I would also do a cleaning of your empty backplane with a good vacuum cleaner
nozzle. It is not uncommon for bits of conductive detritus to fall into the
backplane over time. Given that the pins are adjacent on the connector it might
be a possible cause.
Vacuuming out any stuff that has accumulated over time will be beneficial in any
event. I have had mysterious 'opens' occur on a card slot when an old sticky
label that lost is stick fell into the slot, and got wedged between a card pin
and the connector.
Don
On 1/22/2017 2:05 PM, W2HX wrote:
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.
FYI
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 IS WRONG
http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp8/pdp8e/Omnibus_legenda.pdf
This file shows the Omnibus signal locations CORRECTLY (extracted from a larger
document)
http://w2hx.com/x/VintageComp/PDP-8e/Docs/Omnibus_Card_Edge_Designations.pdf
-----Original Message-----
From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of W2HX
Sent: Sunday, January 22, 2017 11:28 AM
To: General Discussion: On-Topic and Off-Topic Posts (cctalk@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:
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
