Resolved: Re: 11/34 sanity check

Henk Gooijen henk.gooijen at
Sun Apr 26 13:12:08 CDT 2015

-----Oorspronkelijk bericht----- 
From: Josh Dersch
Sent: Sunday, April 26, 2015 7:58 PM
To: General at ; Discussion at and 
Off-Topic Posts
Subject: Resolved: Re: 11/34 sanity check

Got the following e-mail in my inbox from a listmember who apparently
wasn't able to get his mails to show up on the list:

> John,
> my post on classiccmp is not appearing on the list, so here a direct mail:
> ...
>> So at this point all voltages are spot-on, the AC LO and DC LO signals
>> are high (these are active low, right?) but I can't get the machine to
>> respond at all -- the "DC ON" and "RUN" lights are lit up brightly, and
>> the "SR DISP," "BUS ERR," and "MAINT" lights are glowing dimly. The
>> status register display is dark.  The system does not respond to any
>> keypresses on the panel.
> All LEDs "glowing dark"?
> I've seen this, when the 20 pin cable from M7859 to the console panel
> is plugged in reversed.
> Don't trust the marked pin 1 wire: sometimes DEC manufacturing twisted
> the cables.
> hope this helps,
> Joerg

And sure enough, I flipped the cable around and things are much happier,
the front panel is completely responsive and the CPU and bus seem to be
working but I'll need to spend some more time with it to test stuff out.

That'll learn me to trust Pin-1 markings on DEC stuff :).

Thanks all for the suggestions, it's all been very educational...

- Josh

----- Henk's reply.

Well done Josh, and perfect job from Jörg!
I had the same experience when I was working on a dead M7859.
And I did the same goof up, but I do not remember the result of that.

Lesson learned: never ASSume a red stripe on a ribbon cable denotes pin #1.
DEC is not alone ... the HP 21MX console connects using a 50-wire ribbon
cable attached to a 50-pin IDC header. I soldered a substitute cable ... had
to do it all over, as the red stripe on the HP ribbon cable is wire #50 ! 

- Henk 

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