UNIBUS priority plug

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A UNIBUS priority plug is an insertable plug, physically a DIP, which was inserted into almost all UNIBUS devices in order to perform two separate functions related to the device being able to perform interrupts via the bus:

  • connect the device to the desired bus request line (the UNIBUS provided 4 different priorities of interrupt, with a request line for each - BR4 through BR7);

(Secondarily to the latter, it passed all the other grant lines past the device.)

The approach of using a single plug which performed both of these tasks ensured that one could not get a mismatch between the priority levels used for the two.

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
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.