[Coco] Question about the PIA RAM Size input

Roger Taylor operator at coco3.com
Mon Apr 30 16:31:47 EDT 2007

At 10:27 AM 4/30/2007, you wrote:
>Hi all,
>Does anyone know if any harm can be done by reprogramming the Data 
>Direction Register of PIA 2 such that the RAM Size input becomes an 
>output?  Since this line is often jumpered to either GND or +5V, 
>what happens when the PIA is trying to drive the line?
>The RAM Size input is on the 'B' side of the PIA, and the 6821 datasheet says:
>"As outputs, these lines are compatible with standard TTL and may 
>also be used as a source of at least one milliampere to directly 
>drive the base of a transistor switch."

Ah, one of my most favorite subjects... reusing 6821 lines.  There's 
always the interesting topic as well of how to achieve 7 or 8 D/A 
resolution by killing the printer and cassette input lines, etc.

In all of these CoCo-to-PC projects that are popping up, I'm sure we 
could find some very interesting uses for some existing rarely used 
PIA lines, or lines that can serve multiple purposes that currently do not.

My favorite thing about the 6821 PIA is that you can program each of 
the 8 I/O bits to be Input or Output.  That is quite a nice feature 
that has probably gone unrecognized by many, since 1980!  But... I 
bet more know about it than those who don't.

The Single-Bit sound output part of the CoCo is one of those features 
that could have been done without.  What software ever used it, and 
why?  You can alternate the MSBit of 65312 to achieve the same sound 
as inverting the SBSound bit of 65314.  So, what about cutting that 
trace to the sound circuit and reusing that line as an output?

Roger Taylor

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