Patent GB-178783-A (GB)
Abstract: 178,783. Aitken, W. Jan. 17, 1921. Automatic and semi-automatic exchange systems.-In automatic or semi-automatic systems, the holding circuit is extended over a third conductor to the substation, so that the release of the connection is controlled from the substation independently of the loop talking circuit. The invention is described as applied to a private branch exchange system in which each local line S1, S2, &c. has associated with it connecting relays, as R2, R18 &c., which pre-select an idle one of a number of connecting circuits ACC, BCC, CCC, a single co-ordinate switch associated with which, in response to impulses, completes a circuit for the connecting relay of a wanted local line or an idle outgoing trunk OT1 or OT2 to the main exchange. In either case when connection is completed, a holding circuit is established which extends from the guard relay R5 of the connection circuit in use, through the connecting relay R2 and the cut-off relay R5<1> of the calling line, say SI, over the extended third conductor normally grounded at the calling substation at a switch-hook contact, which is only opened for a short interval, to release the connection, when the receiver is replaced. The same connecting circuits ACC, BCC, CCC, are also used when an incoming call is made over an incoming trunk IT1, connecting relays, as R26, preselecting an idle connecting circuit, the stepping switch associated with which extends the trunk to the wanted local subscriber. An alternative incoming trunk IT2 is provided which takes into use an additional independent connecting circuit which is only used for incoming calls. In the case of incoming calls the holding circuit extends over the connectitig and cut-off relays and grounded switch-hook contact of the called line. When a connection (either local, or incoming from or outgoing to the main exchange) has been built up, the stepping switch associated with the connection circuit in use is restored to normal, and a clean metallic talking circuit is completed. Local calls. Assuming the subscriber S1 desires connection with the P.B.X. subscriber S2 and the connecting circuits ACC and BCC are busy, on removing his receiver the line relay R1 is energiezd in a circuit 1, and a circuit 2 is completed for the connecting relay R2 (contacts 2<1>, 2<11> being open), which closes a circuit 3 for the impulse relay R3. The guard relay R5, a holding winding of relay R2, and the cut-off relay R5<1>, are energized in series over the grounded third conductor in a circuit 5, the connecting circuit CCC and the line S1 being made busy by the opening of contacts 2 and 6 respectively. The calling subscriber now dials No. 2, and the wiper of the stepping switch is rotated in known manner under the control of the magnet M<7> until the terminal of the wanted line is reached. On the cessation of the impulses, the series relay R7 relapses, and if the wanted line is busy, busy tone is applied to the calling line. If, however, the called subscriber is free, a circuit 8 is completed for relay R8' and connecting relay R18 of the called line, the relay R18 locking up in a circuit 9 in which the cut-off relay R9 energizes. Release magnet M12 now energizes to restore the wiper to normal. The relay R10 also energizes in the holding circuit of R8<1> and de-energizes the impulse relay R3. Ringing current is now applied in a circuit 15, including the ringing cut-off relay R15, which de-energizes when the subscriber S2 answers, thereby breaking the circuit of relay R10, which re-connects the impulse relay R3, over which current is fed to both subscribers S1, S2 in parallel. Both subscribers may now converse over a clean metallic talking circuit 18. When the subscribers hang up, the holding circuits over the third conductors are momentarily broken and the relays R5, R2, R5', R18 and R8 de-energize and restore the apparatus to normal. Outgoing calls to main exchange. The operation is the same as that described in the previous paragraph for a local call up to the cessation of the dialling impulses, when (assuming the trunk number is " O " and the subscriber S1 is calling), a circuit 55 is completed for the relay R8' and the trunk connecting relay R55 of the trunk OT1, the latter relay extending the local line S1 to the main exchange over a clean metallic circuit 58, and energizing a relay R56, which switches all the energizing circuits from the tenth contacts in the stepping-switch banks to the connecting relays of the trunk OT2. A holding circuit 57 extends over relays R55, R2, R5 and the third conductor grounded at the switchhook contact of subscriber S1. Incoming calls from main exchange. When the loop across the trunk 1T1 is completed at the main exchange, the line relay R25 energizes, and, assuming connecting circuits ACC, BCC are busy, completes a circuit 26 for the incoming trunk connecting relay R26 of the connecting circuit CCC. The relay R26 locks up in a circuit 27 in which the relay R27 is energized, and disconnects line relay R25, the relay R33 remaining inert. The impulse relay R3 now energizes in a circuit 28 over the main exchange loop, and the stepping switch is operated as previouslv described. Assuming the subscriber S2 is called, on the cessation of impulses, the connecting relav R18 energizes as before in series with the relay R8<1>, the guard relay R5 having responded in a circuit 29. A relay R30 now energizes in parallel with relay R10 in the holding circuit of relay R8<1>, disconnecting the impulse relay R3 and completing the ringing circuit 15. When the subscriber answers, the impulse relay R3 is re-connected as in a local call, and the switching relay R33 responds in a circuit 33 to complete a clean metallic talking circuit 35 to the main exchange. The holding circuit extends over relays R5 and R27 in parallel, and the cut-off relay R9, to the grounded third conductor at the substation S2. If a call is made to a subscriber, say S2, over the alternative individual trunk IT2, on the loop being completed at the main exchange an impulse relay R40 associated with an independent stepping switch is energized, and completes a circuit for the guard relay R41. The stepping magnet M42 now rotates the switch wiper in response to the dialling impulses, on the cessation of which a circuit 43 is completed for relays R43 and R43<1>, both of which lock up. If the wanted line is idle, ringing current is applied over a circuit 46, and when the subscriber S2 answers, the ringing cutoff relay R47 energizes and completes a circuit for the switching relay R48, which connects the called subscriber S2 to the trunk 1T2 over a clean metallic circuit 50. The holding circuit 49 extends over the switching relay R48, connecting relay R43 and cut-off relay R9 to the grounded third conductor at the substation S2.