The best I could find was a few pages and a diagram from an old design book. I'll email it to you directly since it won't display correctly here.

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My Internet search results:

PIAs were poor devices. Their cost and size seldom justified their use. Simply hanging a 74x377/374 etc was always cheaper and simpler. That's probably why PIAs were always quickly discontinued.

That is a very rare part. It doesn't look like anyone is making it anymore.

As I scanned the internet for information, it was amusing that the only people using it tended to be University engineering departments particularly in the electronic labs. (I guess someone should tell them to upgrade their information).

The part was used on some of the original home computers that Tandy made and some of the contacts are about how to access the chips in that circuit. So far, that is the best description I've found on the chip:

http://ftp.logica.com/~burginp/m6821.html

I might have an old copy of the datasheet in my study - let me know if you need more information and I'll see if I can turn one up this weekend and scan it into the computer.

Here are some of the various internet sites that mention the 6821 PIA:

http://www2.fe.psu.edu/~ajg2/eet...sld001.htm
http://www.ee.nmt.edu/~rison/ee...lab10.html

-Jay C. Box