Patent GB-560120-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q1/32
Filed: 1942-09-17
Abstract: 560,120. Automatic and semi-automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., TAYLOR, R., and BAKER, G. T. Sept. 17, 1942, No. 13098. [Class 40 (iv)] In a system wherein calls are set up by an operator by dialling over a long junction, the impulses are received by a pair of grid-controlled gas-discharge tubes GTA, GTB which are coupled to the junction by a transformer T2 and respond alternately at the beginning and end of each impulse, and at the conclusion of dialling the current over the junction is reversed to operate the tube which responds at the ends of the impulses and so energize a switching relay SK which disconnects the transformer T2 and extends the junction to the incoming selector. When the calling operator throws her dialling key, relays RR, RP pull up and battery and earth are connected to the junction by way of relay RS and inductances T1 which with a shunt condenser QD and resistance YG form a low-pass filter. Relay RS energizes and connects up DT, which shunts out RR and makes a circuit for impulse relay A. The battery and earth connections to the junction are now shorteircuited, and the short-circuit is then openedintermittently as relay A responds to dialled impulses. The initial closure of the junction circuit produces no effect since the anode circuit of the tube GTB is open, but the voltage pulse resulting from the application of the short-circuit triggers the tube GTA into operation and energizes relay PD, which connects up KO, CD to seize the incoming selector and brings in a series condenser QH to hold itself temporarily and to extinguish the tube GTA. The grid bias voltages of the two tubes are made slightly different to compensate for their different cathode potentials, and condensers QF, QG and resistances YM, YP are provided to guard against false operation by parasitic surges, as described in Specification 556,326. Each impulse gives rise to two voltage pulses in opposite directions, the first of which triggers the tube GTB and energizes relay PS followed by PB which repeats the impulse, extinguishes the tube GTB and connects relay PZ to the anode of the tube GTA, so that it responds to the second pulse and restores PB. On the return of the dialling key, the resistance of the sleeve circuit is lowered to operate relay MR which reverses the battery and earth connections to the junction and opens the short-circuit across them. The resulting voltage pulse triggers the tube GTA and since PB is now back relay PD operates a second time and connects up KN, whereupon CD falls back and switching relay SK energizes and locks to the test wire. Relay KO releases slowly, and meanwhile there is no circuit over the junction for RS which therefore falls back and disconnects RP (DT having been released by MR) to effect switch-through at the outgoing end of the junction. Relay D responds to the reply and clearing signals in the usual manner. When the operator disconnects, the restoration of MR opens the loop over the junction to initiate the release of the incoming selector and in due course SK is unlocked. If the operator disconnects before completing dialling, the release of impulse relay A produces the same effect as the beginning of an impulse, and the prolonged operation of PB initiates the release of the incoming selector. The relapse of CD causes a momentary operation of SK which opens the circuit for RS to free the junction at the outgoing end. If the incoming selector fails to release, CD is held and SK cannot operate, so that the junction is maintained busy at the outgoing end by RS, RP and cannot be re-seized by re-insertion of the plug. Specifications 551,187 and 560,132 are referred to.