CA1268227C - Data transmission system with data verification - Google Patents

Abstract

ABSTRACT A system for selectively obtaining information from a plurality of remotely located data indicating devices includes an interrogate source and a plurality of transponders each associated with a different one of the data indicating devices. The interrogate source transmits an interrogate signal for effecting the readout of a selected indicating device, the interrogate signal being a high frequency carrier amplitude modulated with a pair of low frequency signals to represent an eight bit sync code word, a four bit command word and a twenty-eight bit identification code word for addressing a selected transponder. Each transponder includes a receiver which detects the low frequency modulating signals and generates a short duration pulse corresponding to the leading edge of each modulating signal, a processor operating under program control to check on a bit-by-bit basis each bit of the sync code word and of the identification code word as it is received by timing the duration between the receiver generated pulses, and to thereafter compare the first eight bits with a stored sync word and the bits corresponding to the identification word with a stored identification word, the processor responding to the interrogate signal to activate a transmitter to transmit the information provided by the associated indicating device only if the signals detected by the receiver are within a preselected range of frequencies and if a valid sync word and a valid identification word are received.