Further 11/40 unibus questions...

[Tony Duell]

> > 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 

Excellent...

> 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

The other important power line is the -15V one, It's not just used for 
the RS232 drivers, it's used for the CPU clock on a lot of the older '11s
 
> (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.

OK, let us know how you get on :-)

-tony