10 Mar
2008
10 Mar
'08
4:21 p.m.
Andrew Lynch wrote:
Hi,
...
When I bought this WMB machine it was completely dead.
It had no response
at all and the Z80 CPU was completely static except for the clock input on
pin 6. I used my oscilloscope to debug the circuit. The PS checked out
fine.
...
What is bugging me in this case though is whether swapping the two CPUs is
actually fixing the REAL problem or just coincidentally doing *something*
which is the making it work but not actually fixing the underlying problem.
I am uncertain if my "repairs" are actually causal or coincidental. The
good news is in the end if the machine works reliably it doesn't matter all
that much but I'd sure like to know for certain what actually happened.
Is there some method to verify a Z80 CPU or PIO is actually bad or not? I
have seen 74LSxxx chip testers which apparently work pretty well but nothing
that can test a Z80 CPU or other LSI type chip.
Thanks in advance for any advice or constructive comments.
Andrew Lynch
Testing a Z80 or other microprosessor based system usually requires more
than a scope, logic probe, and voltmeter. A Fluke 9010 (or 9100) with a
Z80 pod is good, or another CPU based test fixtures. The advantage of
the Fluke is the ability to test the RAM, ROM, and I/O even if you do
not know the memory map of your system. Best if you have a working
system then you can create a memory location based layout of the
firmware that the CPU 'sees'. Knowing this you can test the suspect
machine by punching in the same values and see if the RAM or ROM test OK
or not. If you do not have a working comparison machine you can test the
RAM knowing where the Z80 first looks for RAM and then extrapolate the
size of the RAM bank by the RAM IC part numbers. If you have 8 X 4116s
then you have a 16K X 8 bank of RAM.
Your problem is likely RAM or possibly boot ROM based if a second Z80
CPU gives similar symptoms, but identifying the particular IC is going
to be a bit painful without the right tools.
John :-#)#
PS - I purvey these machines, as do others -
--
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Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
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