Tough to Diagnose issue with PDP 11/40 CPU

Fritz Mueller fritzm at
Wed Jun 29 19:39:01 CDT 2016

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.


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