quick omnibus question

Tony Duell ard.p850ug1 at gmail.com
Sun Jan 22 14:17:18 CST 2017

On Sun, Jan 22, 2017 at 4:27 PM, W2HX <w2hx at w2hx.com> wrote:
> Hi friends.
> I am 100% new to my pdp8e and I am troubleshooting a problem.
> The problem is that whenever address bit 7 I asserted, I also see MD bit 4 asserted.
> I am hoping there is a simple short somehow between these lines somewhere. I
> should mention that my setup has known working boards with the exception of
> M8310 does not work (all of my boards were tested in another machine one by one).
> So I am hoping the problem noted above might be occurring on the M8310 board itself.

Could it be a short on the backplane? Or on the jumpers between the
backplanes if you
have 2 backplanes in the box (The standard 8/e box is designed to take
2 backplanes,
I believe the rear one was optional but almost always fitted).

> I found a document that describes the signals on the bus located here:
> http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp8/pdp8e/Omnibus_legenda.pdf
> I noticed that B1J (MA7) is directly adjacent to B1K (MD4). Could this be related?
> So I decided to pull my 8310 and check the resistance between these signals on the
> edge of the board. However, I cannot seem to square the signals named in the PDF
> and what I see on the card edge connector on the board.
> I am wondering if I am not understanding the PDF correctly. I have the board sitting
>  on my table with the components facing up and the omnibus card edge is at the
>  bottom of the card.  Going from left to right, do I have connectors ABCD or is it
> something else? Maybe DCBA?

With the board like that, Connector A is on the right (so the order is
DCBA from left
to right, pin A is on the right of each connector, and side 1 is the
solder side.

> Other than the reference PDF, is there another PDF that has a more detailed
> description of the bus and the signals?

Do you have the Engineering Drawing/Printset (same thing, just 2
different names)
from bitsavers? There is also an excellent 3-volume PDP8/e (and /f,
/m) maintenance
manual on bitsavers (I have it on paper too). Volume 1 covers the CPU (which is
what you need now), Volume 2 is 'internal options', Volume 3 is
'external options. In
time you will need all of them.


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