An environmental control system for use in greenhouses or other structures requiring the control of a temperature regulating element in response to sensed temperatures. The environmental control system includes a plurality of sensor elements and actuator elements comprising peripheral control elements each of which communicate bidirectionally with individual communication interface units. A central control processor bidirectionally communicates with another communication interface unit. All of the interface units bidirectionally communicate with each other over fixed AC power lines by frequency shift keying the information onto and from the lines. The control processor receives operator inputs which cause it to assign time slots to different peripheral control elements to configure the system whereby each peripheral control element can be interrogated by addressing it during its time slot. In response to an interrogation, a sensor replies with data corresponding to a sensed parameter while an actuator replies with an acknowledgement and awaits control commands. A unique framing character is generated at the beginning of each time slot for alerting all peripheral elements that the next character generated will be an element address and for synchronizing multiple control processors to an identical time slot clock.