Patent GB-253583-A (GB)
Title: Improvements in automatic telephone systems
IPC: H04Q3/42
Filed: 1925-03-16
Abstract: 253,583. Standard Telephones & Cables, Ltd., (formerly Western Electric Co., Ltd.), Moir, R. A., and Hartley, G. C. March 16, 1925. Semi-automatic exchange systems.-A coder for transmitting the digits of a wanted number over an interexchange trunk in the form of code impulses to control a number indicator at a distant manual exchange comprises the following features :-(1) The coder is marked busy under the control of off-normal contacts of the impulsereceiving and code-sending switches so that in the event of premature, release, the coder cannot be seized again until its internal restoration is complete. (2) The code-sending switch is stepped by motor-driven interrupter springs, and an automatic test is made at the commencement of code sending to ascertain whether the springs are in operative condition. This is effected by a twostep relay which will not fully operate if the springs are either permanently open or permanently closed, and in either case an alarm is given. The provision of the two-step relay also ensures that the code-sending switch shall not be operated by less than a full impulse. (3) The incoming impulse wire is opened during release of the coder to prevent any possible operation of the impulse-receiving switch at this time. (4) The resistances which are automatically inserted into the code-impulsing circuit in order to compensate for the varying resistances of outgoing trunk lines are associated with both front and back contacts of the special compensating relays. Detailed operation. When the coder is seized, lamp SL lights over the earthed guarding conductor RT. Impulses incoming over wire PW pass through relay A to the stepping magnet PRM of the impulse-receiving switch PS. Relay A remains operated during the digit and operates relay B. At the end of the digit, the relapse of relay A causes the partial operation of two-step relay M and also connects earth to wipers 2, 3, 4 of switch PS so that if, for example, the first (compensating) digit is 1, compensating relay N is operated and both resistances R2, R3 are inserted in the impulse circuit. These compensating resistances are associated with both back and front contacts of the relays F, N in the manner shown. When relay B falls back, relay M operates fully and a restoring circuit is completed for switch PS over magnet PRM and its interrupter contacts. Succeeding digits are similarly recorded on the relays MA - - MD, &c. and at the conclusion of the last digit the relapse of relay A operates relay S partially and also completes a circuit for relay D which energizes relay J and locks up over the outgoing trunk conductors. Relay J starts the motor for driving the interrupter contacts MTS. When the apparatus at the distant exchange is ready to receive impulses, relay D is unlocked and completes a circuit from the earthed lead RT over contacts C6, S33, wiper 1 of switch CSS, and springs MIS for the upper winding of two-step relay K which operates partially as soon as the springs close and pulls up completely when they subsequently open. The complete operation of relay K completes a holding circuit for relay J, but if the springs MIS are faulty, relay J releases and closes an alarm circuit. Relay K also connects the springs MIS to magnet CRM of the code-sending switch CSS which is thus stepped round, a series of code impulses of varying strength and polarity being transmitted over leads - and + in accordance with the setting of relay registers MA - - MD, &c. At the conclusion of the last digit, relay C operates in series with magnet CRM, operates a meter, earths lead PW to cause the connection to be switched through and unlocks all the recording relays in the coder. Magnet CRM continues to step switch CSS until the normal position is reached, when relav E releases and in turn releases relays K, J. Since the circuit of magnet PRM includes contacts 6 of relay J, this magnet is prevented from operating in the event of an impulse being transmitted over wire PW during release. Since relay E is operated over one or other winding whenever either of the switches PS, CSS is off normal and grounds conductor RT. the coder is guarded against further seizure in the case of a premature release until both switches have fullv restored. The continued energization of relay E operates a delayed alarm and flashes the supervisory lamp SL.