Abstract: A security installation comprises sensors arranged in and around a building which transmit digital information to a central station by radio, in each case preceded by an access code specific to the particular installation; signals inadvertently received from the sensors of an adjacent installation are rejected. The information is transmitted by encoding a multiple bit word incorporating the access code, a code identifying the particular sensor, and the actual data, using Manchester II coding on an FSK transmission. Each such word is sent several times to form a data packet. The master station tests the received data for the correct format and for the correct access code. Recognition is only complete when within a particular packet, at least two (say) sequential words and at least two other words are recognized.