Patent GB-491865-A (GB)
Title: Improvements in telephone systems and automatic switches suitable for use therein
IPC: H04Q3/42
Filed: 1937-03-10
Abstract: 491,865. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., BEALE, C. E., and RICHARDS, 1. T. March 10, 1937, No. 7089. [Class 40 (iv)] A bidirectional group selector having access to long distance trunks, after the selection of a level, does not hunt until it receives a signal which unbalances a differential relay in the incoming loop, this signal being generated in the sender when the operator keys the digit 1 after the exchange designation. The final digit 6 instead causes the selector to hunt over a subgroup of high grade trunks located in the latter half of the level. If any other final digit is received, a particular trunk is selected and if busy is seized when it becomes idle. A busy signal is sent if the selector moves to position 11 without finding an idle trunk, and is cut off in response to a signal from the operator. The operator receives a signal when a trunk becomes idle and she may initiate a further search over the level. The selector described is of the kind which continues its rotary movement to an extreme position to effect release. Hunting over a complete group. Upon seizure line relay A energizes over the loop while CS energizes over wire S and connects up release and dialling relays B, C, and the vertical magnet VM responds to impulses, after which C falls back. If the final digit is 1, the sender momentarily unbalances the loop as described in Specification 491,901, to operate differential relay DF, whereupon test relay TA energizes, locks under the control of the rotary magnet RM, connects a third winding to test wiper P, and operates AR. On the relapse of C, the rotary magnet RM operates in a self-interrupting circuit which is dependent on TA being maintained over wiper P. When an idle trunk is reached, the relapse of TA connects up switching relay HX in series with H. If no idle trunk is found, TA falls back in position 11 and C, SR operate over cam springs S. Relay SR extends battery over wiper PS to storage equipment of the kind described in Specification 444,256, while G energizes and sends back a busy signal. Actuation of the operator's ring call key again operates DF, TA to disconnect G and cut off the busy signal. As soon as a trunk becomes idle, intermittent earth applied over wiper S is repeated back by TF to flash the supervisory lamp. Actuation of the unlock call key connects positive battery to wire S to operate RL, which disconnects SR, holds TA, AR and connects up the vertical magnet VM, the latter being arranged to bring about rotary release in the manner described in Specification 391,123. Relay RL then falls back and completes the original hunting circuits. Hunting over a subgroup. On the relapse of C after the vertical movement, CR, E pull up, transfer the impulse circuit to the rotary magnet RM and re-operate C. Assuming that the final digit is 6, C, CR fall back in turn and connect HS momentarily to wiper P1, which is connected to battery in this case. A circuit is made for TD, which pulls up if the first trunk of the subgroup is idle and connects up switching relay HX in series with H. If however this trunk is busy, TD is shortcircuited over wiper P, E falls back on the relapse of CR since HS is now up, and C reoperates and makes a hunting circuit for the rotary magnet RM under the control of stepping relay G. When an idle trunk is reached, G holds in series with TD. If however all the trunks of the subgroup are busy, G holds to earth over wiper P1 on the last trunk and sends a busy signal on the relapse of HS which holds up during hunting. Actuation of the unlock call key brings about rotary release and initiates a fresh search over the whole group. Selection of a particular trunk. The selector is set as in the preceding paragraph, and the trunk is tested by TD. Since HS cannot operate when the final digit is other than 6, E is held after the relapse of CR and maintains the test circuit, so that the trunk, if busy, is seized as soon as it becomes idle. This is indicated to the operator by the darkening of the supervisory lamp by the inclusion in the wire S of a high resistance in the trunk relay set.