Tough to Diagnose issue with PDP 11/40 CPU

Hey Bill, Do you have a KM11 maintenance card? Guy Sotomayor here sells kits and/or assembled boards at http://www.shiresoft.com/products/km11/KM11%20Replica.html. I built one up myself based on a layout by Tom Uban at http://www.ubanproductions.com/museum.html 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.