Patent GB-895956-A (GB)
Title: Improvements in or relating to electrical signalling systems
Inventor: KNOTT LEONARD FRANK
IPC: H04Q5/00
Filed: 1960-03-04
Abstract: 895,956. Electric selective signalling systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. Feb. 13, 1961 [March 4, 1960], No. 7713/60. Class 40 (1). A selective calling system comprising a main station and a plurality of remote stations accessible over a common signalling channel, the remote stations being identified by multi-digit codes, is characterized in that at each remote station the first digit received sets a uniselector, which, if the digit corresponds to the first digit of the code of the remote station, prepares a circuit for reception of the second digit, and so on, only that remote station corresponding to the transmitted code finally responding. The drawing shows a typical field station, incoming pulses-via wire or radio-actuating a relay CR by means not shown. The first pulse received, a long " seize " pulse, providing it is longer than a delay of 3-6 seconds caused by a circuit TR, C1, picks up a relay B, which, in turn, homes uniselector CS, via contact B1 and the homing bank CS4. This causes relays CD (via contact No. 1 of bank CS1 and contact B4) and HP to pick up and the field station is then ready to receive the field station call signals. The call signal to which a field station will respond is determined by the settings of switches SW1, SW2 and SW3, which are shown set to the call " 481." The digits of a field station call are transmitted successively, each being transmitted as a number of " breaks " in a carrier. The breaks of a digit are separated by a short " make " and the digits are separated by a long make. The first bit of the first digit (the break immediately following the very long make of the " seize " signal) drops out relay CR and advances uniselector CS via AL2, CR1, CD1, HP1, TH1 and magnet CSDM, to its No. 2 contact, which causes bank CS1 to break the circuit for the lower winding of relay CD. At the end of the break, during the inter-bit make period, relay CR picks up and breaks the circuit for the upper winding of relay CD. Relay CD is slugged however, and does not fall-out before the next bit arrives The fall-out time of relay CD will not bridge the inter-digit make periods, which are relatively long, and thus at the end of the first digit relay CD falls out. If, for example, the call 481 is being transmitted, after the first digit has been transmitted and uniselector CS has reached contact No. 5, relay CD falls out. This applies earth to the wipers of banks CS2 and CS3 and causes the first digit recognizing relay AD to pick up and hold, and this, in turn, via contact AD1, causes the uniselector to advance to contact No. 13, whereupon relay CD picks up, and the circuit is then ready to await the reception of the second digit. The second digit is received in a similar manner and if correct results in the pick-up of the second digit recognizing relay BD, via SW2, AD3 and SW3B. The reception of the correct second digit also causes uniselector CS to home to its No. 1 contact prior to the reception of the third digit. The third digit, if correct, causes in a similar way the energization of an alarm relay AL to warn an operator that he is being called. If at any stage an incorrect digit is received the relay CD is not re-energized after it falls out at the end of the digit and further incoming bits are ignored. If a switch SW3 is set to " X " then the circuit will respond to two-digit codes. As shown, the emergency call " 000 " will call every field station. Also the field station themselves may have dialling equipment such as DCC so that they can initiate calls.