If a sub-system is essential for the mission success of the overall system, it should be modelled as a series item, e.g., SCSI MPU, Flight Data Processor, Timing Unit and Workstation in Figure B-1). In Figure B-2, the RBD displaying serial items is indicating that a failure in a serial item will mean a total system failure.

The logic of serial modelling in an RBD is similar to that of an electrical circuit. If a serial component fails, then the entire circuit will fail.