This is the progress report on the Apple 1 we are restoring. I'm wondering if anyone has any insights as to what our problem is. We think the 6820 PIA may be bad. But finding an original looks hard right now so we are going to try the 6821. Will this work? The following is from Larry Brown the engineer doing the restore.
Thanks, Mitch
I’ve been working on the board for a few days but have not had much success. The processor appears to be running and the video display also appear to be working but the data from the character data 6820 PIA to the video display doesn’t seem to work. Also the keyboard data doesn’t seem to reach the processor but it appears to be polled so there in no interrupt to confirm for sure. The PIA IC also has several wires running to the IC socket in the breadboard area so static damage is a possibility.
At this point, all I can come up with is to replace the 6820 PIA. Apparently they were long ago out of production, replaced with 6821 and the newer 68C21. Next thing to do is order a test 6821 IC. If it fixes the problem, we can search for a vintage part. Just in case, a spare 6502 would be good to have. This is turning into quite a project.
6821 PIA
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_43596_-1
Supposedly pin compatible 6502
http://www.mouser.com/ProductDetail/Western-Design-Center-WDC/W65C02S6TPG-14/?qs=sGAEpiMZZMtVFuKNr6IGvpdkwXR9vVB1
LB
Western Design Center makes a part, the W65C21N, which is fitted with internal pull-ups to make it compatible with NMOS-level signals. Supposedly it's a drop-in replacement for older 65/6820/21 parts. It's an option you could consider if you don't want to use old stock/eBay parts. Make sure you get the "21N" model if you go that way, not the "21S"
Unicorn Electronics claims to have 6520s in stock, that's another option to consider.
Out of curiosity, are you using the WDC 65c02 in the system right now instead of a MOS 6502? I'd be a *little* leery of that, as in addition to being not 100% TTL compatible the "C" version of the 6502 responds differently to certain "undocumented" instructions than the original. I don't know that the Apple I uses anything that would trip it up but it's a remote possibility.
Just be careful by brand.
The Motorola 6820 sometimes doesn't like certain keyboards. It can bounce characters by double registering the strobe. That one took drove me crazy working on a keyboard once on a Mimeo.
Supposedly all PIAs are program/pinout identical, but their electrical characteristics vary slightly. The original 6820 is unique because it's fabricated using the "enhancement mode" NMOS process whilst the later 6821 and the MOS 6520 are depletion mode. (Motorola started moving the whole 6800 product line over to depletion mode in 1976.) So if you want the thing to be *exactly* correct for an Apple I you have to find a 6820.
That said, depletion mode NMOS is both faster and more electrically stable. So there's a non-zero chance that swapping your 6820 for a 6520 or 6821 will actually improve its reliability.
Actually NTI boards could have had a Synertek 6520. I have seen a few in pictures and three in person and know for a fact one of them never had a chip replaced from new.
Been doing a bit of research into that topic lately.
If that's the case then I'd totally say go for a 6520. They're harder to find than a 6821 but still relatively easy.
(Anchor Electronics in Santa Clara has 6520As on their price list for $1.95, manufacturer unknown. I think one of my Commodore PETs has a Synertek 6520 instead of a MOS label chip so manufacturer shouldn't strictly matter for functional testing.)