Patent GB-1013593-A (GB)
Title: Improvements in and electronic switching system
IPC: H04M7/10
Filed: 1961-12-08
Abstract: 1,013,593. Automatic exchange systems; gating circuits using diodes and transistors. AUTOMATIC ELECTRIC LABORATORIES Inc. Dec. 8, 1961 [Jan. 30, 1961 (2); Oct. 17, 1961; Nov. 1, 1961], No. 44101/61. Heading H4K. A 100-line pabx, in which each subscriber has access to both the originating and terminating sides of a cross-point switching network, is controlled by common apparatus shared in a time-division-multiplex system. Each subscriber has a fixed time slot in the multiplex cycle as have central office trunks, operator's lines, and conference circuits. For each time slot a line register is provided comprised by a storage row of a ferrite core storage matrix and during each time slot a line register is interrogated and its information may be processed as determined by activity signalled from the associated line circuit. When a line register has enough information to set up or disconnect a call it demands the services of a marker which controls the switching network to make or destroy a connection on a one-at-a-time basis. Special services available to subscribers are the privilege of dialling central office calls direct; the use of a conference circuit to which subscribers may connect automatically and to which trunks may be connected by the pabx operator by prearrangement among the conferring parties; to transfer trunk calls to another subscriber by dialling the other subscriber's directory number; and to call up a third subscriber to join an established conversation, the third subscriber being able to call up a fourth, and so on, thereby effecting a chain conference. Pulses.-A distributer produces thirteen units pulses U1 to U0 and Ux, Uy, Uz, each with a length of one time slot. Also produced are ten tens pulses T1 to T0, each with a length of 13 time slots. Combinations of U1 to U0 and T1 to T0 pulses define the time slots of the 100 pabx subscribers. Combinations of Ux and T1 to T10 pulses define 10 " ordinary " trunks which are available for all outgoing calls and for incoming calls handled by the operator. Combinations of Uy and T1 to T10 pulses define the time slots of 10 " bothway " trunks available for all outgoing calls and over which incoming calls may be dialled from the central office directly to a pabx subscriber. Combinations of Uz and T1 to T4 define four operator's lines, while combinations of pulses Uz and T5 to T0 define six conference lines which subscribers may take into use by prearrangement so as to communicate over the common conference circuit. Each time slot is comprised by 16 intervals during which the associated register is read out to the common control circuit, the information so read out is processed if necessary, the register is fed back with the original or amended information, the marker is instructed if employed, and the control circuit is reset for interrogation of the register associated with the next time slot. Subscribers' line circuits.-A line circuit may be as shown in Fig. 7 where the subscriber with directory number 32 has a time slot defined by AND gate 722 unique in receiving the pulses T3 and U2. During its time slot the line is supervised over the multiplex highways H1, H2, H3. The condition of the loop is supplied to highway H1 or H2 from detector 720 depending on the strapping NR or R employed, the other highway of H1 and H2 being pulsed in the time slot invariably. Strapping NR indicates a non-restricted subscriber privileged to dial trunk calls. Strapping R indicates a restricted subscriber who must obtain the operator for trunk calls and who is directed to the operator if found dialling a trunk call. The third highway H3 is connected for a subscriber providing night service in the absence of the operator. Line circuits not connected to a subscriber are disconnected from the highways and are recognized by the absence of signals as unequipped. Dial tone is connected over gate 722 from a common source DT at the dictation of a signal D32 coincident with the time slot and provided from the common control circuit. An integrator 734 holds the dial tone gate open between time slots. Busy tone is interrupted dial tone produced by interrupting the signals D32. Dialling to the central office requires the subscriber to gate bursts of an 8 kc/s. tone to the trunk circuit and this is achieved by an enabling signal Aq32 from the common control which admits signal from ST under command of the line condition detector 720. Each line circuit is connected to both sides of the cross-point network from the terminals Ld32 and La32. An alternative line circuit, to be found in Fig. 9, not shown, employs a bi-stable transistor circuit in place of the dial/busy tone integrator, the 8 kc/s. signal tone gate is disabled for restricted subscribers, the line condition is detected by a normally conductive inverter transistor amplifier, and the tens and units pulses are inverted for application to a NOR gate. Another alternative, to be found in Fig. 10, not shown, uses an integrator to enable the 8 kc/s. signal tone gate. Trunk circuits.-With an " ordinary " trunk circuit, as shown in Fig. 8A, connections are made to both sides of the cross-point network by leads Ld1x and La1x. Dial tone is connected by means of gate 832 fed from a common tone lead Db and enabled by an integrator 348 pulsed over lead D1x in the trunk circuit time slot. Busy tone is dial tone suitably interrupted. For repeating outgoing dialled impulses, comprised by bursts of 8 kc/s. tone, a detector 830 responds by operating a trigger circuit to produce a signal Df. On connection of a pabx line to the trunk circuit, relays Br and Lr operate from flip-flops Bf and Lf set from the common control so as to seize the central office line equipment. During dialling flip-flop If is set to operate relay Ir to guard the line transformer from transients. Relay Br releases to the first impulse while relay Lr releases to each impulse to repeat the impulses in the inter-exchange loop. With incoming calls relay Cr operates to the calling condition and signals the call to the common control over highways H1, H2, so that ring-back tone may be connected after connection to the pabx operator. During transfer of a trunk from one pabx subscriber to another the relay Ir is operated to silence the trunk. During release, relay Ir is also operated for 100 ms. to mark the trunk circuit busy to the pabx which might otherwise reseize the trunk before the central office equipment has restored. A " both-way " trunk circuit over which dialling impulses may be repeated in both directions (see Fig. 8B, not shown) is provided with a feed relay to detect pulses incoming from the central office and which are repeated over the highways H1, H2, to the common control equipment. Operator's equipment.-The operator's console has a ringing key for an audible alarm to augment the visual signals, a night service key, and a conference lamp. For each line appearance the console also displays an answer key with normal, answer, and hold positions, a conference key, a trunk lamp, and a local lamp. In the normal position of an answer key the operator's instrument is disconnected; in the answer position the instrument is connected to the corresponding line; while in the hold position the instrument is disconnected but the line is held. Operator's line circuits have three flip-flops which record the ten states characterizing the kind and progress of calls connected to an operator. A local call flashes the local lamp which burns steadily when the connection has been made. A trunk call flashes the trunk lamp which stays lighted when the connection is made. If the local lamp flashes when the trunk lamp is lighted the operator is being recalled to a trunk connection. If both trunk and local lamps are lighted the call is a transferred trunk call whether under supervision or not. The conference lamp flahes when the conference circuit is no longer in use and the conference key is made. Operator's equipment and line circuits are described with reference to Figs. 23 and 24, not shown here. Cross-point switching network and junctors.- A four-stage network of four-layer diodes is employed arranged symmetrically about a set of junctors. The marker finds and marks free paths on each side of the network and by setting an appropriate idle junctor breaks down a path from the junctor to the calling terminal on the left-hand side and from the junctor to the called terminal on the right-hand side of the network. In Fig. 3 only one cross-point is shown in each stage and one junctor is shown schematically. The full network has 10 XA matrices, 6 XB matrices, 36 junctors, 6 XC matrices, and 10 XD matrices. Each subscriber has a two-digit address corresponding to his time slot and his position in the XA and XD matrices. Each junctor has a two-digit address depending on the XB and XC matrices it serves to interconnect. The marker sets up a connection by means of link marking circuits LMOU, LMOT, and LMRU, LMRT, to select the calling and called subscribers, and by means of circuits LMB, LMC, to connect the chosen junctor. A suitable transistor marking circuit is described with reference to Fig. 4, not shown. The connection of the isolating diodes such as 321 to a resistance, capacity earth 341, 342 protects the four-layer diodes from rapid voltage changes to which they are sensitive and which could cause them to fire out of turn. The junctors are used to supply ringing tones to the subscribers by way of switch 373 which requires the two halves of the connection to be decoupled by a switch 372. The junctor is marked for use by a switch 371. As shown in detail in Fig. 6, the junctor is symmetrical about capacitor 600. When not in use both flip-flops are reset so that transistor 671 conducts and 100 volts is given earth over the resistors 621, 624, and diodes 613, 614, in parallel which holds the junctor terminals at a low potential allowing no breakdown in