18 Jul
2005
18 Jul
'05
12:24 a.m.
Suppose you
have a non-working HP9810 calculator (well, it's almost a
computer :-)). You have a choice of 2 repairers.
A) Worked ofr HP as a service engineer for many years. He's got the
official service manual (which is a boardswapper guide), and can follow
the instructions which are basically to replace all the plug-in PCBs until
it starts working again. The fact that you can't get replacement boards
any more is another matter...
B) Has never worked for HP or any other computer company. But he was
given a broken 9810 a few years back and spent a couple of months working
out how to repair it. This guy can connect a logic analyser to an
undocumented internal connector and trace the CPU microcode. He's got his
own microcode listings and can thus tell just what the darn thing is
doing.
Now, who do you pick?
B. Component level debugging and repair is a rare and valuable skill. Chances are, B could probably debug the thing down to a dead
logic gate, whereas A
As indeed I have, several times (not all on the same 9810, I hasten to
add :-))
would probably go down to board level and go looking
for a new CPU microcode
board or whatever.
Odd you should pick that one. The CPU control board -- 09810-66513 --
which contains the microcode, is the only logic board I've not had to
repair in a 98x0 machine.... Still, I am sure they can fail.
There's also the point of actually knowing what your tools are telling you
and how to make the best use of the tools. It's no use knowing in how to use
an oscilloscope if you don't know how to interpret the information it's
giving you.
Oh, absolutely. Good test equipment is useful, but it's not essential,
and it's not a substitute for knowing what you are doing. Many people
(not on this list I hasten to add) seem to think there's a magic box you
can connect to a brokent <foo> that will tell you exactly what's wrong
and how to fix it. Of course there isn't.
I've said this many times, but it's worth repeating. In order to repair
something, you should : Make measurements (and otherwise gather evidence
as to the problem); Think about what the measuements are telling you;
Then, and only then, replace anything.
It is often possible to get the evidence you need without fancy test
equipment. The person who moans 'I could fix this if only I had a logic
analyser (or storage 'scope, or...)' is probably wrong. Not because such
instruments are not useful, they are. But becasue even if he had them,
he'd not be able to interpret the display. If he had the skill to do
that, he could probably manage without the instruments.
-tony