AT&T 3B2

[Scott Quinn]

Picked up a 3B2-1000 from Josh about a month ago, and I'm still poking 
around prior to trying to mess with it (hardware looks fine, PSU good, 
tape roller is jelly but that's fixed by now, hard drive died, but 
that's being handled).

Does anyone here know much about these beasts? I have the service 
manual from Manx, but it doesn't say much about the architecture. 
Here's my big question now:

I have a 3B2-1000/80 backplane (CM519B) with a 3B2-1000/70 system board 
(CM518B) (in a 3B2-1000/60 box, to round things out...) 3B2-1000/60 has 
a single µBus slot and several buffered µBus slots. 3B2-1000/80 has 
Pbus slots and fewer buffered µBus slots (Pbus takes "Processor 
Element" multiprocessing cards). 1000/70 might or might not have a µBus 
slot (in the 1000/60 the µBus takes a cache card, the cache is 
integrated in the /70 and /80 system boards).

The service manual explains the µBus and the Pbus both with the blanket 
term of "unbuffered access to the CPU bus on the system board". Does 
anyone know what the difference is?

At this point, I'm trying to find out what the lower slot configuration 
is.