Re: Update on my projects

From: Nicolas Welte (
Date: 2001-07-30 15:13:36

Ruud Baltissen wrote:
> Hallo Daniele,
> > That's a good news, maybe in future you will made a card that replaces
> > the 7501/8501 CPUs ;-)
> You only have to figure out what unknown R/W-IN-gate is.

This input gates the R/W signal with the AEC line from the TED chip. I did
some measurements on it, and IIRC it works in such a way the R/W signal is
latched with the edge of the AEC signal. I'd have to dig through some old
notes to find out exactly ...

Anyway, if you disable this feature by disconnecting the r/w gate input, the
machine still works fine, even in extreme overclocking mode (NTSC mode
enabled with 17MHz crystal), so I don't think it must be emulated.

And 8501 CPUs might be hard to find, but the cost of a replacement board like
Ruud's are still way to high to make it an option. And using a 6526(A)
doesn't seem to be a good idea anyway, replacing an obsolete chip with
another obsolete chip ... A really cheap I/O chip like the 8255 or 6821 (I
know, the registers...) would be much more attractive, but at least the 6522
would work as well, and this one is still easily available.


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