&#34; one pulse before the end of the fourth PCSL of a scan set. 
     TRAIN IDENTIFICATION LEVEL: The train identification level designated CIDL is a six bit code transmitted serially (most significant bit coming first) and each bit is six pulses (PCTP&#39;s) wide. Each identification bit originates one pulse before the end of every sixth PCSL following a chain sync level (CSL) sync signal. The identification bits change state only on the trailing edge of PCTP. This identification will only appear once per train revolution. 
     PRINTER FINAL COLUMN LEVEL: This signal is designated PFCL and this level goes &#34;true&#34; at the trailing edge of the PCTP prior to the information transfer for the last character of the sub-scan. The printer will accept one additional informational bit after PFCL goes &#34;true&#34;. The signal PFCL changes state on the trailing edge of the PCTP. At the end of each sub-scan, the printer final column level PFCL goes &#34;false&#34; when the signal PCSL goes &#34;false&#34;. 
     PAPER MOTION LEVEL: This signal, designated PAML goes &#34;true&#34; in response to format information on the data transfer lines DTXL, and when data control levels DC1L and DC2L=binary 2. The paper motion signal PAML remains &#34;true&#34; until the defined paper advance is complete. PAML changes state only on trailing edge of the pulse (PCTP) when PCSL is false. 
     If the signals DC1L and DC2L are=to binary 2 during a sub-scan (PCSL) then the signal PAML will not change state until PCSL goes false. If the printer receives a space suppress code (format=0), the printer will issue a PAML level with a time duration that will approximate a single line advance, unless PR1L and PR2L are to change state, in which case the printer will issue a PAML level with a minimum time duration of one PCTP clock period on the interface before the printer changes to a &#34;not ready&#34; state, as in Table 31. 
     Whenever the PAML line is &#34;true&#34;, the control device 220 must wait until the printer has completed the operation which caused PAML to become true before issuing any following command to the printer. The signal PAML becoming false indicates the operation has been completed. 
     END OF PAGE LEVEL: This signal is designated EDPL and this level, when &#34;true&#34;, indicates that the &#34;end of page&#34; was sensed during a paper advance in a single or double space mode (DTXL=binary 14 or binary 15). The signal EDPL is reset when paper motion information is transmitted for the next line (except when DTXL=0). The EDPL signal is not synchronized with the printer column timing pulses PCTP. 
     PRINT CYCLE: The printing of characters is controlled by scanning the message to be printed and comparing it with the TIB train image buffer. The character position in the print buffer corresponds to the hammer position in the train printer. The address of the train image buffer (TIB) corresponds to the character positioned over the hammer. If the 8-bit entry in the train image buffer (TIB) is &#34;equal&#34; to the 8-bit code in the print buffer, a &#34;set&#34; level signal is sent to the printer and that particular hammer is fired on the next print cycle of the printer. 
     A pointer (accumulator 212 and register 230) into the train image buffer (TIB) is kept synchronized with the movement of the train characters. During print scans, the print message is scanned and the address of the train image buffer is changed in order to correspond with the character-hammer positions. This continues until all the characters in the print buffer have been printed. 
     In FIG. 5D, there was described the accumulator 212 working in conjunction with register 230 and the column done logic 214. The accumulators 212 together with registers 230, are the elements which act as the pointers heretofore mentioned in FIGS. 5A and 5D. Table 18 indicates the usage at the accumulators 212. 
     COMPARE LOGIC CIRCUITRY FOR THE DATA LINK PROCESSOR: 
     With reference to FIG. 5A, the compare logic circuitry combines the use of compare logic 226 (as receipt for signals from source A and B) with latch circuit 215 and multiplexor 83 m . FIG. 5L shows, in schematic form, the compare logic circuit relationships. 
     The compare logic consists of two 4-bit comparator chips F1 and G1. These chips are configured in a tandem mode to provide comparison between two 8-bit sources designated source A and source B. 
     Source A is the compare logic for the 8-bit compare latch (215) comprised of two 4-bit counter chips D1 and E1. Source B is the 8-bit output of the two quad 2-1 multiplexor (83 m ) chips D o  and E o . 
     The 16-bit input to the multiplexor chips D o  and E o  comes from the RAM buffer output from RAM 22. These 16 bits are divided into &#34;top&#34; and &#34;bottom&#34; 8-bit portions. The SL inputs to the multiplexor chips (D o  and E o ) (83 m ) are tied together and are used to control whether the &#34;top&#34; or the &#34;bottom&#34; 8-bit portion is selected. The SL inputs are tied to SELTHBH (high signal=&#34;bottom half&#34;) which is a micro-code signal. 
     The chips for the compare latch register D 1  and E 1  have LD/ (load)/) inputs which are tied to the low active term LATPRDAT (load the compare register with printed data). The term LATPRDAT is micro-code generated. Data from the multiplexor chips (D o , E o ) is loaded onto the compare register (D 1 , E 1 ) on the leading edge of 8 CLK/3 when the term LATPRDAT is &#34;low&#34;. 
     Data from the compare register (D 1 , E 1 ) also forms a data path to format control. 
     The output of the compare logic (F 1 , G 1 ) is the term TIB=PIB, which signal is fed to the 8-1 multiplexor chip P 4 . 
     The micro-code may use the term TIB=PIB in order to generate the term AO by bringing up the micro-code outputs #BRANCH5 and #BRANCH2. The term #BRANCH5 will effectively drive the line SEL 2/ &#34;low&#34; which will enable the 8-1 multiplexor chip P 4  causing the signal AO to reflect the state of TIB=PIB. The signal AO is shown on FIG. 5A as address line generation 221 p  which connects to PROM 13 and stack register 11. 
     LINE TURN AROUND LOGIC CIRCUIT: In regard to the line turn logic shown in FIG. 5M, this circuitry controls the direction of data flow between the distribution card 20 od  (via the backplane) and the Train Printer-Data Link Processor, as seen in FIG. 5A. 
     (a) In one direction flow, the Drivers 228 take data from the DBUS and supply it to the distribution control card 20 od  through the backplane connections 20 OB . 
     (b) In the other direction with the line turned around by line turn logic, the Receivers 110 receive data from the distribution card and feed it on the DBUS where it transmits: to the vertical parity check generator 217; to the longitudinal parity word generator 218; to the OP decoder 83 d  and to the INRAM bus which goes on line 100 to RAM 22. 
     In the direction (a);--the data multiplexor 224 provides data to the DBUSnn which feeds Drivers 228 so as to convey data to the distribution control card 20 od  via the backplane 20 OB . 
     However, in direction (b), the multiplexor 224 can &#34;shut off&#34; so that the distribution control card transmits data to the Receivers 110 which provides data to the DBUS of the Train Printer-Data Link Processor where the data is conveyed to elements 217, 218, 83 d  and 22 of FIG. 5A. 
     Thus, the line turn logic shown in FIG. 5M controls the flow of data: 
     (a) from the distribution (DC) 20 od  Card to the Data Link Processor: 
     (b) from the Train Printer-Data Link Processor (via the multiplexor 224) to the Drivers 228 and thence to the distribution control card 20 od . 
     Referring to FIG. 5M, the line turn logic constitutes a PROM 302 having six input signals described below: 
     (1) OFFLINE/: this signal, when low, signifies that the DLP is in &#34;local&#34; mode for maintenance and check out purposes. 
     (2) LOCAL/: this signal, when low, indicates the maintenance card has access to the Train Printer-DLP. 
     (3) IOSF: this is a signal from the I/O send flip-flop to the PDB 80. 
     (4) CONECT/: this signal, when &#34;low&#34;, indicates a DLP is connected to the main host system via the distribution card 20 od . 
     (5) SW1/: switch signal from maintenance card. 
     (6) PERF/: this signal is from the Parity Error flip-flop on the CFE card 10 c  and used to kill the signal to the Train Printer. 
     The four outputs of PROM 302 are fed to a hex-register 304. Two of its outputs are fed to a tri-state buffer 306 which has outputs as follows: 
     (a) RCV/: this &#34;receive&#34; signal, when &#34;low&#34; turns &#34;on&#34; the Receivers 110 (DATAnn lines) to receive data from distribution card 20 od . 
     (b) SIMRCV/: this signal, generated by the line turn logic in PDB 80, is used during maintenance testing to simulate the DATAn lines to Receivers 110. 
     The hex-register 304 has two outputs (c) and (d) defined as follows: 
     (c) XMIT/: this &#34;transmit&#34; signal, when &#34;low&#34; enables Drivers 228 in order to send data to the distribution card 20 od . 
     (d) LOCDIS/: this is the &#34;local display&#34; signal which activates the data simulate (DXIMxn) lines to the maintenance card. 
     Thus, the four outputs (a), (b), (c) and (d) will depend on the input states as follows:______________________________________1.     XMIT/is low when all of the following condi-       tions are met:       a. CONECT/ is low.       b. SW1/ is high.       c. IOSF is high.       d. PERF/ is high.       e. OFFLINE/ is high.2.     RCV/ is low when all of the following condi-       tions are met:       a. CONECT/ is low.       b. SW1/ is high.       c. IOSF is low.       d. PERF/ is high.       e. OFFLINE/ is high.3.     LOCDIS/ is low when one of the following con-       ditions is met:       a. All of the following conditions are met:       (1) SW1/ is high.       (2) IOSF is high.       (3) OFFLINE/ is low.       (4) LOCAL/ is low.       b. All of the following conditions are met:       (1) SW1/ is high.       (2) OFFLINE/ is high.       (3) LOCAL/ is low.4.     SIMRCV/ is low when one of the following       conditions is met:       a. All of the following conditions are met:       (1) SW1/ is high.       (2) IOSF is low.       (3) OFFLINE/ is low.       (4) LOCAL/ is low.       b. SW1/ is low.______________________________________ 
     It will be understood that the preferred embodiments described herein are only exemplary and that they are susceptible of many modifications and variations in construction, arrangement and use, without departing from the spirit of the invention as defined by the appended claims.