Re: strange 2001N fault

On 02/20/2018 07:28 PM, Gerrit Heitsch wrote:
> On 02/20/2018 07:07 PM, Francesco Messineo wrote:
>> On Tue, Feb 20, 2018 at 5:34 PM, Gerrit Heitsch
>> <gerrit@laosinh.s.bawue.de> wrote:
>>> On 02/20/2018 11:53 AM, Francesco Messineo wrote:
>>>>
>>>> Hi all,
>>>> in case someone doesn't read vcfed forums, my 3032 developed a strange
>>>> fault:
>>>>
>>>> http://www.vcfed.org/forum/showthread.php?62202-3032-(2001N)-strange-fault 
>>>>
>>>>
>>>> The thread link is just to recap what I've found and what I did, so I
>>>> don't have to type all again here ;-)
>>>> Ideas welcome, I'm just scratching my head. This is probably where a
>>>> logic analyzer is needed badly.
>>>> It seems to me there're unintended writes on some ram locations (not
>>>> so random, address wise) and it makes me think about VSP crashes on
>>>> C64.
>>>> Any idea where to look for a fault?
>>>
>>>
>>> Address multiplexer? I think on those boards they used the 74LS153 which
>>> contains two 4-to-1 multiplexer. Also check their control circuits.
>>
>> yes, 4 x 153, they carry the refresh addresses and row/column addresses.
>> All 4 look fine, also control signals as far as I can tell.
>> The fault is intermittent, so it's really hard to catch with a dual
>> track scope. I think I'll leave this board alone
>> for the moment, it's that kind of fault  that really requires a logic
>> analyzer I'm afraid.
>> As far as I can remember, DRAM can be corrupted during a read
>> operation (or even refresh?) if addresses change at the wrong time,
>> right?
> 
> Yes, that's why I suggested the multiplexers or their control circuits. 
> If you have a dual trace scope, you can do a lot to hunt down such a 
> bug. First check the Multiplexer inputs against /RAS, the latter should 
> be low for a bit before the multiplexer control line switches.

Oh.... If your scope is analog, try to set it to 'chop' instead of 
'alternate'. The reason for that is that 'alternate' will do a full 
trace of one signal and then a full trace of the other. If you switch to 
'chop', it will quickly alternate between both inputs while doing the 
traces. This way you get to see what is happening on both inputs at the 
same time while 'alternate' will introduce a lag. With digital signals 
that will suggest a relationship between 2 signals that might not 
actually be there.

To see the difference, set the scope to a slow deflection so you can see 
the point wander over the screen and then switch between 'alternate' and 
'chop'.

  Gerrit


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