Abstract: Transponder apparatus and a corresponding method for use in a security polling system having a central controller and a number of remotely located transponders which are polled sequentially to initiate transmission of status conditions, over telephone lines to the central controller for monitoring. 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. Control 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.