Remote line switching extension for telephone exchange
FIELD OF THE INVENTION 
     This invention relates to telephone exchange switching systems, and more particularly, is concerned with a remote switching station which operates directly into a central station. 
     BACKGROUND OF THE INVENTION 
     To reduce the number of line pairs between a central office and individual subscriber phones, various kinds of pair-gain systems have been developed. Remote concentrators have been used which connect a number of subcriber phones to a smaller number of line pairs going to the central office. The shared line pairs are then expanded back to the full complement of subscriber line pairs at the central office. These concentrators may use some priority scheme for allocating the available channels between the remote station and the central office among the larger number of subscriber phones. 
     Because the standard central office No. 5 crossbar switching system already includes a concentrating portion, conventional remote switching systems which provide concentration and expansion are inherently redundant. Prior attempts have been made to take advantage of the concentrating aspect of the central office switching system by physically displacing the concentrating portion of the central office to a remote location. Such a system is described, for example, in U.S. Pat. No. 4,038,498. This patent shows an arrangement which takes advantage of the concentrating effect in the link frames of a standard Western Electric No. 5 crossbar switching system of the type described in U.S. Pat. No. 2,585,904. The input stage of the standard line link frame crossbar switch selectively connects a group of up to 50 subscriber lines connected to the vertical lines of the crossbar switch to any one of a group of 10 horizontal lines of the crossbar switch, thus providing an inherent concentrator function. The output stage of the same line link frame crossbar switch connects the 10 horizontal lines to 10 vertical lines which in turn connect to a corresponding number of junctors extending to the trunk line frames of the No. 5 crossbar switching system. U.S. Pat. No. 4,038,498 provides an arrangement for physically separating the input stage with its concentration from the subscriber lines to the horizontal lines of the crossbar switch, to a remote station. 
     However, the basic design of the No. 5 crossbar switch makes it relatively uneconomical to detach the line link frame or any part of it for relocation to a remote site. The presence of other control circuits in the line link frame, such as portions of the marker circuit, greatly increases the complexity in cost of any interface system which is required to physically relocate any part of the line link frame to a remote location. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved system for providing remote line switching for use with conventional crossbar switching systems. However, the present invention does not require any portion of the line link frame of the crossbar switch to be physically moved or duplicated at the remote station. Rather the present invention provides a terminal which bridges the first stage of the line link frame at the central office, providing full control of the remote switching without interfering in any way with the operation of the crossbar switch at the central office. The line link switch is retained in the central office and uses the crosspoints in conventional manner to make and hold the connections. Call switcing status is derived from the marker at the central office, and this information is then coded and transmitted to the remote terminal for controlling actual switching. 
     In brief, the present invention provides apparatus for remote switching of a plurality of subscriber telephone lines at a remote location to a limited number of line pairs less than the number of subscriber lines extending to and connected through a central office switching system. The central office switching system includes a line link frame having a crossbar switch for connecting a group of vertical three-conductor lines (tip, ring, and sleeve conductors) to a smaller number of horizontal three-conductor lines at each of a matrix of cross points. The apparatus comprises a central office terminal including a first microprocessing unit, first scanner means controlled by the processing unit for scanning in sequence the voltage level status on the sleeve conductors of the horizontal and vertical crossbar lines and storing the status in the processing unit, means controlled by the processing unit for selectively &#34;grounding&#34; the ring conductor of any of the vertical crossbar lines to initiate a connection between the associated vertical crossbar line and one of the horizontal crossbar lines, and interface switching means controlled by the processing unit for selectively connecting any of said line pairs extending to the remote terminal to associated ones of the tip and ring conductors of the horizontal crossbar lines. The apparatus further includes a remote switching terminal including a second processing unit, second scanner means controlled by the second processing unit for scanning the remote subscriber lines to detect a change in status of the lines, matrix switching means controlled by the processing unit at the remote station for connecting a selected one of the subscriber telephone lines to one of the limited number of line pairs extending to the central office. The system further includes a data transfer link connecting the first and second processors for transferring status data on the subscriber lines from the second processing unit to the first processing unit, and for transferring status data on the sleeve conductors of the horizontal and vertical crossbar lines from the first processing unit to the second processing unit. It should be noted, however, vertical crossbar lines apply to subscribers and horizontal crossbar lines apply to trunks. Different crossbar switches and PABX&#39;s use the reverse designations, i.e., subscribers are on horizontal crossbar lines and trunks on vertical crossbar lines. Call initiation can also be obtained by placing a short across tip and ring of the subscriber crossbar line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention reference should be made to the accompanying drawings, wherein: 
     FIG. 1 is a schematic block diagram of the invention as applied to a No. 5 crossbar switching system; 
     FIG. 2A is a schematic block diagram of the central office terminal; 
     FIG. 2B is a schematic block diagram of the remote terminal; 
     FIG. 3 is a schematic diagram of the interface circuit; 
     FIG. 4 is a schematic diagram of the scanner circuit; 
     FIG. 5 is a schematic block diagram of the matrix circuit; 
     FIG. 6 is a schematic block diagram of the central processing unit; 
     FIG. 7 is a flow diagram of the operation of the central processor during initialization; 
     FIG. 8 is a flow diagram of the operation of the central processing unit at the remote terminal; and 
     FIG. 9 is a flow diagram of the operation of the CPU at the central office terminal. 
    
    
     DETAILED DESCRIPTION 
     The preferred embodiment of the present invention is described as an addition to the conventional crossbar switching system, which is an electromechanical switching system that has been widely adapted for central office exchanges throughout the world. This system is described in detail in U.S. Pat. No. 2,585,904, issued Feb. 19, 1952, in the name of A. J. Busch, which patent is hereby incorporated by reference. 
     The crossbar switching system, a portion of which is shown schematically at 10 in FIG. 1, operates to connect a plurality of local subscriber phones to trunk lines which in turn communicate with other central office exchanges, or to connect the local subscriber phones to other local subscriber phones by the crossbar switching system. A larger number of subscriber phones are each connected by a pair of conductors to one of a plurality of line link frames 14. Each line link frame includes a plurality of crossbar switches arranged in two stages, a line switch stage and a junctor switch stage. Each crossbar switch in a line link frame is in the form of a switching matrix in which the crosspoints between a plurality of vertical lines and a plurality of horizontal lines can be completed or disconnected by sets of relays. The crossbar switch in the line switch stage of a line link frame typically connects any one of 50 vertical lines to any one of 10 horizontal lines. Each vertical line is arranged to accommodate one line from a subscriber phone. Thus the line switching stage of the line link frame operates as a compactor, allowing connections to be completed between any 10 of 50 different subscriber lines at one time to 10 horizontal lines of the crossbar switch. 
     The junctor switch stage of the line link frame acts as a distributor for connecting any one of the 10 horizontal lines to any one of 10 junctors, the junctors extending to trunk link frames 16. The junctors in combination with the trunk link frames allow the subscriber phones connected to any one of the line link frames to be in turn connected to any of the trunk lines from any of the trunk line link frames. The path is established by markers 18 which are called to select and establish a path when required. After the marker establishes a path, it is released for establishing another path. A marker is connected, as needed, to a line link frame through a marker connector 20 and the marker controls the crosspoint switching in the line link frames and the trunk link frames through a frame connector 22. 
     The present invention is directed to an arrangement by which a number of remote subscriber phones 24 can be connected into the crossbar switching system 10 by less than one line pair per phone. A remote terminal 26 connected to the remote subscriber phones 24 communicates with a central office terminal 28 that connects directly into the horizontal lines of the crossbar switches in the line switching stage of the line link frames 14. This is accomplished without disturbing or modifying the physical or electrical configuration of the central office equipment. The central office terminal 28 ties in directly to existing readily accessible terminal points in the line link frames 14, in a manner hereinafter described in detail. Thus the present invention provides a method of reducing the number of costly conductor pairs normally required to connect a large group of remotely located subscriber phones to the central office. Typically the remote station provides from a 2:1 to a 4:1 compaction factor between the number of subscriber phones serviced at the remote station and the number of conductor pairs extending to the central office. 
     The manner in which the remote terminal 26 and central office terminal 28 function can best be understood by reference to FIGS. 2A and 2B. Each terminal includes its own microprocessor, indicated at 30 for the central office terminal, and 32 for the remote terminal. The microprocessors communicate with each other over a data link, including a single conductor pair terminated at each end by conventional modems or line pairs 34 and 36. The single microprocessor at each terminal is shown proving control to one line link frame. However, the system can be expanded using a single microprocessor at each terminal for providing connections to as many of the line link frames 14 at the central office as may be required. 
     Each line link frame of the crossbar switching system is provided with up to 10 crossbar switches, each crossbar switch having 10 horizontal lines and up to 50 vertical lines. Each crossbar switch in the line link frame is referred to as a &#34;group&#34;, and the groups are numbered 1-10. The remote terminal 26 and central office terminal 28 operate to complete connections between any one of up to 40 subscriber phones 24 and one switching group in the line link frame. Up to 10 line pairs or carrier channels are required between the remote terminal 26 and central office terminal 28 for completing the connetions to one group of the line link frame. In FIGS. 2A and 2B, only the 10 line pairs or channels for group No. 1 and group No. 10 are shown, as indicated at 34 and 36. 
     The central office terminal 28, under the control of the microprocessor 30, operates to complete connections between the 10 horizontal lines of each crossbar group to the associated line pairs 34. This is accomplished through an interface circuit 38, shown in more detail and described below in connection with FIG. 3. It should be noted that each horizontal and vertical line of a crossbar switch in the crossbar switching system comprises three separate conductors, referred to as the tip (T), ring (R), and sleeve (S) conductors. Hereinafter the horizontal conductors of the crossbar switch will be referred to as HT, HR, and HS, respectively, and the vertical conductors will be similarly referred to as VT, VR, and VS, respectively. External connections to these conductors can be made at readily accessible terminal points on the line link frame. 
     At the remote terminal end, the ten line pairs or channels for each group are connected under control of the microprocessor 32 to associated ones of up to 40 subscriber phones 24 for each group of ten line pairs or channels. The connection between ten subscriber phones and a group of line pairs or channels is completed by a matrix circuit 40. Two, three, or four of such matrix circuits are provided depending upon whether the system is arranged to accommodate 20, 30, or 40 subscriber phones per group of ten line pairs or channels 34. It will be noted that in FIG. 2B only two matrix circuits are shown, respectively, for subscriber phones 1 through 10 and 31 through 40. It will be understood that to accommodate 40 subscriber phones, two additional matrix circuits 40 are required for subscribers 11 through 20, and 21 through 30, respectively. 
     To control the connections between the interface 38 and the line pairs for each group, and to control the matrix switch connections between the line pairs and the subscriber phones, the microprocessors 30 and 32 monitor various input conditions and control various output conditions through scanner circuits 42, which are substantially identical at both terminals and are hereinafter described in detail in connection with FIG. 4. Each scanner, in response to addresses from the associated microprocessor, scans up to 50 input terminals (10 HS terminals and 40 VS terminals) through an input section 44 and provides an output signal on any one of 40 output lines VR through an output section 46. The central office terminal requires one scanner circuit per group, with the input section 44 being connected to the ten HS conductors of a line link frame crossbar switch, and the forty VS conductors of the crossbar switch. The output section 46 of each scanner is connected to the forty VR conductors of the same crossbar switch. The remote terminal has two scanner circuits 42, designated scanner A and scanner B, for each group. The VS terminals of the input section 44 of scanner A are connected to sensors 48 in the matrix circuits 40, which sense the off-hook status of the 40 subscriber phones. The HS terminals of the input section 44 of scanner A are connected to current sense relays 50 which sense whether associated ones of the ten line pairs of the groups are idle or busy. Forty of the inputs to the input section 44 of the scanner B are used to monitor the operation of 10×10 matrix switches 52 in each of the matrix circuits 40. These matrix switches provide direct connections between selected ones of line pairs and selected ones of the subscriber phones. 
     The output section 46 of scanner A at the remote terminal provides up to 40 output signals for controlling the crosspoints of the matrix switches 52. The 40 outputs from the output section 46 in the B scanner are used to control line relays 54, which operate to switch the lines from the subscribers from the off-hook sensors 48 to the matrix switch 52. The line relays isolate the matrix switches 52 during a switching operation to protect the matrix switch contacts from local currents during make or break operation. 
     The operation of the two terminals in completing a connection between a subscriber line is as follows. When a remote subscriber phone 24 is taken off the hook, a signal is provided on a corresponding one of the 40 lines from the off-hook sensors. As the scanner A addresses each of the VS terminals in response to addresses received over the data bus from the microprocessor, the microprocessor senses when the condition of any of the 40 off-hook lines has changed since the previous scan. If it determines from the addressed VS terminal of the scanner A that the associated subscriber phone has been taken off the hook and is seeking a connection through the central office, it stores the status change and address of the particular phone and forwards this information to the microprocessor 30 at the central office terminal over the data link. The microprocessor 30 at the central office terminal then sends an address over its data bus for addressing the appropriate output line VR from the scanner at the central office terminal and establishes a &#34;ground&#34; on the corresponding vertical ring conductor in the line link frame crossbar switch. This activates the crossbar switching system in exactly the same manner as though one of the subscriber phones 12 had been taken off the hook, as fully described in U.S. Pat. No. 2,585,904. 
     The crossbar switching system then operates by means of the markers 18 to set up a path through the particular group of the line link frame 14 in the normal manner. A crosspoint connection is established between the &#34;grounded&#34; vertical line and an available one of the horizontal lines in the line switch stage of the appropriate group of the line link frame. The crosspoint connection results in the VS conductor and HS conductor of the two lines connected at the crosspoint to be tied to ground or other reference potential. 
     As the input section 44 of the scanner in the central office terminal, under control of the microprocessor 30, scans the HS and VS lines in sequence, the microprocessor 30 detects when the two input lines HS and VS from the line link frame have been grounded by the associated line link frame circuit. This status information is relayed to the microprocessor 32 at the remote station over the data link. At the same time the microprocessor 30 controls the interface circuit 38 of the same group to complete a direct connection between one of the line pairs 34 from the remote terminal and the HT and HR conductors of the selected horizontal line of the crossbar switch in the line link frame. Each line pair is connected by its own relay in the interface circuit 38 to an associated one of the horizontal lines of the crossbar switch in the line switch stage of the line link frame. 
     The microprocessor 32 at the remote terminal, in response to information received from the microprocessor 30 over the data link, first sets the particular crosspoint relay in the matrix switch 52 and then activates the corresponding line relay 54 to complete the connection between the pair of lines from the off-hook subscriber phone 24 and the line pair now connected to the horizontal line of the crossbar switch by the interface circuit 38. The connection to the appropriate trunk line is completed by the crossbar switch in the conventional manner. 
     When the subscriber hangs up, the process is repeated to initiate a disconnect by the crossbar switch at the central office and to disconnect the subscriber phone 24 at the remote station. Thus the present invention provides a remote switching station which ties in directly to the horizontal lines of the crossbar line switches in the crossbar switching system. While providing a remote switching capability, the central office terminal and remote office terminal operate in a manner which is opaque to the crossbar switching system. No mechanical or electrical modification to this central office switching system is required to provide the remote switching function. 
     FIGS. 3-6 show the circuit details of the interface 38, scanner 42, matrix circuit 40, and microprocessor 30 in more detail. 
     Referring to FIG. 6, the microprocessor includes a central processing unit 60 which is described using an Intel 8748 processor chip but not limited to this processor. The processor includes a first port P1 of eight conductors. Lines P1(0) through P1(3) form an output data bus for transferring 4 bits coded as addresses and other information to the scanner circuits 42. Line P1(4) provides a scanner strobe signal to each of the scanner circuits. Output line P1(5) provides an output indicating whether the A scanner or B scanner at the remote terminal is being addressed. The pairs of bits on lines P1(6) and P1(7) are decoded and activate one of three output lines designated SEL (0-2). 
     A second input/output port P2 of the computer chip has four lines 0-3 connected to an input/output circuit 62, preferably a type 8243 integrated circuit chip. In response to a control signal from the processor 60, the four lines are connected either to information bus lines 0-3 or 4-7 over which binary coded information is transmitted between the computer and the scanner circuits. Bus lines 0-3 are also applied to two output decoders for energizing one of ten output lines for selecting, respectively, the input circuit and the output circuit for one of the ten groups via the ten lines designated IN GRP SEL and OUT GRP SEL, respectively. Output lines P 2  (4-5) are also applied to a decoder for energizing one of three output lines for enabling the bus connection to scanner A, to scanner B, or to the interface circuit. The three lines are designated SC BUS EN (A), SC BUS EN (B) and IF BUS EN, respectively. 
     Associated with the central processor unit 60 is a Read Only memory (ROM), such as a type 2716, a random access memory (RAM), such as type 8155, and a serial/parallel converter (UART), such as type 8251A, for providing serial communication between the computer and the associated modem 34. The ROM 64 is used to store the program and is addressed by the processor 60 to provide input data to the computer over the port B1 of the processor. The RAM 66 is used as external storage for information from the scanner circuits and to store information received or to send over the data link with the other processor. The RAM has several tables, including a status table and a link table. A plurality of switches, designated the group number select switches indicated at 70, are manually set to indicate which groups (minimum and maximum group numbers) are being used by the remote switching terminal. A display 72 is also controlled by the RAM 66 to provide a visual display of stored system status information. The RAM 66 communicates with the processor over the B1 input/output port, as does the serial/parallel converter circuit 68. A control signal R/W from the computer signals whether a Read or Write operation is being initiated. The microprocessor circuit configuration of FIG. 6 is a conventional processor design. Control of the microprocessor is by a program hereinafter described in more detail. 
     Referring to FIG. 3 there is shown in more detail the interface circuit 38 by which the microprocessor 30 controls the connection between the ten line pairs of a group and the corresponding ten horizontal lines of the line switch stage of the line link frame 14. As shown in FIG. 3, one line pair from the remote station is connected by a pair of relay-operated switches 74A and 74B, actuated by a relay solenoid 74, directly to the HT and HR conductors of an associated one of the horizontal lines of the crossbar line switch in the line link frame. When the relay 74 is energized, the normally open contacts 74A and 74B provide a direct line path between the line pair and the associated horizontal line in the crossbar line switch. When the relay 74 is de-energized, the crossbar horizontal line is isolated from the line pair going to the remote terminal. In addition, a second relay having a relay coil 78 and normally open relay contacts 78A, when energized, connects a resistance 80 across the HT and HR conductors of the horizontal line. The relay 78 is controlled, as hereinafter described, to provide an impedance across the line while the line pair is being connected to the subscriber phone at the remote station, the relay 78 being timed to remain closed for a predetermined time interval to insure that the markers 18 in the crossbar switching system do not see an open line. The pair of relays 74 and 78 are duplicated in the interface circuit 38 for each of the ten line pairs serviced by the interface for one group of the line link frame. 
     The relays 74 and 78 are controlled from the microprocessor 30 by means of the 8-bit word received over the information bus and the least significant bit on the data bus. The interface circuit associated with a particular group is identified by the four highest order bits on the information bus from the microprocessor 30. These bits are compared with a set of four locally coded bits from and IF group number generating circuit 82 by a compare circuit 84. If the comparison is true, indicating the particular interface circuit is being addressed, a decode and latch circuit 86 is activated. The circuit 86 decodes the lower order bits of the information bus from the microprocessor 30 and is strobed by the IF BUS EN control signal from the microprocessor 30. The selected one of the output lines from the decode and latch circuit 86 activates the relays 74 and 78 to complete the connections described above. A timing circuit 88 causes the relay 78 to drop out after a predetermined interval. The timing circuit 88 also causes the relay 78 to be disabled in response to an OSCD signal from the microprocessor, which is used to signal when the call did not originate with a subscriber phone at the remote switching terminal. 
     Referring to FIG. 4, the input section 44 and output section 46 of the scanner 42 are shown in more detail. The forty input lines, such as the conductors VS from the line link group, the forty off-hook lines from the off-hook sensors 56 in the matrix circuits 40, and the forty monitor lines from the switching matrix in each of the matrix circuits 40, together with the ten input lines, such as the ten lines HS from the line link group, or the ten lines from the current sense relays 50, are arranged in groups of eight input lines going respectively to eight different switching circuits 90, two of the eight being shown in FIG. 4. Each switching circuit connects one of the eight input lines selectively to a common output line. The eight output lines from the eight switching circuits 90 are connected respectively through drivers 94 to the eight bit lines of the information bus of the associated microprocessor. The switching circuits 90 are controlled by the four bits on the data output from the microprocessor. The driver circuits 94 are strobed by the microprocessor in response to the presence of the IN GRP SEL line, which selects the appropriate scanner, and the SC BUS EN line which strobes the transfer of the binary information to the bus with the proper timing. It should be noted that the eight switching circuits 90 can accommodate sixty-four input lines. Since only fifty are needed for the VS and HS inputs, the additional lines are available for test or diagnostic purposes. 
     It will be seen that the scanner input, in response to an address from the microprocessor, provides an eight bit output, each bit being set by the level on one of the input terminals. Eight addresses scan all 64 possible input terminals. The three bit address is called the scan number (SC Address #). The eight bit output is called a status table word. Each bit by its position in the word and in combination with the scan number uniquely identifies one input terminal and its status. 
     The output section 46 of the scanner 42 includes six decoding circuits 96, designated 0 through 5. Each decode circuit has eight output lines which respectively control eight transistor switches 98 for connecting the associated output terminals to a reference potential. Forty-eight transistor switches 98 are provided in all, controlled respectively by the eight outputs of each of the six decode circuits 96. Forty of the transistor switches are connected to the forty output terminals VR of the scanner, the remaining eight switches being used to control other functions if needed. 
     Selective operation of any one of the forty-eight switches is controlled by a combination of the data lines and the SEL (0-2) lines from the processor. The data lines are applied to a latch circuit 100 which is activated by a strobe signal from the processor. The output of the latch applies the binary coded information received from the data lines to each of the decode circuits 96 which, in response to the information of the data lines, select one of the eight outputs. Only one of the six decode circuits is activated at a time by one of six output lines from a decode circuit which decodes the binary information on the SEL lines from the processor. The decode circuit 102 is activated in a selected one of the scanner circuits by the OUT GRP SEL line from the processor, which output selects one of the ten groups, and by the OUT A/B, which determines whether the A scanner or B scanner is being selected. 
     Details of a matrix circuit 40 are shown in the schematic diagram of FIG. 5. The ten line pairs from the central office are connected through a set of ten current sense relay circuits 50, one of which is shown in detail. The relay is balanced by having two coils 104 and 105 connected respectively in series with the R and T conductors of an associated line pair. When the the line pair is busy, current in the line activates the relay to close normally open contacts 106 to provide a reference voltage on one of ten output lines, designated SC (0-9). These lines connect to the ten HS terminals in the input section 44 of the associated scanner (A) of the remote terminal. 
     The ten line pairs then connect respectively to the horizontal line pairs of the 10×10 matrix switch 52. The matrix switch 52 also has ten vertical line pairs, with any one of the ten vertical line pairs being connected to any one of the horizontal line pairs by a relay-operated switch. Thus ten such relays, indicated at 108, are associated with each vertical line pair, there being ten groups of ten relays in all. Each relay operates a pair of normally open contacts 108A and 108B for completing a connection of a line pair through the matrix switch. The coils of each group of ten relays 108 is controlled by an associated decode circuit 110, there being ten such decode circuits. Each decode circuit receives a binary input from an associated latch 112 which is set by bits on the data lines from the output of the microprocessor 32. There are ten sets of latches 112 in the matrix circuit, which are selectively activated by associated ones of ten strobe lines from the output section 46 of the scanner A. 
     The selected vertical pair is in turn connected to the subscriber phone line pair by one of the line relay 54, there being one line relay 54 associated with each of the ten subscriber lines connected to the vertical lines of the matrix switch 52. One of the ten line relays is energized in response to a corresponding one of ten RELY INHIB lines from ten of the VR terminals of the output circuit 46 of scanner B. When the line relays are not energized, the normally closed contacts of the line relay switches 54A and 54B connect the R conductor of the subscriber phone pair to a reference potential through a resistor 114. At the same time, the T conductor is connected by the normally closed contacts to the base of a transistor switch 116. Depending upon whether the subscriber phone is on-hook or off-hook, the transistor switch 116 will be biased off or on to provide an off-hook indication which goes to the input section 44 of scanner A. A line relay is operated before a matrix switch in the same vertical column so as to interrupt any current through the matrix switch during a switching operation. 
     In order to monitor the matrix switch to determine if the switch contacts are making the desired connection, operational amplifiers 118, each having one input connected to one vertical pair of the switch matrix and the other input connected to a voltage reference, are provided. The level at the output of the amplifiers 118 is an indication of the status of the relay switches 108A, 108B. 
     Both the microprocessors 30 and 32 operate in response to a program stored in the respective ROM&#39;s 64. The same program is stored in both microprocessors. A computer printout of the program stored in the ROM 64 for operating the microprocessors at each terminal is attached as Appendix 1. The program is designed to test, during an initializing routine, whether a terminal on the microprocessor is grounded, indicating a central office terminal, or ungrounded indicating a remote terminal. This information is then stored as one bit in a system control word SYSW in the data memory DM of the processor. After the initializing, the program branches to either a central office terminal routing MNCOTP (see SEQ 981 of Appendix I) or a remote terminal routine MNTRP (see SEQ 1305 of Appendix I). 
     Referring to the flow diagram of FIG. 7 for the initializing routine, the microprocessor at each terminal is initialized by first operating a manual reset switch 130 (See FIG. 6). As shown in block 132 of FIG. 7, this causes the processor 60 to set a program counter to 0, a stack pointer to 0, the BUS to a high impedance, select Memory Bank ZERO (MBO), and select Register Bank ZERO (RBO), disable any interrupts, stop the timer, and clear the logic. The serial/parallel converter (or UART) 68 is set to Idle and the external RAM 66 has all ports set to input. As indicated at 134, the program then begins by setting the data memory DM in the processor to 0, setting all eight bits of the port P2 to 0, setting bits 0 through 5 to 0, and bits 5, 6, and 7 to 1 at port P1. 
     It should be noted that throughout the program definition, the eight bits forming a byte are divided into two groups of four bits, called &#34;nibbles&#34;, each coded in hexadecimal. The sixteen coded states of each nibble are designated 0-9 and A-F. In setting the port P1 to E0, as indicated in block 136, the lower order four bits are set to 0 (0000 in binary) and the higher order four bits are set to E (1110 in binary). The timer TO in the processor is enabled, as indicated at 138. The setting of the group number select switches 70 is then read into the processor, as indicated at 140, and the system word SYSW is stored indicating whether the processor is a central office terminal or a remote terminal, as indicated at 142. If the processor is part of a central office terminal, the relays in the interface circuits 38 are all turned off, as indicated at 144, but if at a remote terminal, the processor turns off the crosspoint relays of the matrix switch 52, as indicated at 146. The initializing program then initializes the parallel/serial converter or UART 68, causing it to initiate a search for sync, as indicated at 148. The program updates the status table in memory to store the current status of all input and output terminals scanned by the respective scanner circuits 42, as indicated at 150. The program then branches either to a central office routine or a remote terminal routine. 
     Referring to FIG. 8, the number at the top of a block indicates the starting sequence number SEQ in the program printout in appendix 1 at which the indicated routine is described. As indicated at 152, the program enters a routine CALSW to first compute a status word which identifies the group number of a scanner (4 bits) and the scan address (3 bits). The scan address points to one of the eight inputs to each of the switching circuits 90 in the input section 44 of a scanner 42. Once the status word is computed the program then sets the SC BUS EN (A) line from the microprocessor to each of the scanners to 1, and the program then enters the SCANS routine, as indicated at 156. This routine in the remote terminal looks for changes in the off-hook state as applied to the forty VS input terminals to the scanner from the off-hook sensors 48, and for a change in status of the trunk loop current as applied to the ten HS input terminals of the scanner from the current sense relays 50. This is done by using the group number of scan words to select the appropriate scanner and using the scan address to read out the appropriate eight-bit scan sample from the input section of the scanner. The eight bit sample is stored as a status word in a status table in the RAM. Each bit position in the status word identifies with one of the switching circuits 90 and thus, in combination with the scan address, uniquely identifies the status of one of the input terminals (MS and VS) of the scanner 42. The status is compared with the old sample stored during the previous scan of the same scan address. The new sample and the old sample are compared and used to generate an error word EW. Each bit position in the error word indicates whether or not the status of the corresponding input terminal changed status between scans. 
     The program then tests the error word, as indicated at 158 in FIG. 8. If there has been no change, (all bits are 0), a determination is made to see if the scan address portion of the scan word has been reduced to 0, as indicated at 160. If not, the scan address in the scan word is decremented and the scan routine is repeated until either a change in status is detected or the scan address is reduced to 0 before a change in status is detected. 
     If a change in status of one of the HS or VS input terminals is determined changes in the status of the HS and VS terminals are processed. A change to Busy at a vertical (VS) terminal indicates that an off-hook subscriber is requesting service. A vertical change to Idle indicates a central office relay has closed due to a crosspoint closure in the crossbar switch. A change to Busy at a horizontal (HS) terminal indicates current flow in the line pair to the central office. A horizontal change to Idle means a line current has been interrupted. In processing changes in the vertical or VS terminals and the horizontal or HS terminals of a scanner input, the program utilizes two tables in memory, the status table and a link table. The status table stores eight status words for each group. The status words are addressed by a status table address, four bits of which identify the group number and three bits of which identify the scan address number. 
     The link table contains 100 bytes, ten bytes for each of ten groups of ten line pairs per group extending from a remote terminal to the central office. Each word in the link table is addressed by a group number and a line pair number. Each byte in the link table has a format in which the highest order bit, bit position 7, indicates whether the particular line pair is busy or idle. The three bits in positions 4 through 6 of the byte identify the error bit position number, that is, one of the eight bit positions of an error word or a status word, and therefore one of eight input terminal groups. Bit positions 0, 1 and 2 are the scan address number, thus identifying one of the eight groups of input terminals. In other words, each byte in the link table identifies one of the scanner input terminals. Thus a word in the link table identifies, for example, which of the forty subscriber phones of one group at the remote station is linked with a particular one of the line pairs to the central office. 
     During the DCPRT routine 162, if there has been a change in an HS or horizontal terminal, the status table data is updated with the eight-bit sample from the scanner input data, and the link table data is modified to set the busy/idle bit to indicate the type of change, namely from Idle to Busy, or from Busy to Idle. 
     The DCPRT program routine then determines if a VS or vertical input terminal of a scanner has changed status after testing for a transient condition. If the change has been from Busy to Idle, the status table is updated and the main program is rejoined. If the status change has been from Idle to Busy, indicating a pending off-hook subscriber, the status table is updated and the program enters the XDCOM routine, indicated at 164. This routine causes the transmission of a vertical address word VAW, to the central office terminal. The VAW identifies one of the forty vertical terminals associated with the subscriber phone that went off-hook. The group number is also transmitted to the central office. A transmit stack and a receive stack are provided in memory to buffer the data link, which operates at a much lower rate than the processor. 
     The program then enters the RXDCOM routine, as indicated at 166. This routine decodes the data received from the central office terminal and either calls an MXDIS routine or an MXCON routine, as indicated at 168. These routines operate either to connect (MXCON) or disconnect (MXDIS) the appropriate crosspoint in the matrix switch 52. These routines use the vertical address word (VAW), which identifies a particular vertical line going to a subscriber, and a group horizontal link word (GHLW) which identifies the group number and horizontal link number, to control the scanner and provide the appropriate signals on the STRB lines and the RLY INHIB lines from the output sections of the A and B scanners to complete a connect or disconnect at the appropriate crosspoint in the matrix switch 52. 
     If there is no communication received but there has been a change in status of a horizontal, a debounce routine, as indicated at 170, after a delay interval, determines that there is no line current even though a connection has been completed through the matrix switch 52. If so, the MXDIS routine 172 is initiated to disconnect the crosspoint. Such a locally initiated disconnect may result from some erroneous operation at the central station resulting in a line not being connected through to a trunk, for example. The RTSRT program is then repeated to continue the scanning operation and to service changes in status originating at the remote terminal or at the central office terminal. 
     Referring to FIG. 9, there is shown the main flow diagram for the operation of the microprocessor at the central office terminal. Many routines of the central office terminal program are essentially identical to routines in the remote terminal program, as described in connection with FIG. 8. Thus on entry of the program, a scan word is calculated by CALSW, as indicated at 152. The input terminals VS and HS are scanned by groups, during SCANS 156, to determine if there is any status change. If a status change is detected, as indicated at 158, the program enters the routine designated DCPCOT, indicated at 174. This routine processes VS or HS transitions detected by the scanning operation. Change in status of an HS or VS terminal indicates either a disconnected or connected call. Changes in HS terminals are processed first. A group horizontal link word containing the group number and the horizontal line number is called from the data memory. If a horizontal sleeve conductor HS has been connected by the crossbar switch, the group horizontal link word GHLW is stored, the status table is updated, a timer is started, and there is a call to rescan the same group to find the corresponding vertical sleeve VS transition, which must occur within a predetermined period of time after the HS has been detected in order to be accepted. 
     If the change in status is an HS disconnect, the link table is set to 0 and the associated relay in the interface circuit 38 is released. The disconnect status is then transmitted to the remote terminal by calling the XDCOM routine 164. 
     If a vertical sleeve (VS) connect or disconnect change is determined within a predetermined time of an HS connect, the number of the VS terminal is determined and the status and link tables are updated accordingly. A determination is made of whether the call is a terminating or originating call and the OSCD signal to the interface 38 is generated so that operation of the relay 78 by timer circuits 88 is either enabled or disabled, as described above in connection with FIG. 3. Change in status of the particular VS terminal is transmitted to the remote terminal. The relays 74 in the interface circuit 38 are also controlled during this routine to either connect or disconnect the selected line pair with the HT and HR horizontal conductors of the crossbar switch. The program then enters the RXDCOM routine 166 during which it receives information from the remote terminal. 
     During the RXDCOM routine, the microprocessor controls the output terminals of the scanner to apply or remove a reference potential of the selected VR line in the crossbar switch, as described above in connection with FIG. 2. This is accomplished by an SOCPGM routine during which, based on the group number and vertical address word, the appropriate VR output terminal is selected and controlled, using the information received from the remote terminal. The main routine of the central office terminal then returns to the Start. 
     As noted, the attached printout of the program in Appendix I defines fully the program for operating the microprocessors at both the central office terminal and the remote terminal. It is believed evident from this detailed program description how the program functions to operate the processors and to control the respective terminals. 
     To summarize the operation of the switching system as controlled by the programmed processors, when a subscriber phone at the remote terminal goes off-hook, the change in status of the corresponding VS terminal of scanner A is detected. This change in status is processed during the DCPRT routine. The group number and vertical address word VAW are then transmitted to the central office terminal during the XDCOM routine. At the central office terminal, the received data is processed during the RXDCOM routine and a &#34;ground&#34; is applied to a corresponding one of the VR output terminals of the scanner at the central office terminal. This activates the crossbar switching system in the same manner as if a phone had locally gone off-hook at the line link frame. The crossbar switch completes a path through the line link frame in the process grounding the HS and VS conductors of the horizontal line in the crossbar switch. The central office terminal during the DCPCOT routine detects this change in status in the VS and HS input terminals of the scanner and activates an appropriate relay in the IF circuit 38 to connect the activated horizontal of the crossbar switch to the associate line pair back to the remote terminal station. During the RXDCOM routine the central station transmits a group horizontal link word GHLW and a vertical address word VAW back to the remote terminal. 
     At the remote terminal, during the RXDCOM routine the received data is used to initiate an MXCON routine to select and operate one of the relays 108 to complete a connection at the appropriate crosspoint of the matrix switch 52, thus completing the connection between the remote subscriber to a trunk line through the crossbar switch. 
     When the remote subscriber phone goes back on-hook, this change in status is detected during SCANS, resulting in a local disconnect of the matrix. The crossbar switch also detects this on-hook condition and removes ground from the HS and VS terminals, terminating the call connection through the central office. The central office terminal then communicates the change in status back to the remote terminal. If the call originated at the remote terminal, this information is redundant. However, if the call originated at the central office and the remote subscriber phone never went off-hook, this information from the central office terminal is used to disconnect the crosspoint in the matrix switch. Otherwise a disconnect of the subscriber phone at the matrix switch results during SCANS from the change in status of line current sensor by the interruption of line current by the disconnect at the central office terminal. 
     From the above description it will be seen that a remote terminal switching system is provided which is compatible with the existing electromechanical type crossbar switching systems, and which allows a number of remote subscriber phones to be serviced by such a central office switching system over a relatively few number of line pairs. The equipment is completely opaque to the crossbar switching system, and in fact the same line link frame can service both local subscriber phones and remote subscriber phones using the same crossbar switch. In other words, some of the vertical lines of a crossbar switch can be connected directly to local subscriber phones while other verticals can be, in effect, dedicated to subscriber phones serviced by the remote terminal switching system of the present invention. The central office crossbar switching system does not distinguish between a call initiated by the grounding of a VR conductor in a vertical line of the crossbar switch by an off-hook condition of a local subscriber phone, or by a ground applied by the output of a scanner circuit at the central office terminal of the remote terminal switching system of the present invention. The present invention allows solid state circuits to be used in combination with the electromechanical circuit of the existing crossbar switching systems. ##SPC1## ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6##