A security system provides for bidirectional communication between a controller and a plurality of addressable transponders. At least one transponder includes a sensor circuit for detecting motion, which motion is indicated by the transponder signalling back to the controller. During a walk test the controller sends a signal back to the transponder, completing the circuit for the visible output indicator so that subsequent motions during the walk test sequence will be immediately displayed at the transponder, without necessitating successive communication through the loop including the controller. In systems where the controller itself reports upwardly in a hierarchy of units, the walk test can confirm operability of the entire system, including other condition-indicating signals.