Re: Ultimax questions

Hello!

Rob Clarke wrote:

> Yeah, I had kind of excluded the MAX basic from the list for exactly
> that reason. As you know, the cartridge port on the Max is subtely
> different and you don't have the same select lines exposed so it seemed
> easiest to use the /exram line to gate the latch. Maybe you have an idea
> for how we can use alternative lines that exposed that will leave the
> /exram free for it's intended purpose?

Yes, actually I have :-)

But first, you need to start supporting 16kB cartridges which you don't
at the moment, and the MAX BASIC cartridge is 16kB. Actually this is
quite easy. The only problem is that you need to AND the /ROML and /ROMH
lines and feed this as the /CS line to your EPROM. You can use two
diodes and a resistor to do that - so no additional chip needed. Then,
either /ROML or /ROMH needs to go to A13 line of the EPROM and you're
good to go.

Then, you have two lines from the CIA chip at the expansion port - SP
and CNT. This is CIA's shift register. So instead of a latch sitting at
/EXRAM, you can tie a resettable shift register to these lines and latch
a correct banking value through the shift register from CIA. I haven't
searched the TTL catalog to see if such chip exists, but probably it
does; if not, as a last resort you can cascade two 4-bit counters.

You have also the P0 line from the CPU if yoou need something more, but
I think the above concept should be sufficient for all your needs.

If you build a prototype of this and verify that it works, I'll be happy
to design a PCB ;-)

Regards,
Michau.




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