From: Spiro Trikaliotis (trik-news_at_gmx.de)
Date: 2003-09-23 08:49:21
Hello Nick,
On Tue, Sep 23, 2003 at 12:09:07PM +0800, ncoplin@orbeng.com wrote:
> Is there anything wrong with using a 1N4148 diodes in a wired-OR config to
> detect when all top 15 address lines are zero (that is A1-A15). This one
> output can then be feed to a CPLD along with A0 thus saving 14 inputs to the
> chip.
I don't think there to be any problems using diodes. You might need to look
at which diodes are best - they have to be fast, but not change the
behaviour of the address lines very much. Increasing rise- or fall-time of
the lines is not an option, that might be the reason why noone tried it so
far. I'd suggest testing it yourself.
> The other question is buffer/line drivers. Are 74245 chips needed to drive
> the address circuits from the cartridge port.
Yes, since most processors (including 6502) don't allow their lines to go
tri-state. The 74245 gives the processor this opportunity, which is needed
in order to let the VIC access the RAM.
HTH,
Spiro.
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