Patent GB-476813-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q5/16
Filed: 1936-06-15
Abstract: 476,813. Exchange systems. AUTOMATIC ELECTRIC CO., Ltd., (in Liquidation), and PEARCE, O. A. June 15, 1936, No. 16669. [Class 40 (iv)] In a party-line system, sub-stations are provided with two step-by-step switches, one of which is stepped, on initiation of a call, by impulses automatically transmitted from the exchange until the calling party is connected up, the other being stepped on incoming calls to connect up the calling device of the wanted party. The impulses transmitted on initiation of a call are repeated to a switch at the exchange to select an individual metering circuit or, in a manual system, to identify the calling party. The impulses are of high voltage, the two switches being operated by impulses of opposite polarity, and a gas discharge tube is connected in series with the switch magnets. A sub-station busy indicator is provided. Outgoing call. The a wire is normally connected at the exchange to a grounded relay shunted by a rectifier, the b wire being normally connected to a battery-connected relay. When the receiver is lifted, the line is looped over SH1 and as rectifier MRE is in conducting direction, the battery connected relay at the exchange pulls up but the relay shunted by a rectifier does not. High potential (100 volts) positive pulses are now connected to the b-wire and owing to the polarity of rectifiers MRA ... MRD operate switch magnet F but not magnet C. The first pulse operates busy indicator MP1 which can only be restored by an impulse of reversed polarity. Switch F on its first step only operates off-normal contacts of which FN1 shunt the indicator to prevent interference therewith and FN2 open the original loop and, at other sub-stations, prevent calls from being initiated. When F has been advanced to a position individual to the calling sub-station in which contacts F1, F2 are closed so that the calling subscriber's set is connected across the line, the positive pulse passes also over the a wire to operate the rectifier-shunted relay at the exchange which stops the pulses and switches through to an ordinary line circuit for signalling an operator or for setting up the connection automatically. When the calling party hangs up, positive pulses are again applied to the b-wire to home the F switches at the sub-stations whereupon FN1, 2 close, FN1 making F slow to release so that it does not respond to further pulses of the train which is of a fixed number sufficient to home the switches from any position. Negative pulses are now connected to the b-wire and step the C switches, the first impulse restoring the indicators MP1 and the switches being stepped around to the home position. A neon tube NT isolates the switching equipment as regards speech thereby improving speech transmission. Incoming call. A single positive pulse followed by negative selecting pulses are sent over the b-wire by the operator or final selector. The positive pulse steps the F switches offnormal and operates the indicators, whereas the negative pulses step the C-switches and, at the wanted station, connect up the bell and prepare the loop at C2, Cl. Ringing is applied to the a-wire and the connection completed in usual manner. On release, the F and C switches are stepped home in turn as previously described, but the indicator is not released until the first negative impulse after C has homed and removed the shunt at CN1. Revertive call. The F switches operate on initiation of the call and the number is given to the operator or dialled to set a revertive call switch. The calling party hangs up and the C-switches are operated to select the wanted party, the indicators being unaffected by negative pulses owing to the shunt at FN1. Both parties are rung in parallel. The exchange equipment for controlling the transmission of the pulses may be a simple modification of similar equipment described in Specification 452,038.