Serial bus circuit for AC64  allows transformation of the contact of a "Switch Alarm" into a signal interpreted and managed by the unit as if it were a radio device includes counting of impulses (1-3-5-8) and counter reset if no programmed impulses arrive within 10 seconds. In the event of a failure with opening of the switch for more than ten seconds, an alarm signal is sent. Connection to unit with three wires. Manages signals for alarm, supervision, line control and power supply.