Abstract: A responder unit for communicating with an interrogator unit which sends an RF interrogation pulse thereto is described. The responder unit includes an energy accumulator which stores the energy contained in the RF interrogation pulse to be used to power the responder unit in the absence of any RF interrogation signal. The responder unit also has a memory for storing read data and a RF threshold detector for detecting termination of the RF interrogation pulse. A RF carrier wave generator under control of the RF threshold detector is operable to activate upon detection of the termination of the RF interrogation pulse. A modulator is provided in the responder unit to modulate the RF carrier with the read data from the memory.