Patent GB-740424-A (GB)
Title: Improvements relating to telephone systems
Inventor: LONG DOUGLAS PERCY
IPC: H04Q3/00
Filed: 1953-09-01
Abstract: 740,424. Automatic exchange systems. SIEMENS BROS. & CO., Ltd. Aug. 20, 1954 [Sept. 1, 1951], No. 24209/53. Class 40 (4). In a system in which a normally by-passed feed bridge circuit placed before the final selector is switched into circuit in response to a backward signal, when the final selector is one giving access exclusively to party-lines having a single appearance in its banks the feed bridge circuit is provided with means for responding to code digits for the selection of the appropriate ringing code and for controlling ringing. As shown, the final selector, Figs. 4, 5, is of the motor-driven type controlled by marking from a common control circuit, Figs. 6, 7, substantially as described in Specification 592,989. The normally bypassed feed.bridge circuit, Figs. 1, 2, 3, is switched into circuit by the operation of relay BC. One of 20 ringing codes is selected in response to two code digits which may be 1 .. 4 and 1 .. 5 respectively. Reception of digits. Upon seizure of the final selector relay A energizes alone if the hundreds digit received at the preceding selector is even or, if it is odd, in series with P which controls wiper-switching relay WS Guard relays B, BA and dialling relay CD pull up, as well as coupling relay K in the selector. The tens and units digits are repeated by A to step switches DA, DB, changeover relay PA being connected up at the first relapse of CD and disconnected at the second. Wire o is thus earthed momentarily to operate BC in the feed circuit and the incoming loop is connected to relay AA which pulls up followed by guard relay BB and dialling relay DC. The 10-point switch CS stops in response to the first code digit, and, when DC falls back, transfers its record to storage relays WA ... WD. Relay XA pulls up, CS steps to position 5 and then takes the second code digit. If an excessive number of impulses are received for either digit RS operates and returns N.U. tone. After the final setting of the switch CS, relay XB energizes and loops wires L, J to operate latch magnet MU and start the final selector. Wanted line free; ringing. Test relay T energizes when the final selector stops on the wanted line followed by TA, TB and switching relay H, the common control circuit being disconnected. If the call originates from a local subscriber, TD energizes when the tens switch DA moves off-normal and H connects resistance earth to wire m. If, on the other hand, the call is set up by a toll operator TD is short-circuited over wire p, TA connects battery to that wire to signal that the line is free and H connects direct earth to wire m. In the former case relay XC responds followed by RG, XD and BS. Relay RS is connected to a code ring start wire and ringing takes place under the control of code relay CR in accordance with the setting of the switch CS and storage relays WA ... WD. In the case of a toll call, RG, XD are short-circuited over wire m. and the operator starts ringing by earthing the + wire to energize differential relay TR, whereupon RG operates over its left-hand winding. When the called party replies, earth applied to the centre point of the telephone instrument operates relay D in a simplex circuit. Ringing is cut off and the connections to the incoming loop are reversed. Wanted line busy. The final selector goes to its last position in which T energizes in series with LC (TA being differential does not respond), and the latter transmits busy tone and also busy flash if TD is back. Resistance battery is connected to wire n to cause the release of BC which is differential and thus switch out the feed bridge circuit.