Patent GB-248899-A (GB)
Title: Improvements in circuit arrangements for automatic telephone systems and the like
IPC: H04Q3/00
Filed: 1925-02-09
Abstract: 248,899. Siemens Bros. & Co., Ltd., and Patterson, W. G. Feb. 9, 1925. Automatic exchange systems.-A rotary line switch of the normal type which fails to find an idle outgoing line moves to special " congestion " contacts at which dialling tone may be transmitted and is there arrested until impulse transmission commences when it moves to further special contacts at which a busy signal is given. If the switch, having seized an idle outgoing line, is freed therefrom whilst the calling line still remains bridged (e.g. if the subscriber fails to dial or a fault exists on his line). the switch moves on without seizing a further line to the " congestion " contacts at which it may give the dialling tone and await impulse transmission as in the case when all trunks are busy. Alternatively, it may pass over the congestion contacts and move directly to the busy-signal contacts. In further modifications, a special " number unobtainable " tone or a " howler " tone or no signal at all may be transmitted in the case of release from a seized trunk. First arrangement, shown in the Figure (in which the references dc indicate disconnected contacts). When the calling subscriber removes his receiver, his line relay R energizes over confact t<4>, wiper A which bridges its end and normal contacts, the calling loop, wiper D and its normal contact, and contact t1. The stepping magnet RM operates over the circuit r1, t5, C, r2, and hunting commences, relay R and magnet RM remaining operated, as long as busy lines are encountered, over contacts r1, t5, and the test wiper C. If all outgoing lines are busy, the switch passes to the " congestion " contacts (fourth from the bottom) at which relay R releases to cut the drive and relay T operates over contact r2 and locks up over contact t5, wipers C, A, contact t2, the calling loop, contact t3, wiper B, and dialling-tone transformer DS. When impulsing commences, relay T releases, magnet RM operates over contacts r1, t1, and wiper D and moves the switch to the next contacts at which relay R operates over wiper D and completes a further circuit for magnet RM which moves the switch to the busy-signal contacts. Relay T now operates again, and relay R remains locked up over wiper A, the calling loop and wiper B, a busy tone BB being transmitted over this circuit. Relay R being slow to release does hot respond to further impulses, and the switch remains without further movement until the subscriber hangs up, when it moves a further step to normal. If a free outgoing line has been seized and the line switch is released therefrom owing to the call being due to a fault, relay T releases, magnet RM operates over wiper D, and since relay R is de-energized and relay T is therefore unable to respond in passing over further free lines, the switch moves to the " congestion " contacts and thence continues to the further contacts from which the busy tone is transmitted. Modifications. In order that the switch may stop on the " congestion " contacts and the dialling tone be transmitted in the case of release from the seized apparatus ahead, the contacts preceding the " congestion " contacts are, in the talking and test banks, left disconnected and, in the bank of wiper D, connected to the contact t4 so that relay R operates when these contacts are reached whilst relay T operates at the " congestion" contacts, cuts the drive, and remains locked up over the loop until impulses are transmitted. In another arrangement, the " congestion " contacts are sixth from the end of the bank and the contacts fourth from the end are associated with a " number obtainable " tone, the latter being transmitted over the B-wiper when a release occurs at the switch ahead with the calling loop still closed. In modifications of the latter arrangement, the B-wiper may be connected directly to earth, in which case no signal is given or may be connected to a relay which connects up a " howler " tone and may close a delayed-alarm circuit. Specification 241,655 is referred to.