A radio frequency communication system comprises a control module (1) and communications modules (2, 3) the modules communicating with each other by means of radio frequency signals. Initially, the control module (1) is allocated address number (0) and the communications modules (2, 3) are allocated address numbers (1) and (2) respectively. The system shown in FIG. (1) is set up as a loop arrangement, in which signals are transmitted from communications module (3) directly to the control module (1), so that the control module is also allocated address number (3). When a new module (4) is to be inserted into the system at the location shown in FIG. (1), the new module (4) is caused to emit a first message, in a set up mode, at low, medium and high power levels, as shown in FIG. (2). The message transmitted by the module (4) at each power level contains data identifying whether it has been transmitted at high, medium or low power level.