number of which are 1s and the rest of which are 0s, with a 1 being defined as the transmission of a Barker code in a gap and a 0 defined as the lack of transmission of a Barker code in a gap; (5) receiving a third reply message from said central unit when said central unit receives said start bit and said ID code and detects therein a predetermined number of 1s indicative of the fact that no othe