Digital load control system providing power and communication via existing power wiring

A controller comprises a controllably conductive device adapted to be coupled in series electrical connection between an AC power source and a load control device. The controller also comprises a control circuit coupled to the controllably conductive device for rendering the controllably conductive device conductive each half-cycle of the AC power source to generate a phase-control voltage. The control circuit is operable to render the controllably conductive device conductive for a portion of each half-cycle of the AC power source. The control circuit is operable to transmit a digital message to the load control device for controlling the power delivered to the load by encoding digital information in timing edges of the phase-control voltage, where the phase-control voltage having at least one timing edge in each half-cycle of the AC power source when the control circuit is transmitting the digital message to the load control device.