A large number of irrigation system devices connected to a common two-wire cable can be powered and individually controlled from a central location by transmitting over the cable DC pulses of alternating polarity. Control information is conveyed by transmitting a command pulse train consisting of a series of pulses, separated by short no-power intervals, whose polarities indicate logic ones or zeros. Following a command pulse train, a selected watering station decoder acknowledges receipt of instructions by drawing current during a predetermined pulse of an alternating-polarity power pulse cycle, while a sensor decoder returns binary data by drawing current during one or the other of the alternating-polarity pulses of a series of power pulse cycles.