[Coco] Re: Technical: PIA Help

jdaggett at gate.net jdaggett at gate.net
Mon Dec 5 15:45:01 EST 2005


The MC6822 is an industrial version of the MC6821. That is both 
Port A and B are open drain circuits and will drive 15 Volt CMOS 
loads as well as TTL loads. 

The MC6821 has Port A pulled up with an internal resistor. Port B 
uses a class B push/pull NMOS transistor stack. The MC6821 has a 
fan out of about 1.5 TTL loads. I believe they are  not as robust as 
the Intel 8255 chip. Also the MC6822 has less dcurrent drive than 
the MC6822.

IF my memory serves me correct, the Coco 3 uses a special 
version of the MC6821 for the Keyboard PIA. I don't think it is a 
MC6822. Motorola has done that with standard off the shelf parts. It 
could be that the Coco3 part has a m inor tweek in the output 
transistor geometry size to provide more or less current dirve. This 
is a minor change, this part would get a special marking like 


On 5 Dec 2005 at 20:34, Torsten Dittel wrote:

To:             	coco at maltedmedia.com
From:           	Torsten Dittel <Torsten at Dittel.info>
Date sent:      	Mon, 05 Dec 2005 20:34:37 +0100
Subject:        	[Coco] Re: Technical: PIA Help
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> 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! :-)

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