GB1080622A - Improvements relating to automatic telecommunication switching systems - Google Patents

Abstract

1,080,622. Automatic exchange systems. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. June 22, 1965 [July 2, 1964], No. 27368/64. Heading H4K. Permanent calling condition, resulting in the disconnection of holding earth from the P-wire over primary switch PS, causes the release of relay KB but not of KA so that a gate PG allows scanning pulses on gate GX<SP>1</SP> to open gate GY<SP>1</SP> to produce signal L2. The line circuit exhibits calling condition on resistor R4 to prime gate CG so that scanning pulses on gate GX open gate GY to produce signal L1. Earth on the P-wire operates KA and KB, cut-off relay KK following. Release is effected with negative battery on the P-wire to drop both KA and KB with KK following. Permanent calling condition with KB back but KA held up, leaves KK in a circuit with insufficient energizing current. So long as scanning finds signals L1 and L2 to indicate permanent calling condition, no action is taken, but as soon as the line goes on-hook to remove signal L1 the line circuit is connected up for the return of negative battery over the P-wire to release KA. The relays employed are preferably reed relays and the circuit is preferably constructed in modular form. Where the system is such that, with resistive negative battery applied to the P-wire to release the line circuit, a following call causes the concurrent connection of earth to the P-wire before KA has released, a Zener diode in the path between KA and its negative battery supply produces a drop of a few volts sufficient to balance the voltage drop across the diode MR1 which shunts KB. Relay KA is thereby allowed to release despite the following call.