1. A controller for use in a load control system for controlling power delivered from an AC power source to an electrical load, the controller comprising:a controllably conductive device adapted to be coupled in series electrical connection on a power wiring between the AC power source and a power device of the load control system;
a control circuit coupled to the controllably conductive device for rendering the controllably conductive device conductive during each half-cycle of the AC power source to generate a phase-control signal, the control circuit configured to transmit a forward digital message to the power device by encoding digital information in timing edges of the phase-control signal, and to generate a reference edge at a reference edge time during a first half-cycle of a reverse digital message, the reference edge time being after a zero-crossing of the first half-cycle; and
a reverse receiving circuit coupled in parallel electrical connection with the controllably conductive device for receiving a controller-drop voltage developed across the controllably conductive device, the reverse receiving circuit configured to receive a reverse digital message from the power device via the power wiring, the reverse digital message transmitted by the power device in response to the forward digital message;
wherein the control circuit is configured to:
receive, via the reverse receiving circuit, an acknowledgement (ACK) pulse in a window time period during a second half-cycle of the reverse digital message, wherein a start time of the window time period during the second half-cycle of the reverse digital message is based on the reference edge time during the first half-cycle; and
receive, via the reverse receiving circuit, a data pulse in a window time period during a third half-cycle of the reverse digital message, wherein a start time of the window time period during the third half-cycle of the reverse digital message is based on the reference edge time during the first half-cycle.