GB1473291A - Communication systems - Google Patents

Abstract

1473291 Electric selective signalling L W BRIGGS E L JONES and H R McRAE 26 April 1974 [29 Jan 1973] 59986/72 Heading G4H A communication system comprises a central station coupled by a common line to a plurality of transponders which each send, in response to the receipt of a voltage change of sufficient magnitude, a coded signal after a delay peculiar to the transponder. The central station is arranged to supply direct current via the common line to provide all the power for the transponders, to send an interrogatory signal in the form of a substantial voltage change across the line, and to identify signals from the transponders according to the delay of their replies. Each transponder (Fig. 1, not shown) comprises a binary code generator (7) triggered by the end of the quasi-stable output from a monostable (4), each transponder also including a sensor (e.g. for fire or burglary) operative to disturb or inhibit operation of generator (7). The central station (Fig. 2, not shown) is arranged to execute, after each interrogatory signal, a timing cycle during which signals from the transponders are directed to respective latches (51, 52, ...) according to their time of arrival, gates (41, 42, ...) being opened sequentially by the outputs from timers (30, 31, ...). The contents of the latches are compared with corresponding reference codes from generators similar to those in the transponders, any lack of correspondence therebetween causing an error signal to be generated. If two error signals occur within a given interval then an alarm is actuated.