quick omnibus question

Paul Anderson useddec at gmail.com
Sun Jan 22 15:05:40 CST 2017

Hi Eugene,

You might want to test the over the top connectors. I don't remember the
part #, but they do go bad. They can be labeled and swapped with a spare if
have one. Just look to see if anything has changed after every change.

Dirty contacts and bad connections are always a problem.

I would remove the backplane. The foam under it is probably goo. I like to
lay the backplane upside on white paper, and lift one side up about an inch
and drop it.
This loosens up a lot of junk that air and other things don't get out.It's
amazing what you might find on the paper.

clean and inspect before replacing.

Good Luck!!

On Sun, Jan 22, 2017 at 10:27 AM, 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.
> 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?
> Other than the reference PDF, is there another PDF that has a more
> detailed description of the bus and the signals?
> Thanks
> Eugene

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