2 Jun
2009
2 Jun
'09
6:06 p.m.
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.
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.
-tony