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.
Priority plugs were manufactured by DEC for all four priority levels:
|Level||DEC part number|
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.