On Feb 5, 2010, at 2:40 PM, Chuck Guzis wrote:
Even so, the
knoweldge that TTL inputs float high, and can more
easily
be pulled high than low was very well known at the time. Whoever
designed that chip was not thinkibng aobut how it was going to be
used
IMHO. I am not talking aobut somebody finding an application later on
for which the original design wasn't perfect, but actually I can't
think of a single applicaiton where the behaviour of the 8255 is
desireable. I can think of some where it doens't matter, but none
where you'd avtually want a chip that behaves that way.
That being said, apparently few think that it's an issue, as a web
search doesn't turn up much in the way of grumbling. But how many
8255s are installed in an application where mode switching is used?
Darned few, I would guess.
The 8255 was obsolete years ago. It's slow and most of the desired
functionality can be gotten more easily with other means, either with
programmable logic or even SSI.
So we're really beating a dead horse.
8255s are frequently used to interface to IDE drives, and mode
switching is required in that application, and it does get in the
way. Mostly people have dealt with it by using inverters on some of
the lines.
Why people don't just choose a better chip for that is beyond me.
It's much easier to do with a Z80 PIO if you're using an appropriate
bus. I'm sure the 6821 would be easy too.
-Dave
--
Dave McGuire
Port Charlotte, FL