Patent GB-768953-A (GB)
Title: Improvements in telephone exchange systems
Inventor: FOSTER WILLIAM GORDON
IPC: H04Q3/00
Filed: 1954-09-09
Abstract: 768,953. Automatic exchange systems. TELEPHONE MANUFACTURING CO., Ltd. Sept. 9, 1955 [Sept. 9, 1954], No. 26205/54. Class 40 (4). In a small exchange system privileged subscribers can obtain direct access to certain lines through bridges incorporating finger magnets such as are described in Specification 703,931. Direct-access call. The privileged subscriber closes his loop and operates one of keys 1-5 up or down according to which of ten lines he wishes to call. One or more of relays W, X, Y, SW (but not W and Y in combination) energize to operate FP and one of the finger magnets FM1-FM5, the operated finger magnet interposing its finger btween the armature of the bridge, Fig. 3B, and the bank of associated contacts b1-b5. FP energizes relief relay FPR which energizes magnet CB of the bridge. CB therefore attracts its armature in the manner described in Specification 703,931 to close all the contacts, say b1, corresponding to the finger magnet energized. Depending on whether the key 1-5 was raised or depressed so relay SW was operated or not to select the upper or lower set of contacts in the bridge. Magnet CB locks up and connects interrupted earth through rectifiers MR1 to one side of relay H whose other side is connected to the P wire of the wanted direct-access line. CB also disconnects the privileged line from its line relay equipment, Fig. 2, and connects it to the communication bridge. Wanted line free. If the wanted line is free, H pulls up to battery on the P wire. H releases FP to apply ringing current and tone. When the called party answers F pulls up, cuts off ringing current and tone, energizes A and switches through. Relay A re-operates FP. Relays A and F serve as calling and called station feeds respectively. Wanted line busy. Breaking in. If the wanted line is busy, H is unable to operate and lamp LP2 at the privileged station flashes. If the caller wishes to break in he depresses key TNT which operates W and Y to energize PR. PR locks up to busy earth on the P wire of the wanted line and connects a 400 p.p.s. tone to its + wire. Pr also releases FPR slowly. A triangular connection is thus obtained. When the two parties to the original call hang up, earth is removed from the P wire. PR therefore falls back to re-energize FP and connection to the wanted line is made as in a call to an idle direct-access line. Holding incoming call while calling a directaccess line. If a call is made to the privileged subscriber over the dashed-lines in the left part of Fig. 2, earth on the P wire is extended through relays IC, L and K. Ringing current energizes RG through rectifier bridge MR2 to operate buzzer BZ. When the privileged subscriber answers ringing is tripped and RG restores. If the privileged subscriber now wishes to call a direct-access line, he operates the appropriate key 1-5. Magnet CB eventually operates, as previously described to disconnect the privileged line from the incoming line, contacts cb1 connecting resistance battery to the + ve incoming wire to hold the connection. When the key 1-5 is released the connection to the incoming line is restored. Modification. Figs. 1 and 4 may be replaced by Fig. 5, in which the keys 1-5 operate finger magnets FM1-FM5 directly, in series with FP. If the key is depressed W operates, whereas if the key is raised W and SW operate. Break-in key INT operates directly on PR.