Patent GB-780387-A (GB)
Title: Improvements in or relating to telephone selector systems
IPC: H04Q5/00
Filed: 1955-05-18
Abstract: 780,387. Party-line systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. May 18, 1955 [May 18, 1954], No. 14413/55. Class 40 (4). In an interconnection system each substation, Fig. 1, on a party-line has a selector H and a bell B provided with means to delay its operation, and two or more control circuits such as that shown in Fig. 2 are connected to the partyline. . Call from substation A, Fig. 1, to another substation on the line. The calling subscriber lifts his hand-set, establishing a listening circuit over condensers C1 and C2, and if the line is free he presses a push-button K. This loops the line, and in each control circuit a relay S2 operates, breaking the circuit of a normally-energized relay S10. The different relays S10 are arranged to have different release times, and the first to release brings up a relay S3 in its associated control circuit, followed by relays S4, S1, and S5. Relays S2 and S3 then fall back slowly. In the other control circuits, where the relays S10 are still operated, the reversal of potentials on the line, caused by the operation of S3 in the first control circuit, brings up relay S14 followed by slow-releasing relays S12 and S13. When S3 releases, the consequent release of S14 brings up relay S11 which locks while S12 and S13 fall back slowly. Thus the relays S1- S10 are disconnected from the line in all the control circuits except the first, where a buzzer SU sends dialling tone over relay S1 to the calling subscriber. Calling subscriber dials wanted number (two digits). The first dial impulse releases relays S1. and S4 at the first control circuit, but S5 is sufficiently slow-releasing to hold during the impulsing. Relay S6 is energized and holds to the end of the impulse train, bringing up relay S7 and charging a condenser E1. At the end of the first dial impulse, relays S2, S3, S4 and S1 re-operate, and S2 and S3 then fall back again slowly. The operation and release of S3 puts another impulse of reverse potential on the time as described above. At each substation stepping-magnet SE moves selector H one step, holding-magnet SP being energized and holding during the impulse train. In control circuits other than the first the impulse of reverse potential operates relays S14 and S12, and the release of S14 at the end of the impulse brings up, during the slow-release period of S12, relay S13 which holds throughout the impulse train. The remaining dial impulses of the first digit produce similar effects, the selectors H stepping on at each impulse. At the end of the first digit, the holding-magnets SP all release. The selectors H return to normal under spring pressure, except at substations belonging to the same group as the wanted substation, where projections p and q engage each other to prevent the release of the holding-pawl v. In the first control circuit, relays S1 and S4 remain operated so that S6 releases, bringing up relay S8. At the beginning of the second digit, S1 and S4 again release, bringing up S6 which now energizes relay S9. Other operations are similar to those occurring during the first digit. At the end of the second digit the relapse of S6 re-operates S3 so that the selectors H at the substations take a further step. Ringing. At the substation corresponding to the dialled number, a contact a on the selector H now engages a fixed contact b. Lamp L lights and as it heats up its resistance rises, allowing D.C. bell B to start ringing after a short delay. This delay prevents the ringing of the bell during the stepping of the selector H. The calling subscriber hears the ringing impulses induced in the upper winding of magnet SE. Meanwhile, relay S7 in the first control circuit is held operated for 3-4 seconds by the discharge of condenser E1. When S7 finally falls back it is followed by S8, S9 and S3, and all the selectors H return to normal on the release of their holding magnets SP. In the other control circuits, relays S14, S12 and S13 are released and S11 holds. The call now proceeds in normal fashion, relays S1 and S4 remaining energized in the first control circuit. Release. When the calling subscriber hangs up and releases his push-button K, relays S1 and S4 fall back. Relay S6 operates, bringing up S7, and condenser E1 is charged, while S5 releases slowly, followed by S6. Relay S3 is energized and puts a long impulse on the line as before while S7 is held by the discharge of condenser E1. In the other control circuits, relays S14 and S12 operate so that S11 is shortcircuited and falls off slowly. All operated relays are released following the relapse of S7 when condenser E1 completes its discharge. Relays S3 and S14 may be provided with contacts for sending impulses to further lines connected to the first.