Abstract: A system for sensing multiple conditions, such as alarm conditions, at each of a number of remote stations, the sensing being accomplished solely through a pair of power lines by means of which the remote stations are connected in a cascade arrangement with a central station. When power is applied to the first of the remote stations, a current pulse is drawn through the power lines and is modulated in accordance with a sensed condition at the remote station. At the end of the current pulse, there is at least one zero-current interval, the duration of which is varied in accordance with another sensed condition at the remote station. The central station, after applying power to the first remote station, identifies the sensed conditions from the modulated current pulses and the durations of the zero-current intervals. Each remote station also includes a line power switch that remains open until the completion of interrogation of the sensed conditions at that particular station.