VAX 4000-100 QBUS cables

allison ajp166 at
Mon Nov 28 09:12:32 CST 2016

On 11/28/16 9:20 AM, Noel Chiappa wrote:
>      > 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.
> 	Noel
Qbus VAXen were all PMI(private memory interface, aka over the top 
cable).  That means all memory
was always adjacent to the cpu and the CD slots used were unique without 
full Qbus signals.
The bus was for IO and allowed devices like RLV11, RQDX2/3, DLV11, 
and many others.

While multibox Qbus VAXen were done it was not a preferred condition and 
the suggested
larger BA123 was preferred to the BA23.  The required cables mentioned 
are needed and
all good care in configuration.  Note both (all) chassis should be int 
he same rack and have
good common grounding to each other for best system stability along with 
a common power
controller (single point power control).

I have two VAXen using a BA23 and a BA123 (both uVAX-II).  The latter 
BA123 is preferred as
there is far more power and space for cards and media drives as well as 
room for cables.


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