I believe that is how the PET is, since the BASIC test routine worked on
the Plus/4.
Good thing I am testing, found some BASIC bugs from the OCR of the
listing... and for some odd reason I could not define the variable
"port" on either the plus/4 or 64... never run into that one before...
I have verified the pinout (The plus/4 always seems to amaze me in it's
'different similarities').. Talk about tight squeeze, I had to file off
a millimeter or so off the side edges of the user port opening to get
the connector to fit.
Next comes the VIC-20 and hopefully the P-500.
Richard Atkinson wrote:
>
> On Mon, 24 Jul 2000, William Levak wrote:
>
> > > but I cannot find an entry for the cooresponding Data Direction Register
> > > which assigns the bits as input or output.... :/
> >
> > 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.
>
> In a sense, the data register also acts as a data direction register. When
> you write a zero to a bit, it becomes an output holding that line low. Any
> reads from that bit will yield a zero. When you write a one to that bit,
> it becomes an input and will be pulled high unless something else attached
> to it is holding it low. Thus when you read from it, it is either a one or
> a zero depending on the voltage on it. This is all due to open-drain
> logic. It's implemented as a buffer (to read from) and a transistor /
> pullup resistor pair (to write to). Reading it returns the state of the 8
> buffers and writing to it latches 8 bits into a register connected to the
> transistors.
>
> Richard
>
--
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Classic Commodore pages at: http://www.jps.net/foxnhare/commodore.html
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