VAX 4000-100 QBUS cables
cctalk at beyondthepale.ie
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.
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