An environmental control system for use in a greenhouse or another structure which requires the control of an ambient condition regulating element in response to a plurality of sensed parameters. The system includes a plurality of sensor elements and actuator elements, which communicate with a central control through communication interface units. In an illustrated embodiment the interface units communicate over previously installed AC power lines by frequency shift keyed signals. The interface units communicate to the peripheral control elements and the central control unit through serial digital signals. The central control maintains a protocol where, in response to operator inputs, time slots are assigned to different peripheral control elements to configure the system. Each peripheral control element is thereby addressed during its time slot and a sensor responds with data corresponding to a sensed parameter and an actuator executes a command. A unique framing character is generated by the central control at the beginning of each assigned time slot for alerting the peripheral control elements an address will next be generated and for synchronizing multiple central controls to a common time slot clock.