Richard, you still respond 'orangey' to my 'apples'. then on top of that you post In that case, your ISR would probably be more like and the make an exact copy of what I proposed in my previous post in order to make the solution yours.

Lets end this Richard will never agree that using the snail is dumb, and I will never agree it is brilliant.

I will make the point that I have stated in various form in many posts "I do not disagree that you CAN use the snail, However I do emphasize it is the wrong component for todays designs".

This is a compromise, that I have proposed again and again, and if Richard, instead of 'citing' will propose his compromise, I think we can close this up.

Now for the fun part
Somehow this discussion made me "take a trip down memory lane" and I remember (was it in '92) when the '51 was new to me (previously I used minicomputers) that someone suggested using the 8255 to me. Well there was no internet and the company was a bit new and did not have a library of data books (do you remember those) so I went 'next door' and asked another engineer for a copy. I got a thermally photocopied (do you remember those) copy of the datasheet and somehow managed to decipher the smeared letters.

Oh, you young whippersnappers, you do not know how good you have it. Virtually any datasheets at your fingertips, if it is printed, it is crystal clear. If you need help, you can post in a forum, or e-mail it to someone, no need to keep fighting or rely on a manufacturers engineer, that can not see you design (how would you get it to him instantly), giving you some fuzzy 'advise' for that reason.

Erik