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Re: How much of a ][ is needed to twitch?

Tristan Mumford wrote:

Matthew T. Russotto wrote:



In article <45ce84be$0$83739$c30e37c6@lon-reader.news.telstra.net>,
Tristan Mumford  <xtristan.xmumford@xgmail.xcom> wrote:



-snipped idea of pulling chips to diagnose machine-


This is probably NOT a good way to diagnose the machine.  The garbage
pattern really says nothing except that the character generator and at
least some of the RAM is working.



I'm hopeful of a solution that doesn't involve me blindly replacing chips
until I find the culprit.


That has a better chance of succeeding.  Using a scope or even just a
multimeter and the schematic will help.  First thing I'd check is
probably to see if the processor is running at all (see if the clock
phase outputs are changing).



You have a very good point there. At least finding whether the 6502 is
receiving the signals. Because one thing I can say is the CPU actually
being in the motherboard makes no visible difference. Ie I pulled it and it
didn't affect the garbage.


Of course, it wouldn't make a difference until it changes some
data on the text page--like storing "APPLE ][" there.  ;-)


If the CPU is fried it's a nuisance but no big deal. The 65816 should work,
and they are relatively inexpensive (although it would probably be a few
months before I could get even that).


IIRC, the 65816 cannot be expected to work in a ][.  The 65802 would
work, but is no longer manufactured.


I wish I had a scope. Kind of makes me wish I could do some kind of
space/time teleport trick. When i was living in near melbourne there was a
small pawnbroker that had the strangest stuff. Including about 6 CROs.


A scope is certainly handy, but since you're debugging circuits that
are known to work unless a part is bad, just tracing signal activity
with a logic probe is often enough.

For the processor to work (assuming that it's good, which it almost
always is), there must be power, clock, and a good reset.  For it to
do anything meaningful, it must be able to read ROM, and read and write
RAM.  There are relatively few parts in that path, so it's not too bad.

-michael

NadaNet networking for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it's seriously underused."