Patent GB-1222118-A (GB)
Title: Automatic telephone systems
Inventor: BUCK JEAN-JACQUES DE
IPC: H04Q3/545
Filed: 1968-03-08
Abstract: 1,222,118. Automatic exchange systems. INTERNATIONAL BUSINESS MACHINES CORP. 8 March, 1968 [13 March, 1967], No. 11298/68. Heading H4K. In an automatic exchange system in which subscribers are automatically connected through a switching network to junctors upon detection of an off-hook condition, the subscriber circuits are scanned at a first rate to detect off-hook condition, the junctors are scanned at a faster rate to detect dialling pulses and each subscriber's line circuit includes a transistor. The Figure shows an electronic subscriber circuit scanner and a single subscriber circuit as detailed below. The switching network which comprises cascaded stages ET1 to ET3 of thyristors, the junctors and junctor scanner are only briefly mentioned in the Specification. A private branch exchange (Fig. 2a, not shown) with operator positions and a public exchange (Fig. 2b, not shown) with exchange transmitters both of rotary and multi-frequency type are outlined. In both exchanges there are subscriber transmitters of both types. The subscriber circuit scanner which is controlled by a central processor UCT not shown, comprises a first counter 33 driven by pulses from a clock 38, a second counter 32 driven by overflow pulses from the counter 33 so as to be stepped once for each complete cycle of the counter 33 and a number of OR gates 26. Counters 34 and 35 are back-up counters for the counters 32 and 33 and are switched in in the event of malfunction of the latter. The switching matrices of the first stage ET1 are divided into groups, each group being associated with a different OR gate 26. The counter 33 is used to specify a particular matrix within each group and the counter 32 is used to specify a particular inlet of each matrix. Thus the two counters identify one inlet and thus one subscriber circuit in each group. The central processor receives signals from the simultaneously scanned subscriber circuits, via corresponding OR gates 26, indicating their calling condition. These signals are derived from terminals such as C. When the handset of the subscriber station shown is on its hook, the hook switch K is in the position 1 and a transistor T conducts. Negative potential supplied via a resistor R6 is passed by the transistor and clipped to ground potential by a diode D6 before being applied to the terminal C which is thus maintained at ground potential. When the subscriber goes off-hook the potential at B goes more negative blocking transistor T and allowing positive scanning pulses from the counter 32, and arriving via a gate 21, to be applied to the point C. When a subscriber has been detected and the processing unit has connected him to a junctor through the network, the current necessary to maintain the network cross-points is drawn via a diode D4 causing the potential at the point C to drop to - 6 volts. If the subscriber is being called ringing tone arrives over a transformer Tr and a ringing potential is applied to a gate 23. A thyristor T 1  responds to these signals to produce corresponding current variations in the subscriber loop.