dec /34a issue

Jay West jwest at
Thu Oct 19 14:34:21 CDT 2006

I just brought up my /34a and apparently it's sick. It has one DD11-PK. 
Configuration is as follows:

1 - M8266 (A-F)
2 - M8265 (A-F)
3 - M9312 (A-B), M7859 (C-F)
4 - M7891 (A-F)
5 - Grant (D)
6 - Grant (D)
7 - Grant (D)
8 - Grant (D)
9 - M9302 (A-B), Grant (D)

What works:
Storing & retreiving various patterns from ram via the front panel works 
fine in all cases.
Looping on CLR PC loops as expected
Looping on BR . loops as expected
Trap catcher works (first pass halts at 1030 filling ram, then a BR . loops 
as expected).

The memory address test program fails though. It halts at 246 indicating a 
memory addressing error. R1 points to 422. Examining memory via the front 
panel shows the following:

420    420
422    177355
424    177353
426    177351
430    177347

So it looks to me like it is able to store 420 in 420, but nothing after 
that. I would normally think there is a problem with the memory board 
(M7891). However, I have replaced that board with 2 others, and all 3 boards 
fail at the same address AND with the same values. I find that likely to 
rule out the memory board as really being bad. In addition, I can deposit 
and examine values to locations 420 through 430 via the front panel just 
fine. It's my understanding that the KY11-LB puts data in memory via the 
unibus, so I would think this makes it somewhat unlikely to be a backplane 
issue. Is it a strong likelyhood that the problem is the cpu set itself 
then, as that's what would be writing the values to memory during the 
address test?

And as I type this, I just noticed something interesting. The numbers stored 
in ram at 422 through 430 are the right numbers, just inverted logic. More 
specifically if you invert all the bits in 177355 you get 422, if you invert 
all the bits in 177353 you get 424, inverting 177351 gives 426 (all the 
latter values being what I'd expect). I'm guessing there's a dead inverter 
on the cpu set somewhere perhaps? But if that's the case, why does 420 get 
set to 420 correctly??

Any advice is most appreciated :)

Jay West 

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