Patent GB-596161-A (GB)
Title: Improvements in or relating to automatic telecommunication exchange systems
IPC: H04Q3/00
Filed: 1945-07-23
Abstract: 596,161. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd., BRAY, F. H., BROWN, L. R., and RIDLER, D. S. July 23, 1945, No. 18826. [Class 40 (iv)] In a system employing finder selector links, cold cathode discharge tubes are employed for detecting a calling condition, for detecting the calling line and for testing the condition of the wanted link. The selectors are 25 point switches which are set in succession by two dialled digits. A subscriber's line circuit and a link circuit are shown in Fig. 1. There are four link circuits serving 25 line circuits and each link circuit comprises a line finder switch FS and a selector switch SS. Battery is connected to one wire of the line over a resistance YLA of 10,000 ohms and the other line wire is connected to resistances YLB and YLC each of 10,000 ohms and the test terminal is connected at their junction. The second wire is also connected over resistance YLD of 50,000 ohms to selector terminals for determining the busy or idle condition of the line, cold cathode tube FDT (Fig. 2) being used for the testing. When a subscriber makes a call, flow of current in transformer TRA in the allotter circuit shown in Fig. 2 ignites a cold cathode discharge tube STT and operates relay ST which connects relays STA and STB to start the allotter finder. When the calling line is reached, tube FDT ignites and FD energizes stopping the finder magnet and operating guard relay B which through relay BB applies earth to the T lead to kick off the allotter. If all link circuits are busy, relay OB prevents advance of the allotter until a link becomes free and connects relay RST in place of ST. Relay B extends the line to relay A and the P and S terminals are marked as busy by negative and positive potentials respectively. Relay CC energizes, prepares the impulse circuit, including series relay A, and operates relay TR which applies earth to a lead to start a circuit of common type and connects discharge tube TGT to relay A and via the lead DTL to a condenser and resistance combination. The tube TGT operates with its circuit as an oscillator to generate dialling tone. The subscribers are numbered 21 ... 20, 31 ... 30 and 41 ... 45. If the first digit is 2, MK energizes over wiper H after the relapse of CA disconnecting TR and holding CC. If the first digit is 3, the selector steps by self interruption from position 4 to position 13 where MK pulls up and re-operates CC which has fallen back meanwhile. If the first digit is 4, relay W energizes in position 5 and permits the selector to go to position 23 where MK operates as before. When the second digit is dialled, as relay MK is now operated, relay SD is connected up by CA so that at the end of impulsing the discharge tube TGT is connected to the P wiper. If the wanted subscriber is busy, the potential on his P lead will be either negative 50 or 17 volts according as to whether he is the calling or called subscriber. One electrode of tube TGT is connected to the P wire and if either of the above potentials are on this wire, the tube does not ignite and busy tone is generated in relay A. If the subscriber is free tube TGT ignites and operates switching relay H. The latter connects negative 50 volts to the called subscriber's P line and the circuit of tube TGT is changed to generate ringing tone. Ring-trip relay F opens on reply and completes the connection. A tone generating circuit is individual to each link circuit and consists of a, series of condenser and resistance combinations connected to leads DTL, RTL and NUT to form circuits with tube TGT for generating tones. A series of relays are also provided which operate in cycles to give the interruptions characteristics of ringing and busy tone. Details are given of the duration and frequency of these interruptions. Abnormal operations. If the calling subscriber dials 1 as the first digit, relay Z energizes and holds TR, and on the relapse of CC the circuit of tube TGT is changed to generate NU tone. Similarly, if a first digit greater than 4 is dialled, the operation of W in position 5 of the selector transfers the impulse circuit to Z which responds to the next impulse, stopping the selector and re-operating TR to send NU tone. If a second digit greater than 5 is dialled after the first digit 4, the selector reaches position 4 in which MK is short-circuited and TR re-operates. In the case of failure to dial, relays SD, PG are operated by time pulses on leads APL, ZPL and the link circuit is left locked to the calling line with only PG, BB, Z up. The link, however, may be freed by the response of relay RST to a calling condition when all links are in use. When start relay ST energizes, it makes a charging circuit for a condenser QB in order to operate relay FR through a discharge tube FDT should the calling subscriber clear before his line is found, thus releasing ST. Specifications 541,768, 567,864 and 567,874 are referred to.