Patent GB-870068-A (GB)
Title: Improvements in or relating to automatic telephone switching systems
Inventor: GALVIN JOHN LESLIE
IPC: H04M15/10
Filed: 1959-02-12
Abstract: 870,068. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Feb. 5, 1960 [Feb. 12, 1959], No. 4875/59. Class 40 (4). In an automatic telephone switching network having main and satellite exchanges and which enables a subscriber to set up calls through the main exchange by transmitting digits in the form of loop disconnections, metering signals are transmitted from the main exchange by reversals of the potentials normally applied to a junction between the exchanges, these signals operating a relay in the satellite exchange and subsequently the subscriber's meter, provision being made for the relay to maintain the loop for the duration of the metering signals. The outgoing relay set OGRS (Fig. 1) is located at the satellite exchange and has access to the incoming relay set ICRS (Fig. 2) at the main exchange over the junction JUNC. Seizure.-A subscriber obtains access to relay set OGRS over the preceding selector multiple PSM, the resulting operation of high-speed relay A closing the loop over the junction JUNC and bringing up relay B which busies the relay set OGRS. The release of A to the first " break " period of the next dialled digit opens the loop over the junction and brings up CD which prepares a clean loop by short-circuiting the righthand winding of transformer TR1. Relay B, although short-circuited during each " break " impulse, remains held. Re-operation of A to the " make " portion of the first impulse completes the clean loop over the junction, CD (although short-circuited) remaining up for the duration of the impulse train. Subsequent digits are repeated over the junction in like manner, the final release of CD enabling supervisory tones to be reverted to the calling subscriber. In the incoming relay set ICRS impulse repetition is similar, AA coming up on seizure, and CDA on the " break " of the first impulse to prepare a clean loop over which AA repeats the dialled digits to the incoming selector ICS. Metering.-When the called subscriber answers a metering signal in the form of one or more pulses of + 50 V. is applied to the P wire of ICS, thus operating DD which maintains the loop to the incoming selector independently of AA, introduces retard I into, and condensers C3, C4, C5 across, the junction loop, and brings up MA. This relay removes a short-circuit from resistors R7 and R8 and energizes the slow-tooperate, slow-to-release relay MB which on operation reverses the polarity over the junction and brings up MC which re-applies the shortcircuit to R7 and R8. In this manner reversal of the polarity takes place with substantially no current surges, thus eliminating clicks during metering. In the outgoing relay set OGRS, relay D responds to the reversal, maintains the loop over the junction independently of A, and brings up DR which applies battery to the meter lead ML. At the end of the meter pulse, relays MA, MB and MC restore in a sequence which prevents clicks upon termination of the meter signal. Should the calling subscriber manipulate his dial or clear while the junction polarity is reversed, the release of relay A is without effect, the release of B is prevented as DR is energized at this time, and the possibility of a clipped meter pulse being applied to lead ML is avoided. If the calling party manipulates his dial and causes A to release just prior to the reversal of the junction polarity, D cannot respond to the reversal. However, when A re-operates at the end of the " break " period of the impulse, D pulls up and prevents further impulsing over the junction. The resulting meter pulse on lead ML will be shorter than normal by the length of the " break " portion of an impulse, but will be effective to ensure correct operation of the subscriber's meter. Calling party release is provided.