Tough to Diagnose issue with PDP 11/40 CPU

william degnan billdegnan at
Thu Jun 30 07:04:16 CDT 2016

On Jun 30, 2016 1:19 AM, "Fritz Mueller" <fritzm at> wrote:
> Hey Bill,
> Do you have a KM11 maintenance card?  Guy Sotomayor here sells kits
and/or assembled boards at  I built one up
myself based on a layout by Tom Uban at
> The easiest way to get to the bottom of this since you have a nice,
short, repro case would be to step through the microcode with a KM11 and
see where it goes awry.  From that point, its fairly easy to come of up a
list of boards to swap and/or chips to check.
> If you want to go to the chip checking stage, you'll need some board
extenders and a logic probe, they are pretty cheap.  Or you can grab a
surplus logic analyzer and some DIP clips off eBay if you want to get posh!
> I just went through this process with my 11/45, it was pretty educational.
>         --FritzM.

It turns out that I was missing the KJ11 my cpu cards were wired to
expect.  On the 11/40 this module is the little M7237 card for space E of
slot 3.

The machine in question  did not come with this card, I assumed it was not
needed until I checked the cpu jumpers and discovered my error.  Should
have done this 1st!

It was impossible to run the CQKC diagnostic for 11/40 or 11/45 -

Instructions using IOT in particular.

Bill Degnan
twitter: billdeg

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