Re: Plus/4 Parallel Port I/O question

From: Larry Anderson (
Date: 2000-07-25 02:38:15

I think I'll wire in a Plus/4 connector on to one connector to test the
Plus/4 with the BASIC version...  Sounds like it will work though.

The Complete Commodore Innerspace Anthology has the B-128 I/O in
registers it, will see if I can get my P-500 to cooperate and give me
feedback as to what registers to use.

"Hársfalvi, Levente" wrote:
> Hi!
> Larry Anderson wrote:
> > > There isn't one.  This is a 6529 single port interface (data sheet on
> > > funet).  When you read that address, all 8 data lines are read.  When you
> > > write, all 8 data lines are written.
> >
> > That's like the PET then, the DDR on it does not do much in relation to
> > this interface (on the VIC and 64 the DDR is very important)... so I'll
> > just define an unused location in the BASIC example.
> In fact the 6529B works by the well known NMOS trick that probably most
> Commodore people know well (some maybe even if just waken up from sleep
> ;-) ). The NMOS outputs can't provide strong HI levels. The 6529B input
> port always gives back the current state of the port lines. If you want
> to use a line as input, first write '1' to the appropriate output bit.
> Since the NMOS high outputs are weak, the outputs can be pulled low by
> an external line. If something pulled the line low, the appropriate bit
> in the data register will be 0.

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