quick omnibus question

W2HX w2hx at w2hx.com
Sun Jan 22 16:05:20 CST 2017

Thanks to several who have made suggestions. Noel, Tony, Jim, and Paul thank you. The system has been very thoroughly cleaned and the PS has been load tested. The front panel has had some bulbs replaced as well.

I took all my boards (except the front panel) to a friend's house with a working 8e and tested each board in succession. I found that most boards worked but M8310 was not. And 8K of 32K was working fine. Above 8K there was a stuck bit in the core. But with 8K working we were able to boot OS8 with all of my boards except M8310 (oh, and I borrowed his serial tty card).

I did know about these MA7-MD4 bits that seem stuck together in my machine. But we did not see this problem in my friend's machine. So I am concluding the problem could be in one of 3 possible places. The 8310 card, the front panel, the bus itself.  Because these are the three components that were not part of the test in my friend's machine.

Next step was to see if these two bus lines are shorted somehow. After wasting time looking at a document that was wrong, I checked the M8310 card for shorts between MA7 and MD4 and none existed. 

I am waiting on a douglas electronics card extender in the mail to check the bus itself. This issue could still be in the front panel or in some logic further upstream. This will be interesting to troubleshoot.

That omnibus_legenda.pdf file on bitsavers is CLEARLY erroneous based on what you guys are telling me and I found another document that indeed indicates that file is wrong. Too bad that this file is stored on bitsavers when it is incorrect. 

I hope this email can be found by the next guy searching for the omnibus details so they can avoid wasting time.

This file shows the Omnibus signal locations CORRECTLY (extracted from a larger document)

-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of W2HX
Sent: Sunday, January 22, 2017 11:28 AM
To: General Discussion: On-Topic and Off-Topic Posts (cctalk at classiccmp.org)
Subject: quick omnibus question

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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