Patent GB-306950-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M5/10
Filed: 1929-02-18
Abstract: 306,950. Associated Telephone & Telegraph Co., (Assignees of Crocker, T. F.). Feb. 29, 1928, [Convention date]. Semi-automatic exchange systems.-Incoming calls at an automatic private branch exchange are established by an operator over cord circuits, which have a common splitting key CSK by means of which she may talk to either party to the exclusion of the other, and individual keys RK which are moved to the right to ring the wanted subscriber and then to the left to complete the connection. The arrangement is such that the operator cannot ring after the called subscriber has answered, nor can she interfere with a connection by plugging into a busy P .B.X. line and attempting to ring or by plugging into a busy trunk and attempting to dial. Call from main exchange to P.B.X. When a trunk 201, 202 leading to the P.B.X. is seized by the connector C<2>, guard relay 110 energizes in series with ring cut-off relay 250 which does not operate at this time, operating visual signal VIS and connecting up relay 115 to mark the trunk busy at the P.B.X. Ringing current from the connector C<2> operates relay 106, which lights calling lamp L<2>. The operator plugs into jack J<2>, and relays 30, 100 energize, while relays 106, 110, 115 release, the trunk now being marked busy by earth through relay 30. Condenser 108 is discharged by short-circuiting to prevent possible operation of relay 106 upon clearing. Relay 41 is operated by ringing current, and lamp L<1> lights. The operator presses cord key CK, and relay 24 energizes, connecting up relays 35, 50 in series. Relay 35 extinguishes lamp L<1> and completes a bridge across the trunk 201, 202 through the common splitting key CSK and repeating coil R. thus effecting the operation of ring cut-off relay 250 and backbridge relay 255 in the connector C<2>. Relay 50 energizes relay 55 and then falls back sluggishly to connect up the operator's set TS after a delay when the current over the trunk has become steady. The operator plugs into the jack J of the desired subscriber and relay 2 lights lamp L, and ringing current is sent by moving key RK momentarily to the right. On reply, feed relay 10 pulls up, extinguishing lamp L, and the operator may talk to the called subscriber by moving splitting key CSK to the right, releasing relay 24 to complete the circuit to her set and disconnecting the trunk therefrom, a holding bridge through resistance 85 being provided. The connection is completed by moving key RK momentarily to the left to energize relay 20, which locks in series with relay 15 and also locks relay 24, and restoring cord key CK which releases relays 35, 55. If the operator does not wish to talk to the called party, she moves key RK first to the right and then to the left, and the connection is completed upon reply by the operation of relay 10. The operator may break in and talk to either party by operating cord key CK and throwing splitting key CSK to the left or right. Release. Feed relay 10 falls back when the called partv hangs up, lighting lamp L to give a clearing signal and releasing relays 15, 20, 24, thus disconnecting back-bridge relay 255 at the connector C<2>. Should the called party again remove his receiver, no talking connection can be completed since relays 15, 20 are de-energized. The operator removes the plugs, and relays 30, 100 relapse, restoring the cord circuit and trunk to normal condition. If, however, the calling party does not hang up, relay 110 re-energizes and marks the trunk busy. A second call may be received over the trunk 201, 202 before the operator clears down, relay 41 responding to ringing current and lighting lamp L<1> to give a calling signal. Calls set up by P.B.X. subscribers. The main exchange trunks are accessible over a particular level of the selector S. When the trunk 201, 202 is seized, guard relay 100 energizes over the test. wire 206 and operates busy signal VIS, and relay 125 energizes over the loop, connecting up release relay 120. A subscriber desiring information may call the operator over a trunk 271, 273. Line and release relays 270, 265 energize, lighting lamp L<3>, and the operator replies by operating key K<2>, connecting up relays 65, 260, and the connection is extended to her set TS as described above. Call to main exchange set up by P.B.X. operator. After plugging into an idle trunk 201, 202, cord key CK and dial key DK are operated, and the wanted number is dialled, the dial being bridged across the trunk by the operation of relay 35. The operator then recalls the P.B.X. subscriber as already described. Preventing interference with existing connections. Should the operator plug into a busy P.B.X. line, relay 2 is short-circuited and does not operate, so that ringing current cannot be sent out. The operator cannot re-ring the P.B.X. subscriber after he has replied, since relay 15 is energized. Should the operator plug into a busy trunk, relay 30 is short-circuited and does not operate, so that relay 35 cannot energize to bridge the trunk. The operator can, however, connect her set TS to the trunk by throwing cord key CK to listen in on the connection.