quick omnibus question

W2HX w2hx at w2hx.com
Sun Jan 22 10:27:56 CST 2017

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:

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?


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