The Parallel Bus Selector (PBS) is a double-wide module that resides in any two adjacent I/O slots. The PBS connects the lead logic controller to the parallel I/O and, on request, switches the parallel I/O from the lead logic controller to the backup logic controller.

A 2-position DIP switch on the printed circuit board is used to place terminating resistors across the RCM A and B switch control inputs.

The PBS receives the +5V supply, ground, and I/O power fail signal from the backplane of the I/O rack in which it is housed. All other connections are made through frontplate connectors.

The frontplate of the module includes two 3-position connectors, one labeled A and one labeled B. Through these connectors, the Redundancy Control Modules control the PBS.