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

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

Tristan Mumford wrote:
> ....what a mess.
>
> Michael J. Mahon wrote:

I'll try to trim it up a bit.  ;-)

> > If there is more than one bank, you might try swapping with bank 0.
> > The odds of more than one failure are pretty low.
>
>
> It's just in possession of the bog standard complement of everything, bar
> the weird 16k RAM expansion.

In that case, you can try swapping the 8 DRAM chips in the row
nearest the processor ($0000..$3FFF) with the chips one row closer
to the front of the machine ($4000..$7FFF).

If the row of DRAM nearest the processor (which I imprecisely
referred to as "bank" 0) is OK, then the processor's minimal
RAM needs are satisfied.

If that doesn't doesn't get you a "beep" on power on, then
either the problem wasn't the DRAM, or there's another problem
as well.

<snip>

> > > 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!
>
>
> I see no reason why it wouldn't work. I know the PIC uCs at least could do
> it. I kind of like the ability to configure all logic states and
> powersupply in software. I wonder if there would be a market for something
> like that. It could even be used to generate truth tables from a chip,
> which would be the easiest way to test a chip anyway. Come to think of it
> that means it could also do things like help expose the darkest secrets of
> closed chips, and also dump most types of ROMS. I really have to build one
> now.

I also like the direct power approach, since it eliminates the
mess of transistors to switch the power.  Microcontroller I/O ports
with significant current sourcing capability are a nice solution.

> > > 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.

I applaud your restraint.  ;-)

> I tested my "secret" project today. It didn't work. When I start seeing
> results / make it look less pathetic / pose less danger to people's
> precious hardware I'll reveal more. Or I guess if I'm asked nicely.

Pretty please...with chocolate fudge and sprinkles on top...  ;-)

> > > > > 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.
>
>
> Shame. Such a card would be very useful.

I misspoke.  ;-(

A slot card can take over ROM space ($D000..$FFFF), but it cannot
take over RAM space using slot signals.

Sather describes an elegant (if that can be said of a jumper-based
approach) method for taking over RAM space, but it is not a native
capability.

-michael

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