Patent GB-536058-A (GB)
Title: Improvements in telephone systems
IPC: H01H67/26
Filed: 1939-10-25
Abstract: 536,058. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Oct. 25, 1939, No 28694. Convention date, Oct. 31, 1938. [Class 40 (iv)] In a small, e.g. 100 line system, each group of (say) 20 lines has a cross-bar switch over which all calls to and from the group are made via link circuits common to all the switches. When subscriber A, Fig. 4, calls, the selecting magnet 1FG1, 2FG1 &c. corresponding to the incoming side of the next link to be used, as determined by apparatus 100, is energized and is followed by the individual operating magnet L11 of the calling line. Minor switches individual to the link then respond to the digits of the wanted number to operate the selecting magnet 1CG1, 2CG1, 1CG2, 2CG2 &c. corresponding to the outgoing side of the link in the cross-bar switch of the wanted group and then the individual operating magnet such as L31 of the wanted line. The connection is maintained by the operating magnets, the selecting magnets being released. Construction of switches, Fig. 6. Selecting magnets such as 622 raise a number of sets of flexible contacts such as 601, 603 into the path of depressions in the ends of co-operating contacts such as 604, 606 controlled by operating and holding magnets such as 628. The contacts 604, 606 are so shaped that no connection is made if magnet 628 is operated before magnet 622. Circuit details. Line relay 160 energizes group start relay 101 which, provided that none of the link circuits is testing a wanted line and all relays 255 therefore are de-energized, energizes 150 in the allotting circuit and this is followed by the relays 120, 130 &c. of idle link circuits and the selecting magnet 1FG1, 2FG1 &c. corresponding to the earliest of the operated ones of these relays. This magnet, 1FG1 in the case assumed, makes a holding current for the line relay in series with the individual operating and holding magnet L11 which energizes cut-off relay 170 and extends the talking and holding circuits to the link circuit in which line and release relays 210, 220 pull up releasing the selecting magnet and relay 120. Relay 160 is shunted out by the holding ground from the link and releases 101. The first digit is repeated to magnet 305 of a minor switch 300 which over wiper 301 picks out the selecting magnet corresponding to the link in the switch of the wanted 20 - line group. On the relapse of dialling relay 215 at the end of the digit, 230 which was previously operated partially makes the second step and transfers the impulsing circuit to magnet 315 of the switch 310 which in response to the units digit picks out the operating and holding magnet of the wanted line, wiper 311 or 313 being effective according as the tens digit was odd or even. At the beginning of the units digit, 20 is energized and 240 makes its first step and at the end, 240 operates fully and 230, 250 fall back in succession to provide a testing interval during which 255 is operated to energize the selecting magnet chosen by wiper 301. This completes a test circuit via relay 280 and the operating and holding magnet of the wanted line. If the line is busy 280 is unable to operate and busy tone is sent back. If it is idle, the operating magnet pulls up in series with 280 which substitutes the ring-back tone for the busy-tone circuit. In both cases the relapse of 255 releases the selecting magnet. Tne line is rung in the ordinany way and the connection is completed by the ring-trip relay 270. Back-bridge relay 260 puts an alternative ground on the holding wire. When the calling.party hangs up the switches 300, 310 are restored and the connection is released.