6800/6820 Address decoding help...

[Josh Dersch]

ard at p850ug1.demon.co.uk wrote:
>
> It sounds like what you need is a 6820 PIA data sheet (a 6821 one would do
> as well). This shouldn't be hard to find.
>   
These are actually in Vol. 2 of the service manual, at the back :).
> >From what I remember, the PIA occupies 4 bytes of the memory map, selected by
> a pair of address inputs on the PIA chip called RS0 and RS1 (Register Select).
> These are normally linked to CPU address lines A0 and A1 (and I think that's
> the case in the 4051) so each PIA takes up 4 contiguous bytes.
>
> Each PIA has 2 ports (Port A and Port B), each consisting of 8 data lines
> (which can be inputs or outputs) and 2 control lines (CA1, CA2, CB1, CB2). The 
> data sheet will show you how you read/write the port lines, select their
> direction (in or out), control/read the control lines, etc.
>
> There seem to be some useful tables at the start of the second volume of the
> service manual saying what each of the port lines is linked to. Given that, 
>   
<slaps forehead>  I didn't even see that section of the service 
manual... just jumped straight to the schematics.  More fool me... this 
is quite helpful.

> and the schematics it should be very easy to work out what address does what.
> I've just started looking at the scheamtics and I can't see anything difficult
> here.
>   
I think I've got most of it worked out now... just not sure how to work 
out where the control lines (CA1/2, CB1/2) fit into this whole thing.

Thanks...
Josh
> -tony
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