6820 vs 6821 PIA

[mspangler@gemsi.com (Michael Spangler)]

I recently acquired a pair of Tektronix 4105 computers, one working
and one dead. The dead one has been dismantled, and has 8k of odd
sized static RAM, (neither 74LSxx width or 6502 40 pin width, but
somewhere between) and 5 ceramic cased 6820 PIAs. And a 6800 CPU, and
a stack of non-erasable ROMS.

Anyway, do any of you hardware hackers out there know of any real
difference between the 6820 and the 6821? I have docs on the 6821,
didn't find much on the 6820.

On a related question, some, but not all comments I can find on doing
Apple II IO cards says to use 74LS245 tranceivers on the data bus.
Some, but not all IO cards use the '245 as specified. How much are
they really needed? I have at least one card without a '245, and it's
using a 6821 directly on the bus.

Thanks;

Mike S.