Re: Plus/4 Parallel Port I/O question

From: Hársfalvi, Levente (
Date: 2000-07-25 00:05:33


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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