Linear time division multiplexed conferencer
TECHNICAL FIELD 
     This invention relates to digital conferencing of time division multiplexed (TDM) linear voice sample and, more particularly, to a digital network for establishing a conference connection of n telephone lines (n≧2). 
     BACKGROUND OF THE INVENTION 
     It has become common practice to establish a conference connection of n telephone lines using space division technology. This technology uses metallic contacts, contained in an array of relays or cross-bar type switches, to establish a telephone connection between a first subscriber and a second subscriber. Often, as additional subscribers were added to the space division connection, the voice quality of the connection was degraded due to the loading of the individual lines on the connection. 
     This problem was alleviated somewhat by limiting the number of connected conferees and by using operational amplifiers with resistor networks to build low impedance summing bridges. 
     Currently, it is common for telephone connections to pass through some form of digital time shared circuitry or channel, such as the Bell System&#39;s well known &#34;T1&#34; carrier system, a time division multiplexing switching system. 
     Time division multiplexing is a type of digital switching in which a line is time shared among multiple users, and in which multiplexed time slots are allocated for establishing communication sessions. 
     The art of digital time division switching also includes systems known as Time Slot Interchangers (TSI), in which digital message samples from a given input time slot are switched to a different output time slot. TSI is now used to establish two party connections. 
     U.S. Pat. No. 4,119,807; entitled &#34;Digital Time Division Multiplex Switching System&#34; and issued Oct. 10, 1978; is an example of a time shared digital switch arranged to establish conference connections without restriction as to the number of conferees that may participate in a conference session. This patent discloses an arrangement for sequentially summing digital message samples from 128 lines connected respective to 128 time slots during a first time frame and outputting the summed samples to each line of the connection, respective to its assigned time slot, during a second time frame. 
     A digital switching arrangement of this type uses two summation memories, which are alternately loaded and unloaded respective to a time frame. In addition, third and fourth memories are alternately loaded in step with the summation memories, and are used for the storage of digital message samples respective to assigned time slots. Each summation memory is cleared at the beginning of its respective storage cycle or time frame in preparation for the storage of a new series of summed message samples. 
     Digital switching systems disclosed by the prior art, including U.S. Pat. No. 4,119,807, are arranged more or less to process 128 time slots at the input and 128 time slots at the output. These arrangements are not readily integratable onto a single integrated circuit chip, especially if the number of time slots are increased to 256. The problems of integrating a circuit of this size onto a single chip occur as a result of increased memory requirements, and corresponding increase in the number of circuit elements. The problem is particularly highlighted if additional functions, such as conference broadcast and monitor arrangements, are added to the system. 
     Further, a digital switching system must account for signal propagation delay due to cascaded circuit elements before the size of a digital switch system is arbitrarily doubled. Such signal propagation delay could result in a loss of incoming digital signals if not properly addressed. The memory requirements for a digital conferencer could be proportionately reduced by reducing the number of bits used to digitally encode voice signals. However, such a reduction in the number of bits used to encode audio signals will proportionately degrade the final resolution of digitally encoded voice signals back into analog voice signals. In fact, a well designed digital conferencing circuit uses more than eight bits to digitally encode audio signals for improved resolution of decoding digitally encoded signals back to analog signals. 
     SUMMARY OF THE INVENTION 
     The problems and inadequacies exhibited by prior systems have been obviated by sharing a sample memory between two accumulator memories in order to reduce memory requirements. This memory sharing technique effectively reduces memory requirements by approximately thirty percent without a reduction in signal processing capability. Also, the affect of propagation delay on a 256 time slot conferencer is offset by using synchronized memory prefetching techniques at both the input and output. 
     Prefetching from memory is accomplished by generating two sets of sequential time slot addresses. The first set leads the actual incoming time slot addresses by one-half time slot and the second set leads the actual incoming time slot addresses by two time slots. Using this approach for a given conference or group of time slots, each time slot of the conference is prefetched one time slot prior to when it will actually be updated by a &#34;new&#34; message. 
     For example, assume time slots 3, 6 and 9 are conferenced together for a particular call. Then during time slot 8 the summation of message samples contributed by time slots 3 and 6 are prefetched from memory for time slot 9 prior to storing the conference summation for time slot 8. 
     Prefetching in this manner poses a potential problem when two lines of the same group or connection are assigned to adjacent time slots within a frame. Under these conditions the prefetched summation respective to the second adjacent time slot would not include the message sample contribution of the first adjacent time slot. This anomaly is overcome by recognizing adjacency within a time frame and substituting the summation of message samples respective to the first adjacent time slot directly for the prefetched summation. 
     Also, the production of leading time slot addresses allows a single sample memory to be shared between two accumulator memories, since a priorly stored message sample can be prefetched prior to storing a new message sample. Further, our invention provides capability for establishing broadcast and monitor connections in addition to providing capability for establishing conference connections. Still further, unlike prior art systems, our invention provides capability for selectively inserting an alternate digital message at both the input and output circuits. 
     The concept of refreshing a summation memory each frame by recognizing the first access to the summation memory with respect to a group of time slots is described in concurrent copending application Ser. No. 391,697, filed 6/24/82. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The operation and implementation of the present invention will be more fully apparent from the following description of the drawing, in which: 
     FIG. 1 illustrates one embodiment of the invention in block diagram form; 
     FIGS. 2 and 3 show a detailed block diagram of a preferred embodiment of the invention; 
     FIG. 4 is a time representative sequence diagram of various clock signals, control signals and time slot addresses with respect to a master clock signal; 
     FIG. 5 is a time representative sequence diagram depicting the manner in which a reset signal is generated; 
     FIG. 6 shows the manner in which FIGS. 2 and 3 should be arranged; and 
     FIG. 7 is an illustration, respective to a cited example, of the manner in which a group or connection of time slots are assigned to an accumulator RAM memory location. 
    
    
     GENERAL DESCRIPTION 
     A brief overview of the conferencing system will now be given with respect to FIG. 1 which shows an example of a linear time division multiplexed conferencer 100 in which the time slot input first passes through buffer 205 to twos complement adder 310 which performs the addition of linearly encoded incoming voice or data samples and passes these summed samples, as will be discussed, for storage in accumulator RAM 510 or 520 respective to time slots of the same conference connection. The conferencer includes circuitry (1600) for generating and holding first and second accumulator RAM 510 (520) memory addresses respective to a first and second time slot in which the second address is used to prefetch and hold in latch 620 a summed message sample respective to a second conference connection and in which the first address is used to store a summed message sample in accumulator RAM 510 (520) respective to a first conference connection. In the event the first and second time slots are adjacent time slots assigned to the same connection, the summed message sample respective to the first time slot is alternatively loaded into latch 620. In this manner the summed message sample prefetched from accumulator RAM 510 (520) includes the message contribution of the first time slot and is properly summed with the incoming message sample respective to the second adjacent time slot. 
     The conferencer 100 may also select one of a multiple of alternate digital messages for storage in accumulator RAM 510 (520) in place of the summed message sample outputted from in-adder 310. An alternate message may be selected in response to the logic state of connection mode bits CMBT and CMBR outputted from connection data circuit 1400, in response to a positive or negative overflow condition resulting from the summation process, in response to a first access to accumulator RAM 510 (520) respective to a connection during a respective time slot frame or in response to a parity error (not shown) detected during the summation process. Twos complement out-adder 640 effectively subtracts the contribution of a time slot from the summed message prior to outputting via data bus 750. The conferencer 100 also may select an appropriate alternate digital message sample for outputting respective to a conferee (time slot). This alternate digital message sample is determined by the mode (mode control bits CMBT and CMBR) externally assigned to each respective time slot of a conference connection. 
     A time slot can be externally assigned to a conference mode, which permits a time slot to send and receive message samples; to a broadcast mode, which permits a time slot only to transmit message samples to the conference connection; or to a monitor mode, which permits a time slot only to receive message samples from the conference connection. The alternate message, in place of the summed message sample, is provided under the control of sample select circuit 1700 in response to the assigned mode, or under control of output control circuit 1800 in response to the time slot mode or in response to an error detected by the subtraction process performed by out-adder 640. 
     Each conference connection is assigned a unique accumulator RAM 510 (520) memory location (address) which is used for accumulating summed message samples contributed by each time slot of the connection during a respective time slot frame. The assigned accumulator RAM 510 (520) memory location and mode information is stored on a per time slot basis in connection data circuit 1400. Connection data circuit 1400 is sequentially addressed by address generator circuit 320 respective to a time slot. 
     The memory locations associated with connection data circuit 1400 for the storage of connection data are loaded and changed via service interface circuit 241 responsive to serial input BUSDAT. 
     The connection data, which comprises mode bits CMBT and CMBR, and accumulator RAM 510 (520) memory location assigned to a conference connection, is the means by which respective incoming time slots are identified for connection to output bus 750. 
     Encoded voice samples respective to a time slot are stored in a sample RAM 610 memory location respective to a time slot address. Sequential memory addresses are provided by address generator circuit 320 respective to each incoming time slot and are applied to sample RAM 610 via address bus DRAD. 
     Accumulator RAMs 510 and 520 are alternately loaded with summed message samples from input control circuit 1500 and alternately unloaded for supplying summed message samples to the output. RAM control circuit 1600 in response to time frame pulse TSYNC and clock signals from clock generator circuit 220 is the means by which accumulator RAM 510 and accumulator RAM 520 are alternated between a storage cycle and a supply cycle respective to a first and a second time frame. For example, during a first time frame each time slot of a connection is sequentially addressed and its message sample (via IBDAT) is summed by in-adder 310 with summed message samples received from previously addressed lines of the conference connection during a first time frame. The new summation from input control circuit 1500 is then stored in accumulator RAM 510. Simultaneously therewith, summed message samples stored in accumulator RAM 520 during a preceding time frame are unloaded and supplied to the output via RAM latch 630 and out-adder 640. 
     During a next time frame, summed message samples are stored in accumulator RAM 520 and simultaneously therewith, priorly summed message samples stored in accumulator RAM 510 during the preceding time frame are unloaded and supplied to the output. 
     In order to offset propagation delays and facilitate the process of summing an incoming message sample respective to a first time slot assigned to a connection, a summed message sample respective to a second time slot is prefetched from accumulators 510, 520 and stored in RAM select latch 620 prior to loading the summed message sample respective to a first time slot. RAM select circuits 540 and 550, under control of RAM control circuit 1600, are used by accumulators 510, 520 to prefetch a second summed message sample prior to loading a first summed message sample. 
     The read/write addresses respective to second and first time slots from accessing RAM 510 (520) are supplied to RAMSEL 540 (RAMSEL 550) from connection data circuit 1400 via address has RARAD and address bus WARAD respectively. In turn, RAM control circuit 1600 causes the read address respective to a second time slot (RARAD) to be gated through RAM select circuit 540 (550) to accumulator RAM 510 (520) during the first half of a first time slot in order to prefetch a summed message sample respective to a second time slot. During the second half of a first time slot, RAM control circuit 1600 causes accumulator RAM 510 (520) write address (WARAD) respective to a first time slot to be gated through RAM select circuit 540 (550) in order to store a summed message sample respective to the first time slot into accumulator RAM 510 (520). In this manner, accumulator RAM 510 (520), during a storage cycle, is alternately addressed each time slot for prefetching a summed message sample respective to a second time slot and for storing a summed message sample respective to a first time slot. Simultaneously therewith, accumulator RAM 520 (510) is addressed each time slot for supplying summed message samples to output bus 750. 
     The prefetched summed message sample(s) held in RAM select latch 620 is inputted to in-adder 310 via the PSDAT bus for summation with a respective incoming message sample. A reset bit (bit 16 of PSDAT) is processed by RSB processor circuit 260 for determining whether an incoming time slot of a conference connection is the first access to accumulator RAM 510 (520) for the connection respective to a time frame. If the incoming time slot is the first access to accumulator RAM 510 (520) then RSB processor circuit 260 directs input control circuit 1500 to load the message sample respective to the first accessing time slot into accumulator RAM 510 (520) in place of the summed message sample outputted from in-adder 310. In this manner accumulator RAM 510, 520 is refreshed each respective storage cycle by starting a new conference sum thereby eliminating the problem of pausing at the beginning of each time frame. 
     Input control circuit 1500, in response to mode control bits CMBT and CMBR or in response to an error in the summation process, selects an appropriate alternate digital message sample for storage in accumulator RAM 510 (520). For example, an alternate digital message sample is selected and stored in accumulator RAM 510 (520) for an incoming time slot assigned to the monitor mode in place of the incoming digital message sample respective to the time slot. In this manner, a time slot assigned to the monitor mode is prevented from transmitting to the conference connection. 
     Input control circuit 1500 also selects an appropriate alternate digital message sample for storage in accumulator RAM 510 (520) in the event a positive or negative overflow results from the summation of message samples by in-adder 310. 
     DETAILED DESCRIPTION 
     Drawing FIGS. 2 and 3, when arranged with respect to each as shown in FIG. 6, illustrate how the various elements of the invention cooperate to provide a linear time division multiplexed conferencing network. The various circuit elements are oriented with respect with each other in FIGS. 2 and 3 in a manner analogous to that of FIG. 1. 
     Referring to FIGS. 2 and 3, a digital input time frame having n time slots supplies at input 210 (FIG. 2) a parallel input of linearly encoded data or voice samples in two&#39;s complement format for processing by the conferencer. Thereafter, in accordance with an assigned connection mode, a time slot assigned to a connection is extended to output data bus 750 (FIG. 3). 
     A time frame is typically the 125 microsecond Bell System T1 carrier frame, and herein a time frame will be assumed to be 125 microseconds represented by a frame sync signal (TSYNC of FIG. 4) externally supplied via lead 230 (FIG. 2). It is also assumed that an approximately 4.096 megahertz clock signal (SCK4T) is externally supplied to clock generator 220 via lead 200 and outputted via lead 4T for operating the input and output paths synchronously with one clock signal and for division into a 2 MHz clock signal 2T (FIG. 4) outputted via lead 2T. Clock generator 220 also supplies a second 2 MHz clock signal 2F (FIG. 4) outputted via lead 2F which is an inverted non-overlapping replica of signal 2T, and a second 4 MHz clock signal 4F outputted via lead 4F which is an inverted non-overlapping replica of clock signal 4T. Clock generator 220 further supplies a third 2 MHz clock signal LTC outputted via lead LTC having a logic one state which overlaps the last quarter and first quarter of adjacent time slots to form a clock window. Clock signal LTC enables, during the window, new data to be set up on the &#34;D&#34; inputs to latch circuits 620, 630 and 735 (FIG. 3) in order to update the respective latch on the next rising edge of clock signal 4T occurring within the LTC window, as will be discussed. 
     ADDRESS COUNTER 
     Sequential address generator 320 (FIG. 2) in response to the falling edge of clock signal 2T generates an eight bit some slot addresses (CRAD), representing time slots 0 through 255, for addressing control RAM 430 over address bus 317. In response to the rising edge of clock signal 2T, address generator 320 generates time slot addresses (DRAD) 0 through 255 for addressing sample memory 610 (FIG. 3). TSYNC is the means in which the counters are synchronized with respect to incoming time slots (IBDAT). 
     Briefly referring to FIG. 4, there is shown a timing representation of time slot addresses CRAD and DRAD in which time slot address DRAD leads (starts before) incoming time slot associated with input data bus IBDAT by one-half time slot and time slot address CRAD leads incoming time slot IBDAT by two time slots. This sequence permits prefetching message samples from sample RAM 610 and accumulator RAM 510 (520) as will become more apparent further on in the detailed description. 
     CONTROL RAM 
     Returning to FIG. 2, control RAM 430 has 256 memory locations respective to incoming time slots. Time slot addresses (CRAD) sequentially generated by address generator 320 are extended to the address input of control RAM 430 over address bus 317. Control RAM 430 is the means by which selected incoming time slots from input buffer 205 are connected to output time slots at output 750. In this respect, each time slot participating in a conference connection is assigned (externally) the same memory location in accumulator RAM 510, or in accumulator RAM 520 (FIG. 3). The accumulator RAM 510 (520) memory location address assigned to a connection, mode control bits CMBT and CMBR, plus a parity bit are stored in control RAM 430 (FIG. 2) for each respective time slot of a conference connection. 
     The corresponding between time slot addresses and accumulator RAM 510, 520 memory locations is made external to the conferencer by a central processing unit (not shown). For example, assume that a two party call is comprised of time slots 8 and 15 respectively, and further assume that accumulator RAM 510 (520) memory location address 20 is assigned to the connection. To control the processing of the connection by the conferencer, a seven bit binary representation of address 20, plus two mode control bits, plus a parity bit is stored in memory location 8 and memory location 15 of control RAM 430. 
     The external central processing unit would serially transmit the connection control data, plus time slot address to service interface circuit 241 via BUSDAT lead 240 under the control of a bus synchronization pulse (not shown). Interface circuit 241 is a latched serial-to-parallel converter circuit, which extends the time slot address to address comparator circuit 330 via bus 242 and extends connection control data to control RAM 430 via bus 243. 
     Interface circuit 241 also includes a decoder circuit for decoding a multibit operation code appended to connection data received via lead 240. The operation code specifies whether the data send via lead 240 is to be stored in control RAM 430 (new connection data) or is to be used for auditing or maintenance purposes (maintenance data, not shown). Service interface circuit 241 upon decoding an operation code that specifies a write to control RAM 430 changes the logic state of the wc lead to select latch 440 from a logic zero to a logic one. The logic one state extends new conference information to processing circuitry while the respective control RAM 430 memory location is being updated, as will be discussed shortly. 
     Address comparator circuit 330 compares the time slot address on bus 242 against time slot address CRAD (Control RAM ADdress) contained on bus 317. When the time slot addresses on bus 242 and bus 317 compare, address comparator 330 causes R/W control circuit 340 via lead 331 to switch control RAM 430 from the read state to the write state via lead 343. On the falling edge of clock pulse 2F the connection control data on bus 243 is caused to be written into control RAM 430 respective to the time slot address on address bus 317. 
     With respect to the above example, in which time slots 8 and 15 were assigned accumulator RAM 510 (520) memory location 20, the external central processing unit establishes connection control as follows: first, the slot address 8 and accumulator RAM 510 (520) address 20, together with the mode control bits and a parity bit are serially transmitted by an external processor over lead 240 for acceptance by interface circuit 241. Interface circuit 241 converts the data to a first parallel output on bus 242 (time slot address) and a second parallel output on bus 243 (accumulator RAM address 20, mode bits plus parity); second, when address counter 320 generates time slot address 8, the R/W control circuit 340 in response to comparing circuit 330 causes the connection control data on bus 243 to be written into address 8 of control RAM 430 on the falling edge of the 2F pulse; third, the same procedure is then executed for time slot 15 completing the sequence for establishing conference connection data for the connection. 
     As discussed above, the connection control data stored in control RAM 430 for each time slot of a connection includes: the address of a location in accumulator RAM 510 (520) assigned to the connection; two mode control bits and a parity bit. In the illustrative embodiment disclosed herein, it is assumed that the two mode control bits, CMBT and CMBR, are in accordance with the following table definitions: 
     
                       TABLE 1                                                     
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CMBR     CMBT          FUNCTION                                           
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0        0             Data Connection                                    
0        1             Broadcast Connection                               
1        0             Monitor Connection                                 
1        1             Conference Connection                              
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     In accordance with the definitions described by Table 1, mode control bit CMBT set to the logic one state declares a broadcast or transmit connection in which a time slot has permission to transmit messages to a conference connection. Mode control bit CMBR set to the logic one state declares a monitor or receive connection in which a time slot has permission to receive messages from a conference connection. 
     A single mode control bit set to logic zero prevents a time slot from having access to the respective function. For example, if the state of the mode control bits is 01 for a respective time slot, then that time slot can only broadcast to the remainder of the connection. When both mode control bits are set to logic one (11) a respective time slot has permission to both broadcast and receive messages from the connections. The default state of mode control (00) can be reserved for a special type of connection. 
     Returning to FIGS. 2 and 3, control memory RAM 430 is normally held in the read state by the R/W control circuit 340 via lead 343. Sequential time slot addresses (CRAD) for accessing respective locations of control RAM 430 are generated by address generator 320 in response to each falling edge of the 2T clock signal and are outputted via bus 317. 
     On the falling edge of the 2F clock signal, the eight bit time slot address (CRAD) applied to the address input of control RAM 430 via bus 317 is internally latched by control RAM 430 and a memory location represented by time slot address CRAD is read. The contents are latched into control RAM 430 output latch (internal) on the following rising edge of 2F (not shown). The connection control data respective to a time slot is passed from control RAM 430 to select latch circuit 440 via data bus 441. 
     SELECT LATCH 
     Select latch 440 is a selector followed by a two stage dynamic register of D-type flip-flops and includes a ten bit selector for selecting data from either data bus 343 or data bus 441. During normal processing select latch 440 accepts data from bus 441. Otherwise, data is accepted from bus 243 upon initialization of a time slot via service interface circuit 241. 
     As discussed above, during initialization of a time slot the decoder circuit of service interface circuit 241 causes the wc lead connected to select latch 440 to go to a logic one state indicating a write to control RAM 430. Also, a comparison of address CRAD and the address of the initialized time slot address on bus 242 forces lead 343 from r/w control 340 to a logic zero state. The coincidence of a logic one state on the wc lead and a logic zero state on lead 343, both of which connect select latch 440, is reclocked by select latch 440 on the next rising edge of the 2F clock forming a window to allow select latch 440 to select data from bus 243. The new connection data on bus 243 comprises a seven bit accumulator RAM address and two mode bits (and parity). Select latch 440 extends the accumulator RAM address to RARAD address bus 442 and extends both the accumulator RAM address and mode bits (and parity) to the first stage of the two stage dynamic register. The data is clocked into the first stage of the two stage dynamic register 440 on the rising edge of the 2T clock, which occurs during the middle of the respective time slot. Thereafter the ten bits of connection data is clocked into the second stage of the two-stage dynamic register on the rising edge of the 2F clock. 
     In this manner, new connection data respective to an initialized time slot assigned to a connection is stored in control RAM 430, and simultaneously therewith, is accepted by select latch 440 for immediate use by the conferencer. Otherwise, the conferencer&#39;s access to the new connection data would be delayed one time frame. 
     For processing message samples, select latch 440 holds in its second or output register the accumulator RAM 510 (520) connection write address (WARAD) on bus 443 and mode control bits on leads 444 and 445 respective to a particular time slot and, simultaneously therewith, holds accumulator RAM 510 (520) connection read address (RARAD), outputted from control RAM 430 via bus 441, on bus 442 respective to a next time slot. Using this approach, a summed message sample respective to a second time slot is prefetched prior to storing a summed message sample respective to a first time slot in accumulator RAM 510 (520). 
     The accumulator RAM 510 (520) address portion of the connection control data presented to select latch 440 via bus 441 is first outputted by select latch 440 as accumulator RAM 510 (520) read address (RARAD) on bus 442. Thereafter, on the rising edge of clock signal 2F the connection control data outputted from control RAM 430 via bus 441 is clocked into select latch 440 and held in the second register stage of select latch 440. The connection control data is then outputted by the second register stage of select latch 440 as an accumulator RAM 510 (520) write address (WARAD) on bus 443. Mode control bits, CMBT and CMBR, are outputted on leads 444 and 445 respectively. 
     INPUT SELECT BUFFER 
     Input select buffer 420 (FIG. 2) is a combinational gate circuit for checking parity (not shown) and for selecting, under control of input control circuit 410, one of a multiple of alternate digital words for storage in accumulator RAM 510 (520). Inputs PSDAT, IBDAT and SMDAT are defined, respectively as, previously summed message samples removed from accumulator RAM 510 (520); inputted message sample at 206 respective to an incoming time slot; and the sum of PSDAT and IBDAT generated by in-adder 310. Alternate digital messages PFS, NFS and IDLECODE (IC) are fixed codes that are hard wired internally at input select buffer 420 and are selected at the direction of input selector control 410 for storage in accumulator RAM 510 (520). PFS is a digital two&#39;s complement representation of the maximum positive value including sign of a summed message sample, NFS is a digital two&#39;s complement representation of the maximum negative value including sign of a summed message sample, and IDLECODE (IC) represents a two&#39;s complement digital message sample of zero value. 
     Input select buffer 420 also includes combinatorial logic circuitry for appending (upon initialization) or changing the state of bit 16 (reset bit) of outputted data to SELDAT bus 450 for storage in accumulator RAM 510 (520). The function of the reset bit RSB&#39; will be detailed further on in this description. 
     IN-ADDER 
     In-adder 310 is a twos-complement combinatorial circuit adder for summing incoming message samples (IBDAT) inputted via bus 206 with accumulator RAM 510 (520) previously summed message samples (PSDAT) outputted from select latch 620 via PSDAT bus 311 to in-adder 310. In-adder 310 includes combinational saturation logic circuitry for checking a summation for positive or negative overflow in the well-known manner, and includes combinational circuitry for performing circuit maintenance functions (not shown) at the direction of an external central processing unit (not shown). 
     INPUT CONTROL 
     Input select control 410 is alerted by in-adder 310 via either lead POFLO or lead NOFLO in the event the saturation logic circuitry contained in in-adder 310 detects a positive overflow or negative overflow resulting from a summation of message samples. Input select control circuit 410 in response to an active POFLO or NOFLO lead from in-adder 310 directs input select buffer 420, via leads POF or NOF, to select either hard wired digital message PFS or digital message NFS respective to the overflow condition for outputting to bus 450. In this manner, an excessive summed message sample is clamped to an established maximum positive (PFS) or maximum negative (NFS) value prior to storage in accumulator RAM 510 (520). 
     Mode control bits, CMBT and CMBR, extend from select latch 440 to input select control 410 over leads 444 and 445 respectively for controlling input functions at input select buffer 420 in accordance with the permissions detailed in Table 1. 
     A weighted binary value of zero-one (01) (broadcast) or one-one (11) (conference) for mode control bits CMBR and CMBT cause the input control circuit 410 to permit, via lead SMD, the summed message sample (SMDAT) from in-adder 310 to be gated through input buffer select circuit 420 to SELDAT data bus 450. This sequence is in accordance with the function of the mode bits since a time slot which is permitted to broadcast has its message sample summed with the message sample contributed by the remaining time slots assigned to the connection. 
     A time slot which has permission only to monitor a conference connection (CMBR and CMBT=01 respectively) causes input select control 410 to permit via lead PSD previously summed message samples (PSDAT) to be gated through the input select buffer 420 to SELDAT data bus 450. This sequence is in accordance with Table 1 since a time slot having permission only to monitor a conference session is denied permission to transmit to the session. 
     In the event a time slot has permission only to monitor a conference session, and that time slot is the first time slot of a connection to access accumulator RAM 510 (520) during a time frame, then select control circuit 410 in response to the INIT lead extended from RSB circuit 260 permits, via lead IDLEC, symbolic input IDLECODE (IC), which has a digital value of zero, to be gated through input buffer select circuit 420 to SELDAT data bus 450, rather than PSDAT. This sequence is in accordance with the monitor function since the accumulator RAM 510 (520) memory location assigned to the connection is initialized by loading the message sample from the first accessing time slot, which, for a time slot in the monitor mode is a digital message sample of zero value, into the assigned accumulator RAM 510 (520) memory location. In the event a first accessing time slot has permission to broadcast then IBDAT is selected for outputting to bus 450 via lead IBD from select circuit 410 to buffer circuit 420. 
     
                       TABLE 2                                                     
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                         Output to                                        
Select Control 410 Input Bus 450                                          
CMBR  CMBT    INIT     POFLO  NOFLO  OUTPUT                               
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0     0       1        --     --     IBDAT                                
0     1       1        --     --     IBDAT                                
1     0       1        --     --     IDLECODE                             
1     1       1        --     --     IBDAT                                
0     0       0        0      0      SMDAT                                
0     1       0        0      0      SMDAT                                
1     0       0        0      0      PSDAT                                
1     1       0        0      0      SMDAT                                
0     0       0        1      0      PFS                                  
0     1       0        1      0      PFS                                  
1     0       0        1      0      PSDAT                                
1     1       0        1      0      PFS                                  
0     0       0        0      1      NFS                                  
1     0       0        0      1      PSDAT                                
1     1       0        0      1      NFS                                  
0     1       0        0      1      NFS                                  
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     Table 2 illustrates symbolically, in a format similar to the format of the well-known truth table, the states of inputs to select control 410 that control the selection of corresponding ones of digital inputs to input select buffer 420 for gating to SELDAT bus 450. 
     RESET BIT PROCESSING 
     Input select buffer 420 appends or changes the reset bit (bit 16) of the seventeen bits of data outputted to bus 450 prior to storage in accumulator RAM 510 (520). The reset bit (bit 16) of a summed message sample is the means by which the first storage access to accumulator RAM 510 (520) is recognized respective to a conference session. 
     RSB processor circuit 260 is a sequential logic circuit of edge triggered flip-flops for generating reference signal RSB&#39; on lead 261. FIG. 5 shows a symbolic representation of incoming time slots 255 and 0 occurring within three consecutive time frames N, N+1, N+2. FIG. 5 also shows signal RAMSELO which is clocked by the second rising edge of clock signal 4F during the fourth quarter of time slot 255. The generation of RAMSELO will be detailed further on in this description. FIG. 5 further shows signal RSBI which is internal to RSB processor 260 and which is clocked on the rising edge of clock signal RAMSELO. Signal RSBI is effectively one-half the frequency of signal RAMSELO. Signal RSB&#39; is a delayed replica of internal reference signal RSBI and is reclocked in the middle of time slot 0. Signal RSB&#39; is delayed in this manner to ensure that its application by input select buffer 420 occurs sufficiently after the occurrence of time slot 255 so as not to disturb the state of the reset bit during time slot 255. 
     Returning to FIGS. 2 and 3, RSB processor 260 includes circuitry for comparing the logic state of signal RSB&#39; against bit 16 (Reset bit) inputted via lead 265 for each summed message sample placed on PSDAT data bus 311 via accumulator RAM 510 (520) and select latch 620. 
     Referring to FIG. 5, it is noticed that the logic state of signal RSB&#39; changes every two time frames (N, N+2) during time slot zero. The frequency of signal RSB ensures that the reset bit (bit 16) of each summed message sample stored in accumulator RAM 510 (520) tracks the logic state of RSB for two time frames since the accumulators 510 and 520 are alternately loaded between a first and second time frame. Therefore, for time frames N and N+1 (FIG. 5) the reset bit of each summed message sample stored in accumulators 510 and 520 correspond to the logic state of signal RSB&#39; during the respective time frames. 
     During time frame N+2 (FIG. 5) reset signal RSB&#39; is toggled to the logic zero state during time slot zero and remains in that state during time frames N+2 and N+3 (not shown). 
     To reiterate, bit 16 (reset bit) of each summed message sample processed during time frames N and N+1 were set to a logic one corresponding to the state of signal RSB&#39;. The reset bit and summed samples are stored in the accumulator. During time frame N+2, bit 16 of each stored summed message sample (PSDAT) is compared by RSB processor 260 against signal RSB&#39;. If the logic state of bit 16 of a respective summed message sample and the logic state of signal RSB&#39; do not compare then the respective time slot must be the first access to accumulator RAM 510 (520) for the respective connection. This follows, since if a prior access for this frame had occurred the stored reset bit would match signal RSB&#39;. RSB processor 260, upon detecting a first access to accumulator RAM 510 (520), causes the INIT lead to input select control 410 to go to the logic one state. In turn, input select control 410, in accordance with Table 2, selects IBDAT or IDLECODE to be gated through input select buffer 420 for storage in accumulator RAM 510 (520) along with RSB&#39; (bit 16). In this manner, a new input message IBDAT respective to the first accessing time slot or IDLECODE (monitor mode) is used to reset the respective assigned accumulator RAM 510 (520) memory location thereby avoiding the necessity of pausing at the beginning of each time frame in order to clear accumulator RAM 510 (520). 
     If the logic state of bit 16 at 265 of a summed message sample respective to a connection and the logic state of signal RSB&#39; compare then the respective time slot is not the first access in this frame to accumulator RAM 510 (520). In this event, the logic state of the INIT lead is zero and the output from input select buffer 420, under control of input select 420, will be in accordance with Table 2. 
     This invention can be illustrated by way of example. Referring to FIG. 5, assume that a conference connection comprising three telephone lines are assigned by an external central processing unit to time slots 0, 4 and 8 (4 and 8 not shown) respectively. Also assume that the conference connection is assigned memory location 96 of accumulator RAM 510 (520) for the storage of the connection&#39;s summed message samples. Further assume that accumulator RAM 510 is accessed for storage during even time frames (N, N+2), and that accumulator RAM 520 is accessed for storage during odd time frames (N-1, N+1). Also, assume that bit 16 (reset bit) of each summed message samples stored in accumulator RAMs 510 and 520 during time frames N-2 (not shown) and N-1 were set to logic zero corresponding to the state of signal RSB&#39;. 
     In accordance with the outlined assumptions, and with reference to FIGS. 5 and 7, during time slot 255 of time frame N-1 (which precedes time slot 0) the summed message samples priorly loaded into location 96 of even accumulator RAM 510 during time frame N-2 (not shown) is prefetched and presented to latch 620 (FIG. 3). At the beginning of time slot 0 of frame N, latch 620 is caused to output the summed message sample to PSDAT bus for summation by in-adder 310 with an incoming message sample (IBDAT) respective to time slot 0. Also, during time frame N, reference signal RSB&#39; is toggled as priorly discussed to logic one state. As noted previously, the toggling of the RSB&#39; signal is delayed to ensure its input at select buffer 420 occurs correctly (i.e., is toggled within a corresponding current frame). Reset bit (bit 16) of a respective summed message sample of PSDAT bus is extended to RSB processor 260 via lead 265 for comparison with reference signal RSB. Since the logic state of bit 16 is zero (0) and the logic state of signal RSB&#39; is a one (1) there is a mismatch signifying a first access to memory location 96 of accumulator RAM 510 during time frame N. 
     The INIT lead to input select control 410 from RSB processor 260 goes to a logic one state as a result of the mismatch and directs either inputted message sample IBDAT or symbolic input IDLECODE to be gated through input select buffer 420 to SELDAT bus 450 for storage in location 96 of accumulator RAM 510 thereby overwriting old data with new data. The reset bit (bit 16) of selected ones of inputs to input select buffer 420 is updated by input buffer 420 circuitry to correspond to the current logic state of signal RSB before outputting to bus 450. In this manner, the reset bit equals signal RSB&#39; after initial access has been recognized by RSB processor 260. 
     During time slot 3 of frame N the contents of location 96 of accumulator RAM 510 is again prefetched and presented to latch 620. At the beginning of time slot 4 the contents of latch 620 is outputted to PSDAT bus for summation with an incoming message sample respective to time slot 4 by in-adder 310. Bit 16 of PSDAT bus is sampled by RSB processor 260 via lead 265 for comparison against signal RSB&#39;. However, RSB processor 260 in this instance does not detect a mismatch since bit 16 of the summed message sample was changed during time slot 0 by input select control 410 to equal signal RSB&#39;. As such, time slot 4 is not recognized as a first access and the summed message sample SMDAT from in-adder 310 is gated through input select buffer 420 for outputting to bus 450 for storage into location 96 of accumulator RAM 510. The recognition process that was applied to time slot 4 is also applied to time slot 8. 
     During the next time frame N+1, time slot 0 is recognized as the first access to location 96 of odd accumulator RAM 520. 
     The recognition of time slot 0 as the first access to location 96 of accumulator RAM 520 is based on the status of bit 16 of the summed message sample stored in location 96. The last access to accumulator RAM 520 occurred during time frame N-1 in which the state of signal RSB&#39; was zero. As noted previously, input select buffer 420 (FIG. 2) changes bit 16 to equal the state of signal RSB&#39;, which during time slot N-1 was a logic zero. Therefore, RSB processor 260 upon comparing bit 16 of the summed message sample stored in location 96 of accumulator RAM 520 against the state of signal RSB&#39; detects a mismatch. The mismatch, as noted previously, signifies a first access. The processing of time slot zero includes changing bit 16 of summed message sample to correspond to signal RSB&#39; to preclude recognizing time slot 4 and 8 as a first access during the N+1 time frame in the manner described above. 
     ACCUMULATOR RAM 
     Referring to FIG. 3, accumulator RAM 510 and RAM 520 are identical dynamic random access memories each having 128 memory locations by seventeen bits enabled by 4T clock signals. Summed message samples stored in accumulator RAM 510 (520) memory locations comprise 17 bits as follows: a summed message sample of 14 magnitude bits and a sign bit, a parity bit and a reset bit. 
     FIG. 4 shows four transitions of clock 4T for each period of a time slot. The first falling edge of clock 4T (A) initiates an accumulator RAM 520 (510) read or prefetch. During the next (first) rising edge (B) of the 4T clock signal the prefetched data is clocked into accumulator RAM 520 (510) output register (internal, not shown). During the second falling edge (C) of the 4T clock signal a summed message sample from PSDAT bus 311 is written into accumulator RAM 520 (510). This sequence is depicted by the AR1AD accumulator RAM 520 address sequence (FIG. 4) respective to time slot 253, which shows that a time slot is divided into two distinct operations, namely, first, prefetching summed message samples respectively to a second time slot (254) and second, storing a summed message sample respective to a first time slot (253) during the period in which RAMSEL0 is a logic zero. 
     TS-255 (TIME SLOT) DETECTOR 
     TS-255 Detector 455 FIG. 3 is a circuit arranged for detecting the occurrence of incoming time slot 255 respective to system frame sync signal TSYNC (FIG. 4), which occurs once each time frame and which overlaps time slots 253 and 254 as a means of signifying an approaching end to a respective time frame. Signal TSYNC applied via lead 230 is toggled and clocked into the first D-type flip-flop of detector 455 on the falling edge of clock signal 2T occurring at the end of time slot 253. TSYNC is then clocked into a second D-type flip-flop of TS detector 455 on the falling edge of clock signal 2T occurring at the beginning of IBDAT time slot 255 and is outputted as signal TS255&#39; on lead 451. Signal TS255&#39; on lead 451 is time representative of incoming time slot 255. 
     RAM SELECTOR 
     Accumulator RAM 510 (520) selector circuit 460 is a sequential circuit for generating clock signal RAMSEL0 at lead 461 and clock signal LRSO at lead 463. Clock signals RAMSEL1 and LRS1 at lead 462 and lead 464 respectively are complements of signals RAMSEL0 AND LRS0 respectively. 
     Clock signals RAMSEL0 and RAMSEL1 at lead 461 and 462 respectively are the means for alternating accumulator RAM 510 (520) between a storage cycle during a first time frame and a supply cycle during a second time frame. Derivative RAM select signal LRS0 outputted at lead 463 to RAM r/w circuit 470 and at a logic one state (high) during a first time frame selects accumulator RAM 510 for storage of summed message samples outputted from input select buffer 420 via SELDAT bus 450. RAM select signal LRS0 at the logic zero state selects accumulator RAM 510 for supplying summed message samples for outputting via data bus 750. RAM select signal LRS1 outputted at lead 464 to RAM r/w circuit 470 and at the logic one state selects accumulator RAM 520 for the storage of message samples and at the logic zero state selects accumulator RAM 520 for supplying summed message samples to lead 750. 
     Clock signal RAMSEL0 at lead 461 is toggled by the occurrence of signal TS255&#39; extended from TS-255 detector 455 to Accumulator RAM Selector 460 via lead 451, and the second rising edge of clock signal 4F occurring during time slot 255. In turn, control signal LRS0 is delayed one-quarter of a time slot respective to signal RAMSEL0 and is toggled by the combination of toggled RAMSEL0 and the first rising edge of clock signal 4T. Toggled select signal LRS0 occurs at the start of a time frame coincident with time slot 0. Select signals LRS0 and LRS1 are toggled in this manner following the occurrence of each TSYNC pulse as a means of causing the signals to alternate between a logic one state and logic zero state respectively during a first time frame and a logic zero and logic one state respectively during a second time frame for the alternate selection of accumulator RAM 510 and 520. 
     RAM READ/WRITE 
     Referring to FIG. 4, in particular to time sequence AR1AD, it is shown that each time slot is apportioned into a read (R) cycle for first prefetching a summed message sample respective to a second time slot and a write cycle for storing summed message samples into accumulator RAM 520 respective to a first time slot. Signals RRM0 and RRM1 are the means by which a time slot is apportioned into a read cycle and a write cycle. 
     Returning to FIG. 3, RAM READ/WRITE circuit 470 is a combinational circuit comprised of AND and NAND gates for generating accumulator RAM 510 (520) READ/WRITE signals RRM0 and RRM1 at leads 471 and 472 respectively. Signals RRM0 and RRM1 at leads 471 and 472 respectively are inverted replicas of clock signal 2T for placing the accumulator RAM 510 (520) in the storage cycle. Select signal LRS0 (LRS1) at the logic zero state and inputted via lead 463 (464) disenables circuitry associated with RAM read/write circuit 470 and forces the output at lead 471 (472) to a logic one state, or RAM read state for the accumulator RAM 510 (520) in the supply cycle. Conversely, select signal LRS0 (LRS1) at the logic one state enables RAM read/write circuitry 470 thereby causing the circuitry to output at lead 471 (472) an inverted replica of clock signal 2T. Since select signals LRS0 and LRS1 are logical complements, either signal RRM0 or signal RRM1, but not both, is enabled during a respective time frame. Read/Write control signals RRM0 and RRM1 also connect to accumulator RAM 510 and 520 via leads 471 and 472 respectively. 
     The seven bit accumulator RAM 510 (520) read address (RARAD) and write address (WARAD) extend to accumulator RAM 510 (520) via address bus 515 (525) and via RAM select circuit 540 (550), respectively, which are under the control of signals RRM0 and RRM1. 
     For example, assume that during a first time frame accumulator RAM 520 is operative for the storage of message samples inputted via SELDAT bus and, concurrently, accumulator RAM 510 is operative for outputting message samples stored during the prior frame respective to each time slot to output 750. In this instance, RRM1, which during the assumed first time frame is an inverted replica of clock 2T, gates read address RARAD through select circuit 550 to accumulator RAM 520 via address bus 525 during the first half of a time slot and gates write address WARAD, via address bus 525, to accumulator RAM 520 during the second half of a time slot. Simultaneously therewith, signal RRM0, which is held constantly at logic 1 during the assumed time frame, gates only read address RARAD through select circuit 540 to accumulator RAM 510 via address bus 515 during the respective time frame. 
     During the second (next) time frame, signal RRM1 at lead 472 is held at the logic one state for the entire time frame thereby only allowing read address RARAD to be gated through select circuit 550 to accumulator RAM 520 via address bus 525. Concurrently, enabled signal RRM0, having a logic one state during the first half of a time slot and a logic zero state during the second half of a time slot, gates read address RARAD through select circuit 540 during the first half of a time slot and gates through WARAD to accumulator RAM 510 during the second half of the time slot. In this manner each accumulator RAM 510 (520) alternates between a storage cycle and a supply cycle, and permits prefetching a summed message sample respective to a second time slot during a first time slot. 
     RAM SELECT LATCH 
     RAM select latch 620 FIG. 3 comprises a seventeen bit array of master-slave D type flip-flops for holding prefetched previously summed message samples outputted from accumulator RAM 510 (520) for summing with a respective incoming message sample (IBDAT) by in-adder 310. RAM select latch 620 alternates under control of the RAMSEL0 clock between receiving summed message samples from accumulator RAM 510 (520). A summed message sample is clocked into the master flip-flops of latch 620 on the second rising edge of the 4F clock and then clocked into the slave flip-flops for outputting to PSDAT data bus 311 on the rising edge of the 4T clock signal during the period in which clock signal LTC is a logic one. 
     Referring to FIG. 3, RAM select latch 620 receives and latches via data bus 621 summed message samples from accumulator RAM 510 when the RAMSEL0 lead 461 is a logic one and receives and holds via data bus 622 summed message samples from accumulator RAM 520 when RAMSEL0 lead 461 is a logic zero. A summed message sample stored in select latch 620 is presented both to in-adder 310 via bus 311 for summing with an incoming message sample (IBDAT) on bus 206, and to input select buffer 420 as discussed previously. 
     ADJACENT TIME SLOTS 
     For the occurrence in which adjacent time slots within a time frame are assigned to the same conference, and, therefore, both are assigned the same accumulator RAM 510 (520) memory location, the summed message samples outputted from input buffer 420 respective to a first one of the adjacent time slots is caused to be loaded by the adjacent time slot detector 480 into select latch 620 in addition to being loaded in accumulator RAM 510 (520). In this manner the summed message samples prefetched in preparation of processing the second one of the adjacent time slots includes the message sample contributed by the first one of the adjacent time slots. Without this arrangement the prefetched sum which is removed from memory during the prior time slot would not include the latest sample contributed by the prior time slot. 
     Referring to FIG. 3, adjacent time slot detector 480 is a combinational comparator circuit and register circuit for detecting adjacent time slots assigned to the same conference session. Adjacent time slot detector 480 is disabled by signal TS255 which is time representative of incoming time slot 255, inputted via lead 451 from detector 455. Disablement of detector 480 precludes recognition of adjacent time slots across the boundaries of two adjacent time frames, which is the case for time slot 255 and time slot 0. 
     As discussed previously, accumulator RAM 510 (520) read (prefetch) address RARAD respective to a second time slot occurs simultaneously with accumulator RAM 510 (520) write address WARAD at the output of select latch 440 or address busses 442 and 443, respectively. Therefore, read address (RARAD) respective to a second one of adjacent time slots equals the write address (WARAD) respective to a first one of adjacent time slots occurring simultaneously therewith. 
     Read address RARAD and write address WARAD extend from select latch 440 to adjacent time slot detector 480 via address busses 442 and 443 respectively. Combinational comparator circuitry associated with detector 480 compares the addresses and outputs a logic one via lead 481 upon detecting a match between addresses RARAD and WARAD. At the beginning of the fourth quarter of the first one of adjacent time slots, the logic one outputted from the comparator circuitry of detector 480 is clocked into a detector 480 output register on the rising edge of clock signal 4F and outputted at 481. Detector 480 output register is cleared by the rising edge of clock signal 4F occurring during the first quarter of the second one of the adjacent time slots given that the next subsequent time slot is not adjacent. 
     The logic one outputted from detector 480 register extends to latch 620 via lead 481 which forces the summed message sample respective to the first one of adjacent time slots on PSDAT bus 311 to be latched into select latch 620 thereby displacing the prefetched summed digital message samples respective to the second of the adjacent time slots. 
     RAM OUTPUT LATCH 
     RAM output latch 630 (FIG. 3) holds prefetched summed message samples outputted from accumulator RAM 510 (520) selected for supplying data to the output 750. RAM output latch 630, which is similar to select latch 620 and which is clocked by the 4F and 4T signals, alternates under control of the RAMSEL1 clock signal at lead 462 between receiving and holding summed message samples from accumulator RAM 510 (520). 
     When RAMSEL1 clock at lead 462 is a logic one, RAM output latch 630 receives and holds via bus 621 summed message samples from accumulator 510 for outputting to out-adder 640 via the CSDAT data bus 635. RAMSEL1 clock signal at the logic zero state switches the input of RAM output latch 630 to accumulator RAM 520 for receiving summed message samples via data bus 622. 
     SAMPLE RAM MEMORY 
     As shown on FIG. 4, time slot address DRAD generated by address generator 320 on bus 319 leads a respective incoming IBDAT time slot by one-half time slot and is applied to the memory address input of sample RAM 610 (FIG. 3) for prefetching a previously stored message sample respective to the incoming time slot of a previous time frame prior to the storage of the incoming digital message sample of a current frame. The 2T clock signal directs the read/write cycle of sample RAM 610 respective to clock signal 4T. 
     Continuing in FIG. 4, for example, time slot address (DRAD) 254 precedes and overlaps incoming time slot (IBDAT) 254 by one-half time slot and is applied to sample memory RAM 610 via bus 319. The occurrence of clock signal 2T at the logic one state and the second falling transition of clock signal 4T during IBDAT time slot 253 causes memory location 254 to be read and the contents stored in sample RAM 610 output latch (internal, not shown) on the following rising edge of 4T (not shown) for processing by out-adder 640 via message sample gate (MSG) 625. During IBDAT incoming time slot 254, the occurrence of clock signal 2T in the logic zero state and the first falling transition of clock 4T causes a new message sample respective to time slot 254, extended from bus 206, to be stored in memory location 254 of sample memory RAM 610. The prefetched message sample respective to time slot 254 held in sample RAM 610 output latch is then applied to message sample gate 625 via data bus 616 during time slot 254. 
     MESSAGE SAMPLE GATE 
     Message sample gate 625 is a 16 bit combinational logic circuit under control of sample control circuit 645 for inverting and gating to bus 626 either message samples outputted from sample RAM 610 or digital message IDLECODE, which is internally generated by message sample gate 625. 
     SAMPLE CONTROL CIRCUIT 
     Sample control circuit 645 is a combinatorial logic circuit similar to input control circuit 410 for controlling message sample gate 625 in accordance with the permissions established by the logic state of mode control bits CMBT and CMBR inputted at via leads 444 and 445 respectively. Maintenance signals (not shown) are also inputted to sample control circuit 645 for exercising the conferencer. 
     Sample control circuit 645 selects alternate message IDLECODE to be generated and gated through gate 625 for time slots assigned to the monitor mode. A time slot in the monitor mode has permission to only monitor a conference connection and therefore, message samples transmitted by the respective time slot are not permitted to be operative on the conference summed message outputted at 750. 
     Sample control circuit 645 is operative for selecting a message sample on bus 616 to be gated through gate 625 to data bus 626 when the logical state of the mode control bits CMBR and CMBT inputted via leads 644 and 645 respective to a time slot is either 01 (broadcast) or 11 (conference). A message sample contributed by a time slot in either the broadcast or conference mode is summed at the input by in-adder 310 with message samples contributed by other time slots assigned to the connection and stored in accumulator RAM 510 (520), and therefore, its message sample is allowed to be operative on outputted message samples. Whereas, a monitoring time slot is silent and its message contribution is not summed by in-adder 310 with the conference summed message sample. Therefore, the message sample of a monitoring time slot is not permitted to be operative on summed message samples supplied to output 750. 
     OUT-ADDER 
     Out-adder 640 is a twos-complement combinatorial logic circuit similar in circuitry to in-adder 310. Out-adder 640 includes saturation logic circuitry for detecting positive overflow or negative overflow as a result of summing a summed message sample inputted from output latch 630 via data bus 635 with an inverted message sample from gate circuit 625 via bus 626. Since a message sample inputted via bus 626 is an inverted replica of the message sample previously stored in sample RAM 610, it is essentially subtracted by out-adder 640 from the summed message sample inputted via bus 635 by the twos-compliment addition. A message sample respective to a time slot is effectively subtracted in this manner from the summed message sample to eliminate its message contribution from the summed message before the summed message sample is outputted to the respective time slot at output 750. This circuit sequence ensures that sidetone is not transmitted to a respective time slot, since sidetone is provided by local telephone apparatus. 
     Out-adder 640 notifies output select circuit 710 via leads POOR or NOOR in the event either positive or negative overflow occurs as a result of summing an inputted summed message sample from bus 635 with a respective message sample inputted from bus 626. 
     OUTPUT BUFFER 
     Output select buffer 720 is a combinatorial logic circuit similar in circuitry to input select buffer 420, and is under control of output control circuit 710. Output select buffer 720 includes circuitry (not shown) for generating parity (not shown) over selected inputs outputted to output buffer 735 via data bus 734, and includes combinatorial circuitry for performing circuit maintenance functions (not shown) at the direction of an external central processing unit (not shown). 
     Inputs DFDAT and CSDAT to output select buffer 720 represent respectively the output from out-adder 640 via bus 641 and the output from latch 630. CSDAT is the cumulative conference message sample from accumulator RAM 510 (520) and includes the message sample contribution of the respective time slot. DFDAT is the cumulative conference message sample less the message sample contribution of the respective time slot. Alternate digital messages PMAX, NMAX and IDLECODE are also generated internally by output buffer 720 and were defined previously. Output select buffer 720 generates additional alternate digital messages (not shown) in response to circuit maintenance requests. 
     OUTPUT CONTROL CIRCUIT 
     Output control circuit 710 is a combinational logic circuit similar in circuitry to input control circuit 410 for controlling the outputting of selected ones of inputs to output select buffer 720. The outputs from output select control 710 correspond to respective inputs to output select buffer 720. 
     
                       TABLE 3                                                     
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Output  Select    Control Input   Output to                               
CMBR    CMBT      POOR    NOOR    Bus 734                                 
______________________________________                                    
0       0         --      --      CSDAT                                   
0       1         --      --      IDLECODE                                
1       0         0       0       DFDAT                                   
1       1         0       0       DFDAT                                   
1       0         0       1       NMAX                                    
1       0         1       0       PMAX                                    
1       1         0       1       NMAX                                    
1       1         1       0       PMAX                                    
______________________________________                                    
 
    
     Table 3 illustrates symbolically, in a format similar to the format of the well-known truth table, the states of inputs to output select control 710 which in turn control the selection of corresponding ones of inputs to output buffer 720 for gating to bus 734. 
     In response to a positive or negative overflow signal from out-adder 640, via leads POOR or NOOR respectively, output select control circuit 710 directs output select buffer 720, via leads PFS and NFS respectively, to output alternate digital messages PFS (PMAX) or NFS (NMAX) to bus 734 respectively. IDLECODE is outputted by output select buffer 720 at the direction of output control circuit 710 via lead IDLC whenever the respective time slot has permission only to broadcast (mode control bits=01) to a conference session. IDLECODE is selected for outputting in accordance with Table 3 in this instance to ensure that a respective broadcast only time slot does not receive digital message samples from the conference session. Output select control 710 directs, via lead DFD, output buffer 720 to gate through the output (DFDAT) from out-adder 640 extended to output buffer 720 via DFDAT data bus 641, to respective time slots having permission to receive (CMBR=1) summed message samples from the conference session in the absence of overflow. 
     OUTPUT BUFFER 
     Output select buffer 720 extends the selected digital message sample to output buffer 735 via bus 734. 
     Output buffer 735 is a sequential circuit arrangement of master-slave flip-flops in which data on bus 734 is clocked into the master stage of output buffer 735 on the falling edge of the 4T clock within the LTC enable window; the contents of the master register is clocked into the slave register on the first rising edge of clock signal 4T within the LTC enable window for outputting to sixteen parallel bit bus 750. 
     The progression of a digital message sample through output latch 735 is arranged in accordance with clock signal 4T to allow an outputted message sample to be available for the full period of a time slot, instead of a portion of a time slot. Therefore, in accordance with this arrangement, an outputted message sample via data bus 750 lags the incoming (next) time slot (IBDAT) by one time slot. For example, a summed message sample respective to time slot 254 is available at output 750 at the start of input time slot 255. 
     A substitution of the master-slave register arrangement with standard flip-flops permits the output to be available via data bus 750 during the respective time slot. 
     CONCLUSION 
     It is obvious to one skilled in the art that the invention disclosed herein is not limited to the embodiment disclosed in the accompanying drawings and foregoing detailed description, but can be rearranged through the substitution, addition and/or deletion of components and functions without departing from the scope and spirit of the invention. 
     For example, the arrangement of various data busses can be rearranged by adding or deleting the number of bits which define the digital value of summed message samples. In turn, the accumulator RAMs and sample RAM can be adjusted to reflect the corresponding change in the number of data bits. Further, the number of mode control bits can be easily redefined from what is disclosed herein or can easily be increased in number to define hybrid connections or other functions, such as the insertion of a delay between outputting particular summed message samples. Moreover, alternative symbolic messages can be readily changed to different values without departing the scope and spirit of the invention disclosed herein. Still further, the interface circuit can be readily changed from a serial input to a parallel input which is well known in the art. 
     The linear time division multiplexed switching arrangement disclosed would typically include maintenance functions to perform audits and diagnostics on the circuit elements. The circuit would also typically include arrangements for making parity checks and for appending a parity bit to data. While these maintenance functions are highly desirable, their implementation is well known to the art.