3 Jun
2009
3 Jun
'09
6:13 p.m.
I think
it's time to stop guessing -- I think we've tried all the obvious
things -- and start logical faultfinding. What test gear does the OP
have?
I have a nice Tektronix 1241 logic analyzer and a DMM (and a really
flaky old Tek O-scope). I said in my original mail on this thread that I just wanted to be sure my Unibus config _looked_
sane before I started
digging. (No sense spending hours debugging if all it is is a misplaced
board...)
Sure. It is always best to look for the obvious things first (says the
guy who once spent a long time fioguring out a low contrast fault on a
VT100 video board, only to find the CRT screen needed cleaning...). But
after a bit, it's best to attack the problem logically.
What _I_ would do is first check all the power
voltages, with the boards
in (it's too late to care about a rogue PSU damaging boards :-)). A low
+5V line, or a missing supply to the terminator, will casue all sorts of
problems.
The voltages seem to be fine with the boards installed (5.2V for the +5,
-5.3V for -5). I fixed the ACLO/DCLO problems I was having awhile back (turned out to be a bad contact on one of the many
molex connectors.
That was nice, since I didn't want to pull the supply out again :)).
How many +5V lines are there? Certainly in my 11/45 there are 4 or
5 +5V regulator bricks for the CPU, and you need to check the outputs of
all of them. They are not parelleled, rather one feeds the CPU boards,
another the first expansion backplane, and so on.
Then I'd look at all the Unibus signals both
with the terminator out
(they will still be terminated by the resistors at at the CPU end -- IIRC
on an 11/40 these are on the Unibus jumper between the CPU and first
expansion backplane) and with it fitted. I would guess something is
changing state, let's find out what.
I will start looking at this tonight (assuming I have the time tonight
:)). Thanks.