Abstract: This interrogation system monitors the status of a plurality of protection zone transducers by repeatedly supplying a series of sequential interrogation pulses thereto sufficient to address and scan every transducer associated with a protection zone. Reply pulses are received from a transponder associated with a protection zone transducer and the reply pulses received over a selected number of prior sequential scans form a response pattern from which the condition of each transducer and its protection zone is determined. At least three conditions may be determined from the reply response pattern, including an alarm condition, a normal condition, and an out-of-service condition. Information relative to each transducer and its respective protection zone is stored in the form of bits at memory addresses corresponding to the count of the protection zone transducer from which that information is received.