Re: DD2 & 3

On 6/18/2013 12:47 PM, Gerrit Heitsch wrote:
> On 06/18/2013 07:38 PM, Jim Brain wrote:
>
>> I guess, but dubious.  CBM could have just went and stuffed something in
>> the address map that contradicted the 6821 address space.  IN any event,
>> it was an academic question.  Interesting that they sourced a 6821
>> instead of just adding a '574/'245 or something simple.
>
> Replacing even a single port of a 6821 or a 6522 with TTLs only sounds 
> simple until you think about what this would really mean. I'd say a 
> 6821 or 6522 needs less space on the PCB than even a limited port 
> setup using TTLs in DIP that supports direction change of the data flow.
If just a bidir parallel port was needed, I think a '138/245/574/04 
would have done the job, and could have been put in the same space. I 
also know cost was an issue, over board size.  Still,I can't imagine 
they didn't consider the option, and they did not pursue it.  Thus, it 
gives some additional weight to SD's position that they were looking far 
ahead, or they planned to use some feature in the 6821 in the near term.

Like I said, interesting that they chose a full fledged PIA instead of a 
more tactical option.

Jim




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