GB1475809A - Telephone system - Google Patents

Abstract

1475809 Automatic exchange systems, transmission gates SIEMENS AG 16 Aug 1974 [22 Aug 1973 (4)] 36086/74 Headings H4K and H3T In a telecommunications system employing bidirectional electronic cross-points a register junctor is used as an off-hook detector. In one embodiment shown the cross-points such as TAa to TCb are photo-triacs controlled by thyristors such as TYA to TYC and light emitting diodes LDA and LDC in holding wires each connected between a transistor T and a positive supply, the thyristors being in turn controlled by driving transistors MA to MC. In operation the control unit ZE/WS/M selects a free connection set VI and sequentially connects it via the network to each free subscriber line in turn, the addresses of busy subscribers being stored in the common control. When the junctor is connected to a substation that has just gone off-hook a detector AWA in the junctor responds to the loop and signals to the control stopping the scan, another free junctor being selected and allocated for scanning at this point. The junctor causes dial tone to be transmitted to the caller. As this tone includes zero transistions the triacs in the path repeatedly switch off, but are immediately turned on again by virtue of the hold wire. Dial impulses are generated by simultaneously opening contacts 2nsi, 3nsr producing loop resistance reductions rather than loop breaks. Thus the thyristors remain conductive when dial tone is removed and the hold wire may be de-energized at this time. The impulses received by the detector ASA may be analysed in the junctor or the common control. When the identity of the called party is established it is determined, by the busy subscribers addresses stored and/or by the loop state of the called subscriber as determined by a second analyser on the called subscriber side of the junctor, whether the called subscriber is free. The connection when established is monitored by the junctor a loop break being recognized as a disconnection request. As described the exchange is a PBX with call transfer/enquiry facility and to allow for this in the embodiment shown the hold wire is maintained throughout the connection. To effect a transfer either party operates an earth key ET earthing one wire of the path. The analyser on that side signalling to the control unit that such a request has taken place. The hold wire constitutes a record of the path employed for the original connection and thus the other extension involved to allow the latter to be re-connected when required. In another embodiment using conventional rather than light controlled triacs, the hold wire cannot be maintained during speech and the junctors include subscriber address stores to allow for the re-establishment of a connection.