1,017,504. Automatic exchange systems. GENERAL DYNAMICS CORPORATION. July 17, 1962, No. 27414/62. Heading H4K. A time division multiplex pabx serves 300 extensions and 100 trunk circuits with 32 time slots of 2À5Á sec. each in an exchange cycle of 80Á sec. A calling line store comprised by ten circulating delay lines holds the identity of the called line in a time slot allocated for the call and connects the line to the speech highway in this time slot. A called line store similarly comprised of circulating delay lines holds the called line identity and connects the called line to the highway in the allocated time slot. Supervision is exercised by means of three data highways, 112 for the line circuits, and 116 and 118 for trunk circuits. Conference connections may be set up using separate time slots for each of five extensions. An operator's position is provided. Extensions barred access to trunks are routed to the operator if attempting trunk calls. Incoming trunk calls are ticketed to record the number called and the duration of call. The exchange is described in detail with reference to Figs. 2 to 22 (not shown). The exchange employs diode AND and OR gates; transistor inverters, monostables, and bi-stables; and either/or gates. Scanning for calling line circuits and trunks; line identification.-The calling line store has a binary ring counter marking units on four output leads, tens on a further four output leads, and hundreds on a further two leads. These marks are extended over a translation field to become a 1-out-of-400 mark enabling a single gate in a calling line and trunk matrix 124. In the digit detector and time slot allotter a circulating delay line store records all busy time slots and a second such store records a single free time slot obtained by comparing clock pulses with the output of the busy slot store. As soon as an idle slot is found and allocated for use by entering it in the idle slot store the ring counter is advanced by one to the identity of a line or trunk circuit and this identity is compared with the identities recorded in busy time slots in the calling and terminating line stores. If the identity to which the ring counter has been advanced is already circulating in one of the stores then the line is busy and the counter is advanced to the next identity and the allocated time slot is preserved for further use. If no match is found then it is possible that the circuit corresponding to the ring counter position is in a calling condition and to test for the hook-switch position the circuit is gated to the highways in the next occurrence of the allocated time slot by gating the output of the ring counter to the calling line and trunk matrix. If the circuit is on-hook the time slot is preserved idle and the ring counter is stepped forward to scan the next circuit. If the line is off-hook the allocated time slot is entered into the busy time slot store and the set of ten circulating delay lines register the calling line identity in the allocated time slot to repeatedly connect the circuit to the speech and data highways. The idle time slot store now looks for a new time slot and as soon as one is allocated the ring counter advances to continue the inspection of circuits for calling condition. Connection of dial tone.-The appearance of a time slot in the circulating delay store recording all such busy slots automatically connects a busy tone source to the speech highway in this time slot. A further circulating delay line store records the time slots of all calling parties who have dialled at least one digit and is effective to inhibit the connection of dial tone in those slots. Detection of impulses of dialled digits.- Digit detection is effected by three counters comprised of seven, six, and four circulating delay lines. The counters record impulse duration, impulse spacing, and accumulated impulse totals, respectively, as a parallel array of pulses circulating in the time slot of the calling circuit. The counters therefore deal simultaneously with as many circuits as have connections. A pulse divider, which may require to occupy a time slot for its operation, provides a counter control clock pulse once every 50 exchange cycles. Each break impulse has the effect of connecting pulses in the allocated time slot to the digit detector 120. The counter control clock pulses select every 50th time slot pulse to operate the counters. The first break impulse effects connection of the clock pulses to the seven stage impulse duration counter which after counting two such pulses, clears down the spacing counter, effects release of dial tone, and after counting five such pulses (elapsed time 20 ms.) injects a count of one into the four-stage accumulated total counter. An inhibition is now placed on the further injection of a count into the total counter until the seven-stage impulse duration counter is cleared following the detection of an appropriate spacing. For as long as the loop is closed between impulses the six-stage spacing counter is stepped by the counter control clock pulses so as to differentiate between times of more and less than 128 ms. The appearance of a second break impulse before 128 ms. clears down the spacing counter and the impulse duration counter takes over. If the spacing counter detects an interdigital pause by achieving a count of 31 then both spacing and duration counters clear down and the total in the fourstage counter is transferred to the trunk finder 126. The subsequent two digits are accumulated and transferred in the same way. Utilization of digit values.-A two-stage circulating delay line store in the terminating line store 128 records the order of the digit received. The first digit is routed to the trunk finder where it is translated to a 1-out-of- 10 mark to detect digit 0, which requires connection of the operator; digit 2, which requires connection of the conference circuit; and digit 9 which requires the selection of an idle trunk circuit. Only three other initial digits are required to cater for internal calls and the 1-outof-10 marks indicating these digits are connected to the two circulating delay line stores recording the hundreds digit in the terminating line store so as to enter this digit in the time slot allocated to the connection. The other two digits are sent directly to the two sets of four delay line stores in the terminating line store which record the tens and units digits in the time slot allocated to the connection. The identity of the called party is now circulating in the terminating line store and is set up on a set of ten bi-stable circuits so as to effect a comparison with all other identities circulating in the calling line and terminating line stores. If the called party is already engaged in a call a match will be found and will effect erasure of the called identity in the allocated time slot and injects the time slot into a busy tone circulating delay line store to effect connection of the busy tone to the calling extension. If no match is found the called party may still be busy to the extent of being off-hook. The stored identity of the called party is used to open his gate in the terminating line and trunk matrix so as to connect his line circuit to the data highway in the allocated time slot. The detection of an off-hook condition by this means produces the same response of erasing the called identity and connecting up busy-tone. Called line idle; ringing.-If the tests for busy condition prove negative the time slot is registered in a circulating delay line store to supervise the repeated connection of the called line identity to the terminating line matrix by way of the translation field to produce the 1 -out-of-400 marking. The called line circuit is thereby connected to the speech and data highway once in each exchange cycle in the calling time slot. A pulse is also injected into one of a pair of circulating delay line stores in the allocated time slot to supervise the connection of ringing tone to the called party and ring-back tone to the caller. Since both parties are connected over the same highway in the same time slot it is necessary to alternate the connections of ringing tone to the called party and ring-back tone to the caller. Reply.-Detection of the off-hook condition from the called party by way of the data highway 112 erases the ringing tone stores of the pulse in the call time slot and leaves the connection under the supervision of the calling and terminating line stores which repeatedly open the calling and called circuit speech and data gates in the allocated time slot. Belease.-The on-hook condition of a party is timed by the digit detector 120 the seven-stage impulse duration counter of which measures off a period of 512 ms. during which time slot pulses are uninterruptedly on the data highway. Given such a count in a time slot, release is effected by deleting the time slot from the circulating delay line busy time slot store. The absence of the time slot from the store automatically clears down the calling and called party identities in this position in the calling and terminating line stores 122 and 128. Incoming call.-A calling trunk circuit is detected and identified in the same way as a calling pabx extension. However, no dial tone is connected, and dialling is repeated by the trunk circuit as bursts of time slot pulses applied to the trunk receive highway 116. These pulses are divided by 50 and are dealt with by the digit detector as previously described. Reply has the effect of connecting pulses in the allocated time slot to the trunk send highway 118 and this causes the trunk circuit to revert a supervisory signal to the exchange. Release of the connection is effected when the party at the distant exchange hangs up. The operator may be called into the connection by flashing the switch hook. This facility is confined to connections in which a trunk identity