[Coco] Re: Technical: PIA Help

Torsten Dittel Torsten at Dittel.info
Mon Dec 5 14:34:37 EST 2005

Hi Brad,

just some thoughts produced by a blurry remembrance (might be pure
nonsense): I guess before writing and reading the keyboard you have to
programm the corresponding PIA registers as inputs or outputs. I guess
it's possible to "inverse" the input/output configuration. Maybe one
even could mix it and make some PIA lines read and others write (no idea
what this would be good for).

I remember darkly on first Motorola "CoCo" board designs (using two PIAS
6821) it was even possible to programm both PIAS as outputs and "burn"
the PIA lines by pressing the corresponding keys (kind of short
circuit). Those lines would work only in one direction after being
killed that way. Some software would still work (e.g. BASIC) but other
using the "inverse" PIA setting wouldn't recognize the keys any longer).
AFAIR this was a know problem and Motorola replaced the 6821 by the 6822
which had kind of protection against this (I wonder if it's possible to
kill PIAs in one of the CoCo1 emulators, it would win a price for
accurate emulation... ;-)).

I'd enjoy some comments by all you gurus out there! :-)

Best regards,

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