I think the 5100 is just a baseboard with no internal bus.  My
recollection is that it's modeled after a 3100, but in a larger case
and with some upgraded peripherals, plus maybe an R3000 CPU.  But
that's just a vague recollection.

You'd really need to find somebody at DEC who'd be willing to part
with the documentation...

			       _MelloN_