Abstract: The status of individual plug-in circuit boards is monitored by a central control unit via an associated single service request wire, in conjunction with a serial data link. The central control unit interrogates possible operational states, e.g., removed, power loss or other failure, operating properly or requesting service, of a plug-in circuit board by controlling a resistive load connected to the single service request wire for the associated plug-in circuit board. This is realized by employing a logic circuit which is controlled by a microprocessor to supply different logic states to a resistor connected to the service request wire at the central control unit.

Abstract: Individual equipment units are adapted to be connected to a Telemetry Byte Oriented Serial (TBOS) protocol communication link in a bussed fashion. This allows more than one equipment unit to be monitored and controlled over a single TBOS communication link. Each equipment unit includes a multipoint receiver for receiving request messages from and a multipoint transmitter for transmitting response messages to the TBOS communication link. An additional multipoint receiver is adapted to receive response messages from other equipment units being monitored and controlled over the TBOS communication link. When a particular equipment unit is designated either a backup unit or an auxiliary backup unit, it is enabled to transmit response messages for the other equipment units being monitored and controlled if they do not respond in a prescribed interval.