Transmission-reception system

The invention relates to a transmission-reception system, particularly a transmitter-receiver device contained in the bow of an ignition key. When a resonance circuit receives a carrier wave signal from a transmission-reception ECU of a motor vehicle, a microcomputer is switched to an active state by the output of a power circuit that smoothes and rectifies the carrier wave signal. A detector circuit discriminates an enquiry signal supplied together with the carrier wave signal through the resonance circuit portion, and outputs the signal to the microcomputer. In response to the enquiry signal together with random number data, the activated microcomputer changes the bit length of an identification code stored in an EEPROM, on the basis of the received random number data and a predetermined function, and performs the on-off control of an FET of a modulating circuit in a mode corresponding to the bit length-changed identification code. The microcomputer thus performs control to send back an enciphered answer signal to the transmission-reception ECU.