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

["Michael J. Mahon" <mjmahon@aol.com>]

Tristan Mumford wrote:
> This may seem like an odd question, but how many components can I pull out
> of an apple ][, well an identical clone anyway to see some signs of life?
>
> I'm in the mood to attack my infirm clone to try to get it running again.
> As it is I'm fairly certain the problem isn't the CPU or ROMS. About a month
> ago I was doing a random pull test. Essentially pull a chip, power up and
> see what changed. The most difference that pulling a chip made was making
> the screen all white. Other than that all that really changed was the
> pattern of 'garbage' on the screen. Although I'm sure the garbage pattern
> would probably tell a story to some people it doesn't to me.

This is complicated by the fact that an Apple ][ powers up in
the monitor, and may occasionally require a reset after power on
to start cleanly.

> I'm hopeful of a solution that doesn't involve me blindly replacing chips
> until I find the culprit.
>
> This all reminds me, is there an Apple2 schematic available? It would help
> me to no end... and also help me with a conceptual future project. A
> reduced logic Apple2 clone. Nothing exciting. Just integrate the RAM and
> ROM chips to reduce chipcount, and a few other minor fiddles for my own
> contentment, like AT/X board layout and PS/2 keyboard support. It's not a
> short term project by any means, but I know it's by all means achievable.

All Apple II Reference Manuals have the schematic.  It's a little small,
but still readable.

> Anyway, back to it. The clone ][.
> I used a replacement power supply to eliminate that. I've pulled the ROMs,
> CPU, and chargen with no real effect. Pulling various RAMs affects the
> pattern a little like leaving alternating gaps in the garbage.
> Strangely enough the patches of random garbage slowly dissapear and turn to
> black. After a while no more patches stabilise though.

There are two quite separate parts of the Apple ][--the video generator
and the processor.

If you see a stable raster on the screen (even though it displays
garbage), then the video generator is operating.  It's hard to tell
whether it is operating correctly unless the processor is also working
well enough to change the display.

For this to work, at least the F8 ROM must be working, since it has the
reset routine and the monitor.  It has the code that makes the machine
"beep" when powered on (the ][ does that, doesn't it?).

If you can get the "*" monitor prompt, you can examine and change the
content of memory and, of course, get output on the text screen.  This
will verify that the video generator is operating correctly, at least
for text.

If the processor isn't working (no beep, for example), then you need
to start checking its reset line, clock signals, address and data
busses, etc.  A logic probe can be very useful in finding missing
or stuck lines.

Get a copy of the Reference Manual, and many questions will be answered
and many doors will be opened.  The good news is that they are still
plentiful.  ;-)

-michael

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