Patent GB-706221-A (GB)
Title: Improvements in or relating to testing-circuits for use in automatic telephone-exchange and like systems
IPC: H04Q1/24
Filed: 1951-10-09
Abstract: 706,221. Automatic exchange systems. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Oct. 9, 1951 [Oct. 12, 1950], No. 23457/51. Class 40 (4) In a system in which selectors are positioned by comparing outlet identification impedances with an impedance selected by a marker, a potential distinguishing free outlets from those under test or busy switches on a test current which determines the identity of the outlet, while a further current different from the test current causes the outlet potential to be altered to a busy value so that identification impedances cannot be tested simultaneously by other multiplied switches. As shown in Fig. 1, a marker switch MS1 is set by incoming impulses and when contact al is closed a group selector GK1 is started at drive magnet Dl by operation of a differential relay T1. At free outlets a test relay B1 pulls up over wiper d1 and completes a test circuit to the differential relay second winding over wiper c1. The currents in the two windings of the differential relay are determined by the values of comparison and identification resistors Rm, Rx in the marker and selector respectively and if the outlet required is free this relay is disabled and stops the switch. The test relay in operating short circuits a high resistance winding B11 and the potential at wiper d1 is changed by the voltage drop in a resistor Rb so that corresponding multiplied outlets are marked busy and further test relays cannot respond. In a second embodiment, Fig. 2, the comparison and identification impedances each comprise a parallel combination of a resistor Rm, Rx and a capacitor Cm, Cx, respectively, while the tests determining the free or busy condition of an outlet and its identity are effected over a single wiper d of a selector KS. The test circuit comprises a trigger-pair of valves B1, B2 of which B1 is normally cut off by an open-circuited or earthed cathode connection, and free or busy selector outlets are marked by D.C. potentials V1 (-60v) and earth respectively. At free outlets only the cathode potential of B1 (-60v) is more negative than the control grid potential (-30v) so that the pair is triggered and establishes a testing circuit between the marker MS and the selector outlets over the cathode and screen grid of B1 high resistances R3, R4 and R5 reducing the anode current to a negligible value. When valve B1 conducts, the outlet under test is busied by earth potential arising from voltage drop in resistor Rx. The identity of.a free outlet is determined by an A.C. zero test device WO which compares an A.C. reference potential Vw3 with the A.C. potential at a point P on a potentiometer R7, R8 connected between the cathode and screen grid of B1. This potentiometer has a high resistance so that negligible current is taken at busy outlets and conditions on the outlet multiple are not disturbed. The potential at P is derived from A.C. voltage sources Vw1, Vw2, and when this potential equals Vw3 the test device WO, which may comprise an a.c. amplifier controlling a relay, responds and stops the switch at drive magnet D. The resistors R7 and R8 are graded so that their ratio is the same as that between the comparison and identification impedances at the required outlet. If resistors R7 and R8 are equal, the relation between the A.C. voltages is arranged to satisfy the equation Vw3 = ¢ (Vw1 + Vw2).  Alternatively, if Vw1 and Vw2 are equal and in phase opposition, the potential Vw3 may be omitted. At busy outlets the valve B1 is cut off and due to earth potential on the multiple the voltage at point P becomes Vw2/2 so that the switch continues to rotate. Since valve B1 conducts at free outlets, the potential at point P is determined by the values of the comparison and identification impedances. If the identity test is unsuccessful, the trigger pair is restored to normal as the wiper d moves to the next outlet. An additional valve B3 is also provided which establishes zero A.C. potential at free outlets after completion of the test circuit by valve B1 in order to neutralize the capacitive shunt effect of the wiring to multiplied outlets. This valve amplifies the A.C. cathode potential of B1 and applies a resultant signal in phase opposition to the grid circuit over a transformer T, thereby reducing the apparent internal resistance of the valve. According to the Specification, the circuit shown in Fig. 2 may be used to position a final selector, the tens and units digits being characterized by the-values of the impedances and their direct current resistances respectively. Testing of the two valves may occur simultaneously, the testing device comprising arrangements for measuring the D.C. voltage at point P. When Rx and Rm are equal, the D.C. voltage at P will be ¢ (V1 + V2) and the test device is arranged so that the switch is stopped only if both tests are successful.