2 Jun
2009
2 Jun
'09
7:27 p.m.
Tony Duell wrote:
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...)
Paul Anderson
<useddec at gmail.com> wrote:
I thought that was only true if you were using an M9302 termintor. That
board will assert SACK if it gets a grant (that is, if a grant goes all
the way along the Unibus and isn't 'taken' by some device). The older
terminators (M930, and I think M9300, don't. THey're just resistors to
terminate the bus.
I always used a M930 in the 11/40 and older
machines, and the M9300 in the
11/34 and newer machines. I have no way to check on the differences
now. now. Also, there may be a trick using MOS in an 11/40.
I can't remember that there should be any functional difference between
a M930 and a M9300. The later is just an improved design.
As for the original posters problems. When you get a stuck machine when
the terminator is in, but a somewhat more functional machine when the
bus terminator is out, you have a problem on the bus. Most likely a bus
grant or NPR grant. A third possibility is a
problem in the CPU with the
logic related to these signals.
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?
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 :)).
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.
Josh
-tony