my Data General Nova 4/X, disk woes continue
ard at p850ug1.demon.co.uk
Mon Apr 25 20:09:24 CDT 2005
> Tony wrote:
> > I am never in favour of shotgun replacements. It's the brother of
> > board-swapping in that you don't know what the fault was, you don't know
> > you've found it, so you can't know it's fixed.
> I'm not a fan of shotgun replacements. Board-swapping can be a useful
> diagnostic measure, in that if you swap a board out and the machine
> starts working, you have most likely isolated the fault to that board,
With the emphasiss on 'likely'...
I have actually seen a case like that (somewhat simplified) :
2 modules, A and B. Module A outputs a signal to B. A was marginal, but
withing spec on timing, B was just out of spec. System failed. Replacing
_A_ with one that was less marginal got the system to work. So you spend
a long time working on the old module A, which in fact is working within
spec, missing the fact that B is the problem.
> making it easier to focus your efforts to track down the actual failed
> I've found this technique to be especially useful when the symptom is
> sporadic flakyness.
And that's the one time I'd never use it. Intermittant faults are a pain
to trace because you don't know you've fixed them (Murphey's law will
ensure tha the machine works fro a bit after you've done something to it
:-)). With such faults you _must_ find the real cause and put it right.
Swap modules, you might think you've traced the fault to a particular
module, only for it to come back and bite you later on because it was
somewhere totally different.
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