Unibus / Qbus

allison ajp166 at verizon.net
Sat Oct 22 15:08:34 CDT 2016

On 10/22/2016 03:18 PM, shadoooo wrote:
> Hello,
> we are discussing on separate thread about doing an universal interface for
> PDP11.
> I'm taking all the relevant documentation about Unibus and Qbus busses,
> aiming to check the possibility of doing a board compatible, with some
> adjustments, with both worlds.
> I started to read the 1979 specifications, however it's not all clear to
> me, specially about Unibus.
> What I understood:
> - Qbus is complete on A and B connectors, so a dual card could be done.
> Some backplanes have a true serpentine, while some other has C and D with
> other signals, but those are of particular usage with dual-board interfaces.
> Basically both dual and quad boards can be done, with the latter using A
> and B and simply propagating grant on C and D, supposedly connected in
> standard serpentine.
> Unibus: the specifications are describing A and B, but backplanes are
> complicate than that, and can have Unibus, Modified Unibus, Extended
> Unibus, SPC...
> What for?
> If all the signals are in AB, why they are connected again in CDEF?
> There's some complete documentation about the different backplane types,
> and the standard approach for an Unibus board?
> Thanks
> Andrea

There are later DEC databooks on the net that give a more complete picture.

The biggest total difference is the QBUS the address (a0-15) and data
d0-15 are multiplexed.
So separate boards make more sense for the buses when you allow for the
Qbus being AB
and Unibus minimally quad or hex size.  

FYI the CD and some cases EF width for Qbus was to allow for quad wide
and hex wide cards
for large peripherals or memories (PDP-11 Qbus CORE is hex wide) and
many board sets for
Qbus like RLV-11 (two boards) need CD interconnect to ty both together
but not for CPU access
where the single board version RLV21 is only a single quad wide.

So a Qbus mass storage could be a dual width and can be very simple for
IDE/CF or maybe SD.

Often the larger problem is not building hardware (there was an IDE
design out there for
QBUS VAX or PDP11 using PIO transfers) but a driver for VMS, Ultrix,
was a totally larger project.

So to do that you have two project the hardware is fairly straight
forward (see the
applicable Bus interfacing books put out by DEC) but the software to use
it is a project.
FYI I have never heard of any one recreating the RQDX1/2/3 software
protocol MSCP
as it was nontrivial, proprietary, and copyrighted.


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