4 Nov
2005
4 Nov
'05
5:18 a.m.
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