Patent GB-295446-A (GB)
Title: Improvements in telephone systems
IPC: H04Q3/00
Filed: 1927-05-10
Abstract: 295,446. Standard Telephones & Cables, Ltd., Wright, E. P. G., Baker, J. H. E., and Cameron, A. H. May 10, 1927. Divided on 295,421. Automatic exchange systems.-Single-motion step-by-step selectors set by marking switches each common to a group of selectors are controlled over two-wire inter-exchange junctions. The outgoing end of each junction is provided with a relay group which controls the busy condition of the junction, and when a particular junction is seized all other junctions leading to selectors controlled by the same marking switch are marked busy by signals transmitted back over the junctions to the corresponding relay groups. In one arrangement, identical with part of the system described in Specificaticn 295,421, the marking switches are set by impulses from a register-controller and the junction relay group, which has a part common to all relay groups associated with junctions leading to selectors controlled by the same marking switch, extends a direct metallic circuit. In another arrangement, Figs. 1 and 2, a complete relay group is individual to each junction and acts as a repeater for impulses received directly from a subscriber's dial. The selectors described have two sets of wipers associated with different numerical groups. Normal operaticn of system, Figs. 1, 2. Line relay A and release relay B complete a loop across the junction causing the operation of relay K which associates the common control circuit (below the dotted line, Fig. 2) with the selector and extends the loop to relay 1. Relay 4 operates and places earth over wiper N2 to operate a busying relay BG in each of the other junction relay groups to which the marking switch is common. The impulses are repeated by relays A, 1 to magnet SCDM of the marking switch, and as soon as this switch moves off normal a circuit is completed for main-switch magnet SPM over contacts 5 (K), 1 (9), 1 (8), and wiper SC1. Wiper SC2 connects earth to groups of contacts in the banks CA, CB over which a relay 8 operates to cut the drive if the main switch overtakes the marking switch. At the end of the impulses, the relapse of relay 3 short-circuits contacts 1 of relay 8 so that the main-switch wipers may advance over the wanted group. When an idle trunk is found, test relay HA (or HB) operates over wiper TA (or TB) and bank SC3 and switches through. Relays K, 1, 4 release and a restoring circuit is completed for magnet SCDM, relay 3 operating in parallel to hold the common circuit busy until it is completely restored. At the end of conversation the release of the test relay completes a restoring circuit over wiper N1 for magnet SDM, the mainswitch being marked busy by earth over wiper N2 until restoration is complete. The highimpedance relay I, Fig. 1, is normally shunted by a resistance R which it cuts out on operating, this arrangement preventing the sudden insertion of the full impedance at the end of a series of impulses frcm giving a false impulse. All trunks busy. If no idle line is found in the wanted group, rotation proceeds until the normal contacts are reached when relay 9 operates over wiper CA or CB in series with relay 8, stops the main switch, and connects up busy tone over relay 1. A statistical meter may be operated' over wiper SC4. No impulses received; permanent loop. A periodically earthed lead TP controls relays 5, 7 so that if no impulses are received within a predetermined time relay 7 completes a circuit for relay HA over wiper TA and key PGK. Relay HA releases relay K and the common marking switch, and completes a circuit from earth on wiper N1 for a lamp connected over wiper TB. The selector is not released until the attendant operates key PGK, and during this time the trunk leading to the selector is marked busy, at first by relay B and after the release of relays A, B if the calling party hangs up, by relay BG which operates over earth on wiper A.