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

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

Tristan Mumford wrote:
> Michael J. Mahon wrote:
>
>
> > Tristan Mumford wrote:
> >
> > > Michael J. Mahon wrote:
> > >
> > >
> > >
> > > > Tristan Mumford wrote:

<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.

That sounds unusual, but if the processor isn't initializing the
system, it could come up in HGR mode.

> > > > 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?

I sure did--sorry.  ;-)

> > > > 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.

If there is more than one bank, you might try swapping with bank 0.
The odds of more than one failure are pretty low.

> > > 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?

You could actually save the times of logic transitions to capture
the "waveform" at whatever sample clock your uC can handle.

If you sample multiple lines, you could save the states into a circular
buffer and "trigger" on preset conditions to see what happened before
and after the trigger.

Since most of what happens around the processor is 1MHz, sampling at
5MHz or so would provide reasonable detail.

> 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.

Interesting approach--I like it!

> 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.

Very simple and very handy.

> > > 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!"

I really should have said "main board ROM", since that's the
address space that a RAM card maps to.  It is possible for a
slot card to disable all main board address decoding, and so
take over for main board ROM *and* RAM, but I'm not aware of
any manufactured cards that do so.  This would be the only way
to get page zero, etc., mapped to a slot card.

-michael

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