14 Apr
2009
14 Apr
'09
10:56 p.m.
Josh Dersch wrote:
The KM11 looks like it may be very useful for getting
my 11/40's CPU
running again... anyone have any experiences to share using one of these?
I would say a pair is a necessary tool for debugging the 11/40 if there
is anything wrong beyond power supplies, etc.
My PDP-11/40 had a chafed backplane wire that caused it to fail the
KT-11D diagnostics. I messed around with it for a month trying to debug
the problem. I had no extender cards, so I would tack solder a lead onto
the signal I was interested in and then probe it with a scope while a
good scope loop program was running. I figured out what the problem was,
but not why. Sometimes the Virtual Address was being used when it should
have been the Physical Address. I finally built two KM-11s based on some
information provided by Tony Duell and the real KM-11 schematics.
After I had the KM-11s in hand, I fixed the problem in two evenings.
Single stepping through the microcode indicated immediately that a
signal was stuck in the same state. I think it was BUPP6 H. Then I had
to track down why. I removed all the non-essential cards. Then I
replaced the last driver IC for that signal... and nothing worked. I
then traced down each backplane connection for that signal and found one
that was nicked as it was pulled around a pin. I slipped a bit of Teflon
tubing over the pin and all has been well since.
Here is a description of my home brew KM-11 and a link to Tony's notes.
http://www.chdickman.com/pdp11/Notes/KM11
-chuck