Patent GB-315291-A (GB)
Title: Improvements relating to automatic and semi-automatic telephone systems
IPC: H04Q3/00
Filed: 1928-04-11
Abstract: 315,291. Siemens Bros. &. Co., Ltd., and Long, D. P. April 11, 1928. Automatic exchange systems.-In a P.B.X type final selector that can be used for ordinary calls and for offering and completing trunk calls, arrangements are made whereby a group engaged test is placed on the first line if all the lines in a group are engaged and if a preferred call is to be connected when this condition exists the switch tests and breaks into the first line engaged on a local call. If all the lines in the group are engaged on preferred calls the switch steps to the last line and then returns the engaged signal. If a single line group is called for a preferred call, the switch extends the circuit if the line is free or if the line is engaged on a local connection. If a circuit in a P.B.X. group other than the first line of the group is dialled, the switch will not hunt and will connect only if the line is free. The operation of the circuit for ordinary connections is the same as that described in Specification 284,811 and the P.B.X. hunting is similar to that described in Specification 203,759. An additional wiper and bank D‹, Fig. 1, is provided and the first contact of each P.B.X. group is connected to break contacts in multiple, such as tp, of the preselector relays of the group. The other contacts of the group are permanently earthed. If an operator wishes to connect, a preferred call battery is fed over the a wire and relay TL pulls up, locking up via its second winding TL (ii) and contacts n4. The test relay T is disconnected at tl2 and trunk test relay TK is connected up. Relays A and N have pulled up and relay D operates (n4, D, a2, k3, w1). The vertical impulses are repeated to magnet VV by . relay A and at the end of the train relay D falls. back, G operates (n3, G, pr1 vertical off normal spring k3 made, w1) and D re-operates over g4. The rotary magnet is connected up at g2 and the rotary impulses are repeated as before. The rotary off normal springs w operate and at the end of the train A holds up and D relapses followed by G. If the line is free TK pulls up before G de-energizes (C‹, n3, tl2, TK, g3, d2) and the circuit is switched through. The operator listens on the circuit and hearing no conversation, she presses her ringing key. Relay L pulls up locking itself up over its second winding and l3, and short-circuiting its first winding to connect through a full metallic circuit at l1. If the first line is engaged and one circuit of the group is free, earth is extended from tp over wiper D‹ short-circuiting relay DF. Relay PR pulls up from earth on bank X (da, d4, n2, tk6, t1) connecting the interrupter to the rotary magnet (pr2, a1, g2) and locking relay G at pr1. The switch steps round, relay DF being kept short-circuited by the permanent earth over wiper D‹ and when the free line is reached, relay TK pulls up, breaking the circuit of PR at tk6 and the circuit is cut through as before. If the whole of the lines are engaged, earth is extended over D‹ and relay DF operates (g5, d5, DF, tl3) and prepares to connect wiper D‹ via df2, pr4 and resistance P to coil II of relav TK. If a local call is connected on the line, relay TK pulls up (d2, g3, tf1, TK, df4, P, tk5, pr4) and the circuit is extended. If the line is engaged on a trunk call, relay TK is shortcircuited by battery from another switch over l2, n3, and wiper C. In these circumstances the switch is stepped on as before by wiper da. When a line engaged on a local call is reached TK pulls up from earth on bank D‹, df2, pr4, etc. and the call is extended as before. If all the lines of the group are engaged on preferred calls the switch steps on to the last line of the group and wiper da finds no earth. Relay PR falls back releasing relay G and thus completing the circuit of relay TR at its second winding (g1, w2, TR (ii), a3). Busy tone is connected to the circuit (tr2, tk2, t2, s3, loop, s4, t3, tk3, tr3, battery). If a line other than the first line of a group is dialled, wiper db steps on to the bank X and relay PR is short-circuited. If the line tested is free, the circuit is extended. Otherwise relay G falls back after relay D and busy tone is connected by TR as before. Modified circuit, Fig. 2. The extra bank contacts of a group are not multipled and if all the lines of the group are engaged the contacts are connected to earth via contact gt1. Single line groups are permanently connected to earth as indicated by the dotted line. When an operator enters the circuit to connect a preferred call relay TL operates as before and the wipers of the switch are stepped to the circuit required. If a single line is called and if the line is engaged on a local call or is free, earth is extended over wiper D‹ and relay TK pulls up extending the connection and putting trunk busy potential on the extra bank contact at tk6. If the circuit is engaged on a preferred call, battery is received over wiper D‹ from a contact tk6 of another switch and relay TK is short-circuited. Relay TR pulls up and returns busy tone as before. If the first line of a P.B.X. group is dialled and one circuit of the group is free earth is not extended over wiper D‹. Relay PR pulls up connecting coil I of relay TK to wiper C to test the circuit and connecting the interrupter to the rotary magnet. When the free line is reached relay TK operates extending the circuit. If all the lines of the group are engaged earth is received over wiper D‹ from gt1 and consequently TK operates over its second winding as soon as a line connected on a local call is tested. Specification 15546/14 is referred to.