Any difference between VAX Q-bus and PDP-11 Q-bus cards?

allison allisonportable at
Thu Dec 7 08:18:22 CST 2017

On 12/06/2017 09:55 PM, Jon Elson via cctech wrote:
> On 12/03/2017 10:28 AM, Aaron Jackson via cctech wrote:
>> I'm looking after a VAX 4000 for a friend, which has a SCSI Q-bus card
>> (M5976). If the card did not have the large metal face, would it work in
>> a Q-bus PDP-11? We are not going to potentially ruin a card by trying
>> this, but I am interested to know if this is the case.
> As long as the PDP supports the 22-bit Q-bus, it should work
> perfectly.  The metal plate can be removed.
> Jon

Save the plate though as they are scarce...

MicroVAX used Q22.  The thing to be wary of is the Q-bus spec allows for
slots ABCD (wide)
and also ABAB if its ABAB its easy and no big issues unless....  They
can also be ABABAB
(hex wide QBUS CORE).

Whats that mean... AB is the address, data and control bussed connectors
and CD are usually
parallel bussed but may have odd voltages or order to the signals for
some of the wide cards that
(usually RL and Memory) need them.  Its not unusual for a bus to be
mixed  like BA123 (uVAXII)
where the first 3 or 4 slots are ABCD and the remaining are ABAB.

IF the board requires ABCD such as the two board set RLV11 its important
as the two board talk
between them selves on the CD portion of the bus.

Also many of the uVAX and some later PDP-11 (J11based) the CD bus was
wired differently
for memory.

In most cases the ABCD portion of the uVAX backplanes are only a few
slots and then it reverts
to ABAB.

FYI if your in the ABAB area make sure you keep the bus grants and
interrupt grants continuous.

I know of this as I have PDP-11 and uVAX systems from LSI-11 though
uVAXII/GPX.  They are all Q
but there are more differences than Q16/Q18/Q22 due to the mechanical

Qbus board can be used in any system if they match the bus width (Q22
usually for uVAX) and
you pay attention to where you put them.  This includes a lot of the
PDP-11 Qbus cards. 


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