Patent GB-277298-A (GB)
IPC: H04Q3/62
Filed: 1927-07-04
Abstract: 277,298. Associated Telephone & Telegraph Co., (Assignees of Lomax, C. E.). Sept. 11, 1926, [Convention date]. Automatic and semi-automatic exchange systems.-In working over a two-way trunk line between a private branch exchange and a main exchange, no current normally flows over the trunk, and when the trunk is seized at the P.B.X. the operation of the line switch at the main exchange is initiated by the connection of ground to one conductor, while when seized at the main exchange, a bridged high-resistance relay at the P.B.X. end energizes and applies guarding potential. Call from P.B.X. to main exchange. Associated with the trunk 9, 10 is a trunk circuit TC. When this trunk is seized by a selector over bank terminals 1 - - 3, line relay 13 energizes, connecting temporary ground to the test wire 4, and connecting ground to the upper wire 9 of the trunk to operate the line relay 20 of the line switch LS. Series relay 15 also energizes in this circuit, and connects up release relay 16, which disconnects bridged relays 17, 18. When the line switch LS engages an idle selector, switching relay 19 operates, and relay 12 energizes over the lower wire 10 in series with the line relay of the selector, connecting up switching relay 11 and visual signal D. Relays 13, 12 relapse. The switches at the main exchange are now controlled by the calling subscriber's dial to complete the connection. Upon release, when the calling party hangs up, series relay 15 and the line relay of the connector are disconnected, and the former causes release relay 16 to fall back, removing ground from the test wire 4 and restoring the trunk and trunk circuit TC to common use. Call from main exchange to P.B.X. The trunk 9. 10 is seized by a connector over bank terminals 72 - - 74, and bridged relay 17 is energized by ground superimposed on the ringing circuit, and immediately applies holding ground to the test wire 4. The ring-cut-off relay does not operate in series with relay .17. Bridged relay 18 is energized by ringing current and connects, up relay 14, which locks and lights lamp L. The operator inserts plug P' of the cord circuit CC into jack J. and relays 13. 22 energize in series. Relay 13 bridges relays 12, 15 across the trunk to energize the ring-cut-off relay, and these two relays then energize in series with the backbridge relay of the connector. Release relay 16 energizes and unlocks relay 14, which opens the bridge and extinguishes lamp L. Switching relay 11 energizes and visual signal D operates. Relay 22 now energizes in series with the backbridge relay, lighting lamp L<1> and removing a shunt from relay 24. The operator converses with the calling subscriber and completes the connection by means of the plug P<2> in the usual manner. Sleeve relay 25 energizes and connects supervisory relay 24 in series with the talking conductors, so that it energizes in series with the back-bridges relay upon the reply of the called subscriber, and connects up relay 23. Relay 22 falls back, extinguishes lamp L<1> and replaces the shunt across relay 24 to improve transmission, but the latter relay remains energized. Upon release, when the calling party hangs up, all the main exchange switches release except the connector, which is held until the back-bridge relay de-energizes upon the called party hanging up. When this occurs, supervisory relay 24 also falls back, and lights lamp L<1> to give a clearing signal. and series relay 15 relapses and restores the trunk and trunk circuit TC to common use, even though the cord circuit CC has not been removed. A call to the main exchange may be set up by the P.B.X. operator by plugging into jack J and operating the dial CD. A night jack J<1> serves to connect the trunk 9, 10 to a particular P.B.X line.