Further 11/40 unibus questions...

[Josh Dersch]

Tony Duell wrote:
>> Paul Anderson <useddec at gmail.com> wrote:
>>
>>     
>>> 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 
>>     
>
> 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.
>
>   
>> 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? 
>   
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...)

> 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
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