An RFID tag (200) and similar control/sensing unit (300) for a communications system are described. The control/sensing unit has a control element (301) having one or more states, and include an antenna (207) as well as an electrical circuit (201-206) generating a pulse control signal for controlling transmission from the antenna and varying a parameter of the pulse control signal in dependence on the state of the control element (301). Also disclosed is a receiving communication unit in the form of a base unit or system controller for such a communications system, the unit including a receiver (100) for receiving a pulsed RF signal and an electrical circuit (105-106) for detecting a parameter of the pulsed RF signal. An output circuit (107) then produces an output control signal dependent on the detected parameter of the pulsed RF signal.