[Coco] CoCo3 PIA?

[jdaggett at gate.net]

On 5 Jan 2007 at 13:08, Mike Pepe wrote:

> Don't forget that in most CoCos, the keyboard uses a 6822, which as I
> recall has open drain inputs on the A port.

The Coco1s used an MC6821 PIA  chip for the keyboard. 

Secondly the MC6822 has open drains on both Port A and Port B pins. 

Third, using Port A as an input to read the ROWs of a keyboard matrix would not 
care if the output of each pin on the port is open drain or not. 

I still hold to the belief that LSC81001 is not a MC6822 variation but an MC6821 
variation. My belief is that it has the sink NMOS transistor for port B pins is 
beefed up to be able to sink more current when a port B pin is pulled low. This 
could easily be done with a mask change that would make 8 transistors larger in 
geometry. 

When a port B pin is pulled low it will sink current from a corresponding ROW pin 
on port A. This current is sourced out of the corresponding port A pin, in my 
opinion, a NMOS depletion load that is in parallel with the port A pin. The NMOS 
depletionload acts as a pullup resistor for the corresponding port B pin sinking 
current. 

james