Re: Plus/4 Parallel Port I/O question

From: Larry Anderson (
Date: 2000-07-26 04:27:10

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

01000011 01001111 01001101 01001101 01001111 01000100 01001111 01010010 01000101
   Larry Anderson - Sysop of Silicon Realms BBS  (209) 754-1363 
300-14.4k bps
     Classic Commodore pages at:
01000011 01001111 01001101 01010000 01010101 01010100 01000101 01010010 01010011
This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail

Archive generated by hypermail 2.1.1.