GB994459A - Improvements relating to electrical communication systems - Google Patents

Abstract

994,459. Automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Sept. 19, 1961 [Sept. 22, 1960], No. 32617/60. Heading H4K. In a system using earth return, each subscriber's line circuit being coupled by a transformer to a link path forming part of the speech path, a flow of direct current in a line circuit to signify a calling condition thereof brings about the operation of a relay in the calling line circuit and is caused to modify the potential at an intermediate point in the associated link path, thereby producing a calling signal identifying the calling line circuit to common control equipment. As shown, Fig. 1, the flow of direct current in the looped line wires l1, 12 of a calling subscriber operates relay RA and also causes the potential at the base of a transistor CT to become sufficiently negative to render the transistor conducting, thereby producing a calling signal which identifies the calling substation to a selective electrical marking arrangement M of the type described in Specification 901,837. This operates relay RB to connect common marker equipment CE to the link path LP which is thereupon extended through a local concentrator LC comprising an array of four-layer diodes D, as described in Specification 881,172, to a free supervisory link circuit SLC and register. Return of positive potential from SLC, as described in Specification 994,458 cuts off the calling transistor CT. In a modification, Fig. 2, the calling transistor CT is made conductive by the operation of the impulsing and answering relay RA on completion of the calling loop.