[Coco] CoCo3 PIA?

[jdaggett at gate.net]

On 3 Jan 2007 at 11:58, Mark Marlette wrote:

> 
> Does someone know if IC5, LSC part number is a TTL or CMOS device?
> 
**************

NMOS.

> I'm working on on interface device and I need to make sure that I  
> don't have a I/O conflict.
> 
> Since it is on the keyboard port it appears that a zero represents a  
> key press. So the A side must be inputs and the B side are the drives  
> of IC5.
> 
> With that said then if no key is pressed(N.O. contact configuration)  
> the A side input of the PIA must be pulled high, thus indicating no key.
> 
> This is a weird part number and I'm not exactly sure what is different  
> in this part than a 6821.
> 
*****************

Possible special grade out of an MC6821 or a mask tweek for one or more 
parameters. The part has either extended current and/or voltage ranges. Motorola 
would do a mask tweek to extend one or more paramters to customer needs for a 
minimum quantity order of about 50,000 pieces. 

My guess is that with the Tandy engineers believed they needed more current 
drive out of the B Port of a standard MC6821. This was needed to drive both Port 
A and the 74LS30. Motorola most likely just tweeked the mask in the area of the 
output transistors for Port B to increase t heir current sink and source capbilites. 
Doing that requires another mask set ID number and customer parrt ID number. 

james