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 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);
- insert the device's grant-acquiring/passing circuitry into the matching daisy-chained bus grant line, of the four such available on the UNIBUS (BG4 through BG7).
(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.
