Tough to Diagnose issue with PDP 11/40 CPU

william degnan billdegnan at
Thu Jun 30 09:03:24 CDT 2016

On Thu, Jun 30, 2016 at 9:58 AM, Noel Chiappa <jnc at>

>     > From: William Degnan
>     > The IOT step is bombing (?) and loops through the addresses:
> This may be a pointless question, but just to clear the ground first: the
> is otherwise functioning reasonably well? E.g. it's not dropping the 020
> bit
> when reading words from memory? (That would convert the '220' new PC in the
> vector to '200', and produce exactly the behaviour you are seeing.)
> If it is otherwise more or less working, so this is specific to IOT trap
> handling, I agree with Fritz - a KM11 would be a big help.
>         Noel

It was not apparently dropping the 020 bit.  The problem was a missing
KJ11, I did not notice the CPU cards were wired for one.  When I first got
this 11/40 10 years ago it did not come with it installed, I *assumed* the
machine as I got it was complete.  My error.  Lesson: Always check
@ BillDeg:
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