VAX 4000-100 QBUS cables

Noel Chiappa jnc at
Mon Nov 28 08:20:14 CST 2016

    > From: Pontus Pihlgren

    > The machine also has a qbus which is somewhat peculiar to find in a
    > desktop VAX.

I don't know much about Vaxen, but I think the early desktop ones all had a
QBUS. I'm not sure if it was ever used for memory access; I know some used it
as an I/O bus (to take advantage of all the existing QBUS I/O devices), and
used an 'over-the-back' cable system for the memory bus.

    > However, I don't have the cables necessary to attach a QBUS expansion.
    > Does anyone have a set? What do they even look like?

They are a pair of boards connected with a pair of 50-pin cables. The boards
are different on each end, because the QBUS does not have in/out connectors
which separate from device slots, like the UNIBUS; it only has regular QBUS
slots, and those have grant in on different pin from grant out. So on the card
at the end of one backplane, the grant line is connected to the 'grant in'
pin, and on the other card, to the 'grant out' pin.

There are a whole bunch of different card and variants thereof for this, with
and without termination resistors, etc; the M9400/M9401 pair are for Q18
(which you won't want, I'd be pretty sure), and the M9405/M9405 for Q22. One
set I know of is an M9404/M9405-YB.

There's a fair amount of discussion of multi-segment QBUS systems in the QBUS
PDP-11 processor/etc manuals, as far as the termination, etc goes.

As to finding a set, there are some available on eBait at the moment for a
not wholly unreasonable amount of money.


More information about the cctech mailing list