Plessey PM-D11/SPC-1 backplane documentation needed

[tony duell]

> 
> But I'm trying to figure out exactly what sort of SPC backplane it is. E.g.
> it looks like slots 2-8 _might_ be MUD (the signals from 1A/B are bussed
> across all those slots on A/B, over to 9A/B). But without documentation, or
> a lot of ohm-meter work...

IIRC the difference (at least on 18-bit addressing machines) between the MUD
pinout and the Unibus (as in slots 1A/B and 9A/B) are power and grounds
connections. The logic signals are in the same place. It shouldn't be too
hard to check if there are power lines on 2A/B..8A/B that match up with MUD
pinouts.

-tony