H - Electricity – 04 – Q
Patent
H - Electricity
04
Q
340/70
H04Q 9/00 (2006.01) G08B 26/00 (2006.01)
Patent
CA 1057832
ABSTRACT OF THE DISCLOSURE The present invention relates generally to security systems of the type which includes a central controller connected with a number remote stations. Existing systems have a central controller for polling a number of remote transponders connected to the controller by telephone lines, but these prior systems typically have the capability of inter- rogating only a single alarm condition at each transponder site. More complex systems are well within the state-of-the-art of today's digital communications technology, but a key limiting factor is cost, especially cost of the modem equipment. The present invention overcomes these deficiencies of the prior art by providing an improved transponder system and method of use. The transponder detects polling signal bursts of a particular frequency from the central controller, and counts the bursts to determine its turn for response. Each response includes two sequential signal bursts of different frequencies, the durations of which define, with the frequency sequence, the status condition being transmitted. Con- trol information is encoded into the lengths and spacings of the polling signal bursts, and is decoded by the transponder, to indicate transmission errors, transponder resetting commands, and remote control commands. Means are also provided at the transponder for transmitting a special alarm code when electrical power is first applied to the transponder, thus indicating the possible substitution of an unauthorized transponder at a protected site.
244656
Falck Melvin S. (jr.)
LandOfFree
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