UNIBUS priority plug

A UNIBUS priority plug is an insertable plug, physically a DIP, which was inserted into almost all UNIBUS devices in order to select which daisy-chained interrupt bus grant line, of the four such available on the UNIBUS (BG4, BG5, BG6, and BG7), the device's grant-acquiring/passing circuitry was connected to. As a secondary purpose, it passed all the other grant lines past the device.

As part of the configuration of a device being added to a system, it was necessary to ensure that a priority plug for the desired priority level was installed in the device.

Another name for "priority level" was bus request, usually abbreviated to "BR"; the manuals usually state e.g. BR 5 for priority level 5.

Priority plugs were manufactured by DEC for all four priority levels:

Level	DEC part number
4	54-08776
5	54-08778
6	54-08780
7	54-08782

Most devices also came with a plug pre-installed at manufacturing time, at a level selected by DEC as likely the most appropriate interrupt priority level for that device; they were rarely changed.