Abstract: A method for communicating between a first station and a second station over a wire pair, in which the first station sends an alternating power signal over the wire pair, the alternating power signal comprising a plurality of communication symbols, the second station receiving the alternating power signal and determining therefrom the communication symbols, the second station drawing current from the alternating power signal in a sequence corresponding to at least one further communication symbol, and the first station determining the current drawn in the alternating power signal to recover the further communication symbol.

Abstract: An apparatus for monitoring a device (such as a solenoid) powered by the apparatus includes a power supply for providing an alternating power signal to the device and a monitoring circuit that measures a prescribed characteristic of the alternating power signal. A control circuit is responsive to measurements from the monitoring circuit and is arranged to analyze the measurements to determine the status of the device therefrom.