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

[Tristan Mumford <xtristan.xmumford@xgmail.xcom>]

Michael J. Mahon wrote:

> 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.

Why is that?
I'm not questioning you saying that, I just want to know why.


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

Good to know. Thanks. There are so many references it gets a little
confusing at times.

> 
>> 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.

That's true. If the CPU is a no-go on startup, what would appear on the
screen? Black or garbage?

> 
> 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.

Agreed. It is absolutely rock solid. It's kind of a shame it's not possible
to get a ][ to start directly from a Z80 soft card... is it? Because I have
a known good disk ][ clone that came from the clone in the first place.
Now I'm having evil thoughts of forcing a start, but I know it would be
nigh-on impossible without already having the CP/M booting stuff in RAM
anyway.

> 
> 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?).

the ][ in general does I believe. My instance however does not. Before its
convalescence in the shed I think it used to start. I'm not sure if it had
a failure before I put it there though. 

> 
> 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.

I wish. If I had the monitor I would be happy. It would give me a starting
point to probe it's darkest depths.


> 
> 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.


I'll have to make a logic probe. I think I used to have one a long time ago.
I'm really tempted to cheat and make a uC based one. That way I could have
a few custom functions like a tone or light when a pulse is detected. Just
a thought anyway.

Theoretically, if one were to pull every RAM chip out and make an expansion
card with the correct amount of RAM (or at least disabled in the ROM
region) would that work? Ie would an otherwise healthy Apple start?

> 
> 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.  ;-)

Good idea. Which one though, are we talking about the red book?

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