A smoke and fire detection system (10) wherein a central controller (12) transmits data to remote transponders (24, 26) on a voltage supply line (22) by pulse code modulation (PCM) of the supply voltage, and the transponders (24, 26) communicate with the controller (12) by pulse width modulated (PWM) current pulses. A transmitter (30) supplies a nominal operating voltage and transmits a data word that includes plural data bits to the transponders (24, 26) over the line (22) by switching the voltage from the nominal operating voltage to a first voltage corresponding to a first logical level or a second voltage corresponding to a second logical level. A decoder (84) in each transponder (24) detects the transition and determines the logical level of each bit by measuring the transmitted voltage level immediately after detecting the transition. The transponder (24) returns data signals to the controller (12) having a duration based on the data to be returned.