Abstract: A central controller is connected by a common two-wire communication cable to a plurality of remote zone transponders. Each zone transponder includes a counting means and a resistive network having a detecting sensor therein. The central controller includes pulse generating means for generating control pulses and address pulses, and an analog-to-digital converter means. A gate enabled by the control pulses passes the address pulses onto the communication cable for supplying power to and incrementing the counting means in each of the zone transponders to sequentially render the resistive network in each operable to transmit a responsive current on the communication cable back to the central controller where the disabling of the gate by the absence of a control pulse permits the analog-to-digital converter means to receive the responsive current from the resistive network of each of the zone transponders and convert it into a digital signal indicative of the status of the detecting sensor therein.