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

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

Michael J. Mahon wrote:

> Tristan Mumford wrote:
>> 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 am assuming this is the case because the Disk ][ Controller has
> a reset generator that causes a longer reset pulse.  If there had
> not been an issue, this would not have been necessary.

So that's why that is. It never occurred to me.

-snip-

> Whatever the power-on pattern in the video RAM is, viewed as text.
> 
> Often this pattern is something like:  00 00 FF FF...
> 
Heyyy..... That is most intriguing. viewed as text, as in standard 40col
text mode. What I'm seeing seems to be something approximating HGR, I think
anyway.

I may have to try to take a screenshot for everyone to look at.

>>>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.
> 
> A Softcard relies upon a working 6502, so you can "boot" the Z80
> directly.

Did you mean can't?

> 
>>>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.
> 
> Then the processor is not running the reset code.

Unfortunately what I suspected.

> 
> Could be lots of things, but low RAM, processor, F8 ROM, and reset are
> the first things to suspect.  (Are all eight first-bank RAMs there? ;-)
>

They are all there in body, but I can't guarantee their sanity.

-snip-

>> 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.
> 
> True, and you could even make a simple logic analyzer!

What exactly do you have in mind?

Something came to me last night while trying to get to sleep. One of my
40pin uCs, and some pullups on all the used IO pins. Depending on the
states of the pins, it would be enough to power, earth, provide input and
monitor output for the standard 74 series and maybe some RAMs. Essentially
a programmable tester.
But I already have a "minor" project in the works already. it's a2 related
and it's a secret :) It will be freely available. I just don't want to get
people's hopes up if it doesn't work right.

For now I think a logic probe will be adequate.


> 
>> 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?
> 
> No, since reset will switch to main board RAM.

As a certain jaundiced animated man would say, "D'Oh!"

> 
>>>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?
> 
> No, the much more plentiful little white spiral-bound book.
> 

Gotcha. I'll grab an online copy I think :) 
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