Patent GB-1426919-A (GB)
Title: Switching system
IPC: H04Q3/62
Filed: 1973-05-01
Abstract: 1426919 Automatic exchange systems WESTERN ELECTRIC CO Inc 1 May 1973 [3 May 1972] 20602/73 Heading H4K In a centrally controlled system having subscribers appearing on one side of a switching network and trunks and registers on the other side, the line circuits, the registers and the central control are interconnected by a bus system which is used (a) to identify a calling line, (b) to convey this identity to a register, (c) to apply marking signals indicative of a called line from the register to the called line circuit and (d) to apply marking signals indicative of the calling line from the register to the calling line circuit whereby a connection from the called to the calling line may be set up via a trunk circuit. The system is applicable to a PBX having an outgoing calls barred feature. The system is distinguished from that of Specification 1,108,701 in that the bus rather than the calling- line/register connection via the network is used for conveying the calling line identity when it is wished to set up the final connection via the trunk. The subscribers are arranged in groups of 100 each of which has a line scanner 116, Fig. 1B, that is effective when in operation to mark one out often "tens" leads and one out often "units" leads of the bus system 107 so as to thereby select a unique subscriber. The particular scanner enabled at any one time is designated by a "hundreds" marking in the common control (details in Fig. 2C, not shown). A calling line, e.g. ST00 brings up a line relay (L, Fig. 2B, not shown) in the supervisory 105 of this line circuit LC00 thereby applying an earth marking to lead LDTQ* of the signal bus system 102A. When the common control is available it responds to this marking by enabling each line scanner in turn to step through its 100 positions and marks the "find-calling-line" enable lead DTE. The logic 108 of line circuit LC00 is thus eventually enabled and applies a stop-scanning mark to lead LS00 in a signal bus system 102B each of whose leads includes a decoupling diode. The class of service translation provided by this marking is ignored at this stage of the call set-up procedure. The central control enables a register and trunk scanner 120 to mark in turn the individual leads Tn-R which extend to the trunks and registers and simultaneously enables a register bid circuit 118 to mark the registers 104. The first free register makes its presence known over lead RT thereby stopping the scanning. Thereafter a go condition is applied to lead NETE* so as to permit a network controller 112A to select a path from the calling line appearance to the selected register using the information presented to it from the line and register scanners via the bus systems 107 and 103. In addition, the central control opens gate CNG1 so as to permit the calling line identity data on bus 107 to be entered into calling line number memory 104D in the register. The central control releases from this call during the subsequent reception of dialling signals. Completion of dialling is signalled over lead FOR the common control which when it is ready opens gate CLD1 so as to apply the called number markings from memory 104C on to the line scanning bus 107. (In practice, the control marks the particular register involved and buffers the called number therefrom. It performs an initial translation to deduce local or outgoing call type. The latter is denoted by the initial digit 9 and causes immediate seizure of an outgoing trunk if there is no bar on the class of service. The operation will be clear from what follows. The particular 100's group of a local called party is deduced from a binary/decimal translator so as to enable the appropriate subset of bus leads 107.) The common control also marks control lead LSE which permits the logic 108 in the called line circuit to provide a free/ busy marking on its output lead LI. If the called line is free the central control enables the register and trunk scanner 120 and also the trunk bid circuit 119. The first free local trunk, e.g. 114 responds over lead OT1 to stop the scan. The network controller 112A is then called to set up a connection from the called line circuit to the called side appearance of the selected trunk using the information presented to it over bus leads 107 and 103. With a path set-up as denoted by a mark on PC, the central control causes the register and trunk scanner 120 to mark the calling side, e.g. TO-A lead, of the trunk marking bus 103, releases the calling-line/register connection and gates the calling line identity from the register memory into the relevant line scanner via gate CNGO. The lead LSE is again marked whereby the calling line's logic 108 can apply a mark to lead LS00 so that his class of service can be determined. (It may be noted that as the calling- line/register connection is now dropped, the cut off relay in the line circuit is back so that the marks on LSE &c. can penetrate the logic.) In the local call considered, the scene is now set for the common control to ask the network controller 112A to complete the connection by setting up a path from the calling line to the "A" apperance of the trunk. The control circuit thereafter withdraws. If the called line is busy the common control enables a busy-tone-generator marking lead from the trunk bid circuit 119 and starts the trunk scanner 120. When the first free generator declares itself, the control removes the called number marking from the bus 107 and replaces it with the calling number marking with results as just described for a local trunk connection. Outgoing call.-The initial translation effected on the called number by the common control is effective in this case to cause the calling number to be gated out of the register on to the bus 107. In addition control lead LSE is marked so that class of service can be determined. If the call is to be barred, central control causes the trunk bid circuit 119 and scanner 120 to find a busy tone generator whereafter a connection between the calling line and the generator is set up as described above. If the call is to be permitted, the central control enables the trunk scanner and the trunk bid circuit to find a free outgoing trunk with an obvious result. Common control uses transistorized NAND building blocks (Figs. 3A-3G, not shown) which are used to form self stepping shift registers (Figs. 4A-4D, not shown) and a mode control circuit (Figs. 5A, 5B, not shown). The latter is in the form of a chain of JK flip-flops which is locked at a particular stage when this is enabled so as to produce a significant output. The chain is reset by a common reset pulse.