1393156 Automatic exchange systems PLESSEY CO Ltd 4 May 1973 [19 May 1972] 23685/72 Heading H4K A connection between two subscribers utilizes two substantially identical supervisory units, each unit being coupled to a respective subscriber's line circuit via a subscriber's switching network which acts as a concentration stage, and the two supervisory units are linked through a route switching network of the exchange. Such an arrangement allows for a purely A.C. connection through the route switching stage between the two supervisory units which supply D.C. line current to their respective associated subscribers line circuits. The arrangement is applied in a stored programme controlled exchange having reed-relay crosspoint switching matrices and utilizing a memory map and a marker for path selection through the route switching network. Trunking.-(Figs. 1-4).-A plurality of subscribers SUB having individual line circuits SLC are connected via a three stage subscriber's switching network SSO, to 120 supervisory units each being of the type shown in Fig. 6, the even numbered units being connected to the left hand side of route switching network RSN and the odd numbers to the right hand side. Several subscriber switching networks may be provided and their associated supervisory units are connected to the route switching network in like manner. Also connected to the route switching network are 30 junctions IR for connection to the route switching networks of similar stored programme controlled exchanges of which the processing equipment is connected by a data-link to the processing equipment of this exchange. Incoming junctions 1CJR from other types of exchanges, e.g. stronger and register controlled crossbar exchanges, pass via supervisory units 1SUO and a junction access network JAN and outgoing junctions OGJR to such exchanges include supervisory units OSUO. The supervisory units ISUO and OSUO differ in detail from the units SU6 exemplified by Fig. 6, but are similar in that they provide D.C. isolation of the path through the route switching network from the incoming or outgoing junction thereby ensuring a purely A.C. coupling through the network RSN between two supervisory units. The supervisory units SUO &c. of two subscriber switching networks are coupled by a four wire connection AAP to a number of auxiliary circuits AC0 to AC3, e.g. digit registers (loop disconnect and/or coded voice frequency), coin and fee checking equipment, tone supply devices, ringing equipments, operator's access circuits &c, via a further reed-relay marker controlled crosspoint network AASN. Incoming and outgoing supervisory units 1SUO and OSUO also have access to auxiliary circuits 1ACO-1ACN and OACO-OACN over networks 1AAN and OAAN respectively. Local to local call.-In response to the off hook condition the subscriber line circuit extends its identity and signalling condition to the dataprocessing equipment described in Specification 1,325,194 and a marker is caused to set up a nominated connection to a supervisory unit SU. A four wire path is then set up via network AASN between the supervisory unit and a digit register which returns dial tone on wires - and + of path AAP. In response to the digits dialled a path is set up between the calling supervisory unit and the line circuit of the called subscriber via route switching network RSN, a further supervisory circuit SU associated with the called subscriber, and the corresponding subscriber switching network, it being noted that a line split exists at contacts CA1, CA2 in both the supervisory units. The called subscribers supervisory unit is then connected to a ringing current generator and a "ring-trip" device by wires - + and to a ringing tone generator by wires 1A, 1B before the subscriber is rung the ringing equipment extends wires 1A, 1B to a tone-test circuit and in the calling supervisory unit the wires 1A, 1B are extended through the register to a similar tone-test circuit. A V.F. tone is then transmitted from the calling to the called supervisory unit and an acknowledgement tone of a different frequency is returned, thereby testing the speech connection. In response to a successful test the connection between the calling supervisory unit and the register is dropped and relays AN and CA are operated to connect talking battery to the calling line and to remove the line split, relay AN also connecting in a transistor loop-detecting device LD. Ringing current is supplied via a ring trip device to wires 1A, 1B at the called supervisory unit and ringing tone to wires - + from which it is extended back through the route switching network to the calling subscriber. When the called subscriber answers, the ring trip device is activated to cause release of the connections from the called supervisory unit to the ringing equipment and to cause relays AN and CA to be operated. The call proceeds with the connection between each subscriber and his respective supervisory unit being monitored by a respective transistor loop-detector LD, these detectors being scanned by a scanner/distributor such as S/D. Calls involving incoming/outgoing junctions take place in similar manner. In the above two wire speech connections are used through the route switching network, but a facility for four wire connections may be provided for incoming and outgoing calls involving line carrier and/or PCM transmission links. A number of the subscriber supervisory units may include hybrid transformer circuits in such an eventuality. Two wire in place of four wire connections may be provided between the supervisory units and auxiliary circuits with the supervisory units being modified to allow for the application of ringing tone via an additional winding of the transformer RC.