Patent GB-716141-A (GB)
Title: Electronic line circuit with calling-line excluding facilities
Inventor: OBERMAN ROELOF MAARTEN MARIE
IPC: H04Q3/00
Filed: 1952-01-09
Abstract: 716,141. Automatic exchange systems. STAATSBEDRIJF DER POSTERIJEN.. TELEGRAFIE EN TELEFONIE. Jan. 9 1952, [Jan 12, 1951], No. 725/52. Class 40 (4) In a system in which a potential change in resistors R1, R2, R3 permanently connected to a subscriber's line starts a line finder when the line is looped and identifies the outlet, the potential is disabled by a further potential change due to the firing of a gas tube if a line fault occurs which simulates a calling condition or the subscriber fails to replace his receiver after a call. General arrangement.-In the system described resistors R1, R2 and R3 are equal, and are connected in series to a 60 volt D.C. supply V1. A point X between resistors R2 and R3 is connected through a decoupling rectifier S1 to a call detector common to a group of lines. This detector comprises a double-pentode valve B arranged as a trigger-pair with a normally-released polarized relay T connected in the anode circuits. Line finder.-When a call is initiated, closure of the subscriber's loop circuit changes the potential at point X from earth to - 20 volts so that the valve B triggers and relay. T pulls up. This starts a register (not shown) which positions a link finder (not shown) and a second line finder OZ2. Line finder relay A1 then pulls up and starts a first line finder OZ1 at magnet D. During hunting, the potentials at X are tested by a register test device (not shown) over wiper d of OZ1. When the calling potential of - 20 volts is found, the register removes earth from wiper a of OZ2 so that relay B1 pulls up in series with A1 and stops the switch. Busy earth marking is connected to point X by B1 which also switches through the speech wires. Final selector.-Numerical control marking from the register is located by wiper e of the final selector EK. The line circuit is tested over wiper c and if a potential VI of - 60 volts over resistor R4 is found, the line is free and switch-through is effected by relay B2 which busies the outlet by connecting earth to point X. Defective circuits.-If the subscriber fails to replace his receiver after a call or a line fault simulates a calling condition, the register'in a manner not described releases the relevant line finder or final selector by disabling the associated A and B relays. During the operated period of an A relay a condenser C1 or C2 is charged to - 60 volts and this potential is applied to point X when the relay falls back. A gas tube G which is connected between X and a potential V2 of + 60 volts then has an applied potential in excess of its firing voltage of 100 volts, and the tube remains ignited with an operating voltage of 70 volts as long as the fault condition persists. Point X therefore assumes a potential of - 10 volts so that the call detector, which can respond only to potentials more negative than - 15 volts, remains inoperative. With this arrangement the defective line circuit may still be seized by a final selector unless a circuit between the line wires and wire c is provided as described in Specification 655,682. The gas tube is disabled when the line loop is broken. Busy tone may be connected to the line circuit over the gas tube to indicate that the subscriber's receiver should be replaced. The gas tube may also include a delay circuit, Fig. 2, comprising a resistor R9 and a condenser C3 which maintain the ignited condition of the gas tube for short intervals of line open circuit such as may arise if the subscriber's dial is operated while the line circuit is in a defective condition. A rectifier S2 is included in the delay circuit to prevent condenser C3 discharging through resistor R3.