operates according to a command list stored in its local memory commands the various Remote Terminals to send or receive messages services any requests that it receives from the Remote Terminals detects and recovers from errors keeps a history of errors The 1553B spec dictates that all devices in the system be connected to a redundant pair of buses to provide an alternate data path in the event of damage or failure of the primary bus. Bus messages only travel on one bus at a time, determined by the Bus Controller. Backup Bus Controller Whilst there may be only one BC on the bus at any one time, the standard provides a mechanism for handover to a Backup Bus Controller (BBC) or (BUBC), using flags in the status word and Mode Codes. This may be used in normal operation where handover occurs because of some specific function, e.g. handover to or from a BC that is external to the aircraft, but connected to the bus. Procedures for handover in fault and failure conditions generally involve discrete connections between the main and backup BCs, and the backup monitoring the actions of the main BC during operation. For example, if there is a prolonged quiescence on the bus indicating that the active BC has failed, the next highest priority backup BC, indicated by the discrete connections, will take over and begin operating as the active BC. The Bus Monitor A Bus Monitor (BM) cannot transmit messages over the data bus. Its primary role is to monitor and record bus transactions, without interfering with the operation of the Bus Controller or the RTs. These recorded bus transactions can then be stored, for later off-line analysis. Ideally, a BM captures and records all messages sent over the 1553 data bus. However, recording all of the transactions on a busy data bus might be impractical, so a BM is often configured to record a subset of the transactions, based on some criteria provided by the application program. Alternatively, a BM is used in conjunction with a Backup Bus Controller. This allows the Backup Bus Controller to "hit the ground running", if it is called upon to become the active Bus Controller. The Remote Terminal A Remote Terminal can be used to provide: an interface between the MIL-STD-1553B data bus and an attached subsystem a bridge between a MIL-STD-1553B bus and another MIL-STD-1553B bus. For example,