GB798904A - Conference circuit for private automatic telephone exchange systems - Google Patents

Abstract

798,904. Automatic exchange systems. TELEPHONE MANUFACTURING CO., Ltd. Sept. 8, 1955 [Oct. 8, 1954], No. 29090/54. Class 40 (4). A subscriber is given a conference facility by the modification of a fault circuit associated with a seized finder-selector link and the inclusion in his line circuit of a relay which is operated by a relay in the fault circuit when the subscriber dials a conference digit and makes a connection with a conference circuit while releasing the seized finder-selector link and marking the subscriber's line busy. Figs. 1 and 2 show subscribers' line circuits respectively without and with a conference facility provided by a relay PG which operates in conjuncton with a permanent glow alarm circuit shown in Fig. 3 and a forced release circuit associated with a final selector circuit shown in part in Fig. 4. When either subscriber lifts his receiver a preselector or line finder is started by line relay L to make a connection with a finderselector link causing line relay A to operate and actuate relay B. Relay B returns earth on the subscriber's P wire and makes a circuit for a relay KO which operates after a time determined by Ó delay DE, the circuit being broken when A falls back in response to the first impulse. If a subscriber with the line circuit of Fig. 1 fails to obtain a finder selector link, no earth is returned over the P wire and the cut-off relay K responds and locks up, after a time determined by thermistor TH1, to release L and operate relay PGA which actuates a delayed permanent glow alarm. If, having obtained a finder-selector link, the subscriber does not dial within the time determined by the delay DE, relay KO responds releasing relay A and removing earth from the P wire. With earth on the P wire both L and K are energized in series, but when earth is removed L falls back while K is held over l2 and k1, and then locked up over l4, k4 and l1. With K operated and L released, the alarm is set off and contact k2, by taking battery off the P wire, holds the line engaged to incoming calls. In the circuit of Fig. 2 which has a conference facility, the relay PG pulls up in series with the holding circuit for K over l4, k4 and l1, battery being removed from the P wire by pg4, and the alarm circuit being set off by PGA which is energized over a contact cn. To set up a conference connection a subscriber dials a predetermined digit which by any known means closes a contact x, Fig. 4, to operate the relay KO without waiting on the delay DE, and to operate a relay CN which prevents operation of the alarm circuit and locks up against the release of x over pg3; at this stage K and PG are operated and L is released as described above. Contacts pg1 and 2 connect with a conference line and contact pg4 holds the line engaged by removing battery from the P wire. Parties to the conference are fed with speaking current by way of relays PG and K.