[Coco] CoCo3 PIA?

[Phill Harvey-Smith]

Mark Marlette wrote:
> james,
> 
> If the LSTTL is not being used as the pull up and the 68x21 doesn't  
> have configurable pullups. How does the circuit guarantee a hi level  
> for the CoCo3 keyboard interface?

Port A which I believe is configured as inputs have internal pullups on 
all lines, which operate in either input or output mode. Since a keyrow 
with no keys pressed will not be connected to anything, the internal 
pullup should ensure the 1 level. A connected key on the row being read 
will of course be pulled down to zero, by dint of the output row being 
zero, which will overcome the pullup.

This very behavior is used on a couple of the Dragons to pull up the rom 
select line of a rom, and ensure that it is selected at reset time.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.