quick omnibus question

jim stephens jwsmail at jwsss.com
Sun Jan 22 14:35:13 CST 2017

On 1/22/2017 12:17 PM, Tony Duell wrote:
> 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).
My friend Sherman has a number of 8/E systems he is going to get going, 
and has a setup consisting of a frame, PS, and backplane.  He has a 
couple of single slot fixtures with switches at the moment which he used 
to ring out a potentially working front panel, and found a few failures 

Of course after checking the backplane for shorts, and the PS.

He went to the M8330 (Timing board) and has fixed some things on that 
board.  he will cycle all board sets thru this system setup once he gets 
all the way to the processor.

As tony says, I'd start very basic with the backplane.  Especially if 
there is proximity of the signals there for your specific problem.

One system will be all scrap boards including a front panel which he 
retrieved just a tad too late from a scrap operation that had clipped 
the flip chip connectors.  A strap on Douglas board and rewiring will 
fix that one.

There are a lot of places in a system with most of the boards present 
that you can have failures, so hard to give a lot of suggestions.

I also hope the other main thing to test, the power supply was checked 
out and isn't playing games with you, though it doesn't seem likely it 
would make the short you suggest.


>> 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.
> -tony

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