VAX 4000-100 QBUS cables

Peter Coghlan cctalk at
Tue Nov 29 12:47:40 CST 2016

Noel Chiappa wrote:
>     > From: Peter Coghlan
>     > Can anyone suggest an existing, simple QBUS device that I could study
>     > the documentation of to figure out what a basic QBUS device needs to
>     > have and to give me some ideas on how to implement one?
> Depends. Do you want to be able to do interrupts? Do you want to be able to
> do DMA? Each is a significant increment in complexity.

Interrupts would be great to have and DMA might also be useful down the line
but I have a much better chance of getting somewhere with this if I keep
things as simple as possible for now anyway.

> Later DEC QBUS devices may not be the best things to look at, since they tend
> to use special DEC QBUS control chips (I'm _not_ talking about bus
> transceiver chips here) which are of course no longer available. 

A single chip plus tranceivers solution would be ideal but I couldn't see it
being that easy :-)

> If all you want is master/slave (i.e. the ability to read/write registers),
> try this:
> It implements a single 16-bit register. (Changing it to support a single
> block of registers would of course be trivial.)

This is pretty much exactly what I was looking for. Thanks!

> The switches (and associated comparators) in the lower left allow one to set
> the bus address it responds to; the 3 latches on the right hold the register
> contents; the drivers/buffers below them drive LEDs to display the register
> contents. The control logic is about as simple as it can be; one latch, and a
> couple of gates.

I've studied the circuit a bit and it seems pretty clear what most of it is
doing.  However, I am puzzled by the BPOK and BINIT signals being connected to
U7 even though they do not seem to get used for either input or output.  I
wonder is this just because two tranceivers were left over and they might
as well have something connected to them that might come in handy later or is
it because I am failing to understand something properly?

> You should probably read the QBUS description in any QBUS PDP-11 manual
> before attempting to understand it, but having done that, it should be pretty
> self-explanatory - the signal names should clue you in to what they mean.

Thanks.  I'll do that.

Another thing I noticed is that there are rather more than 50 QBUS signals
listed on the top right of the circuit diagram.  I guess which ones are omitted
from the 50 pin D connectors will become apparant when I find a pinout for
those connectors.  I suppose the power rails and those labeled "spare" are
likely candidates for omission.

I was hoping that there would be lots of signal pairs like SCSI and some 25
pair telephone cable I have on hand might be suitable for the cabling but it
doesn't look like that is going to be appropriate.

Peter Coghlan

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