That darn Intel jingle...

[Chuck Guzis]

On 11/4/2005 at 1:36 AM ard at p850ug1.demon.co.uk wrote:

>Oh come on. It's broken. Fundamentally.
>
>For one thing you can't arbitratily set the direction of individual port 
>lines (virtually all other parallel chips let you do that). And that 
>write-to-mode-register-clears-outputs is ridiculous.

I'll concur with the mode-register write operation as being silly, but
apparently it doesn't get in the way for too many people.  And setting the
direction of I/O pins in groups of 8 or 4 is apparently not a stopper for
most people.    24 bits of mode 0 I/O is pretty cool, no?

Consider what the alternatives were back in--what was it--1974? (anyone
have an exact date?).  You needed parallel I/O, you used an 8212.  The 8255
was a pretty substantial step forward.   It's pretty amazing that it's
still around more than 30 years later.

>I suspect only because it was trivial to link to the ISA bus. Linking up 
>a 6821 (approximate contemporary) was more work.

I've seen 8255's hooked to 6800's.  Also trivial to interface there, as
well as Z-80 and a host of other CPUs.

>I can think of plenty that would be more useful if they were still being 
>made. The 6522 would come high up the list. 

Nothing wrong with a 6522--I've used them myself and found them darned
useful.   But 8255's are easy to obtain and simple to interface to.  

Trivia time:  Anyone notice that in the PC BIOS listing for the printer
port routines, the 8255 is mentioned in a comment?  See line 3082 in the
listing.  Wonder if this means that the original intent was to use an 8255,
but changed later?

Cheers,
Chuck