For many years I have been working off and on to get my pdp 11/40 running. I was having trouble getting past "chase the lights" and simple tests. I was even able to load in a papertape. It was in memory ok but never ran .I got a lot of good advice but nothing seemed to work. I made sure everything was correct on the backplane and RAM was ok. Some important diagnostics simply would not run however. I was stuck. After a lot of spinning my wheels finally I discovered the CPU was wired for a KJ11 module (stack limit register) but I did not have one installed. I was having problems with code that included the stack so there you go makes sense now....but it took a while. Part of the problem is that I always thought the KJ11 and KW11-L (line time clock) were always paired up. This was not a logical reasoning, it was only because I always saw them paired that way and I assumed they were a set. I even had in the past tested the system with both installed but it froze things so I abandoned, never thought to try just one or the other alone. But apparently that would have been the ticket, my system CPU was wired for the KJ11 only. I tried just the M7237 and poing it worked. I finally was able to run complex diagnostics and I even downloaded BASIC from papertape and ran it (much faster than 11/05 btw) Here is the page with more info http://vintagecomputer.net/browse_thread.cfm?id=635 Next I can try to boot an RT11 RL02 pack from an RL11 controller (M7762). If anyone is going through the same thing how to troubleshoot a lot of the UNIBUS stuff let me know happy to try to help. Bill -- @ BillDeg: Web: vintagecomputer.net Twitter: @billdeg <https://twitter.com/billdeg> Youtube: @billdeg <https://www.youtube.com/user/billdeg> Unauthorized Bio <http://www.vintagecomputer.net/readme.cfm>