A monitoring system comprising: a common control unit; supplemental units (20a, 20b, 20c) displaced from the control unit; a primary communications bus between control unit and at least some of the supplemental units; and at least one supplemental communications bus (18a, 18b, 18c) which extends between the control unit and at least some of the supplemental units where at least one supplemental unit includes a bus control unit, coupled to the primary communications bus and the supplemental communications bus, and where the supplemental bus is activated in response to at least one of a sensed failure associated with the primary communications bus, a failure of the bus control unit, or a failure of the common control unit where the bus control unit includes primary and secondary control circuits (34a, 34b) which are electrically isolated from one another; a plurality of ambient condition detectors or output devices are coupled to the bus control unit where members of the plurality communicate with the common control unit using the bus control unit where the primary control circuits communicate with the secondary control circuits to monitor one another, where the secondary control circuits can send a malfunction notification via a bus conductor to the common control unit if a communication request from the secondary circuits to the primary control circuits is not responded to, and, if the primary control circuits do not receive any communication requests from the secondary control circuits, the primary control circuits activate a redundant alarm detection path after a predefined period of time wherein fire or gas alarm notifications can be transmitted directly to the common control unit via an alternate emergency indicating line. a common control unit; supplemental units (20a, 20b, 20c) displaced from the control unit; a primary communications bus between control unit and at least some of the supplemental units; and at least one supplemental communications bus (18a, 18b, 18c) which extends between the control unit and at least some of the supplemental units where at least one supplemental unit includes a bus control unit, coupled to the primary communications bus and the supplemental communications bus, and where the supplemental bus is activated in response to at least one of a sensed failure associated with the primary communications bus, a failure of the bus control unit, or a failure of the common control unit where the bus control unit includes primary and secondary control circuits (34a, 34b) which are electrically isolated from one another; a plurality of ambient condition detectors or output devices are coupled to the bus control unit where members of the plurality communicate with the common control unit using the bus control unit where the primary control circuits communicate with the secondary control circuits to monitor one another, where the secondary control circuits can send a malfunction notification via a bus conductor to the common control unit if a communication request from the secondary circuits to the primary control circuits is not responded to, and, if the primary control circuits do not receive any communication requests from the secondary control circuits, the primary control circuits activate a redundant alarm detection path after a predefined period of time wherein fire or gas alarm notifications can be transmitted directly to the common control unit via an alternate emergency indicating line.