[rescue] Parallel ports [was Re: Slightly OT: ?Bad Cap Saga]

Geoffrey S. Mendelson gsm at mendelson.com
Fri Aug 22 01:16:04 CDT 2008

On Thu, Aug 21, 2008 at 04:45:35PM -0700, Curious George wrote:
>What do you think "parallel port printer protocol" is?
>Centronics has defined how the interface is supposed to
>work.  If you are taking advantage of a particular
>bastardized implementation of that interface, then how
>can you complain that the "new interface" isn't as good
>as the old?

That's an even bigger problem than you think. Centronics ports were not
even TTL, they originaly used some logic family that existed before TTL.
The current specifications use TTL voltage levels (5v and ground). 
I don't know if an original Centronics printer will work with them or

They were eight bit data out and a few control/status lines. They were
never intended to send data to a computer, only receive it. And it was
a pretty simple protocol, wait for the ready line to come "on",
place the data on the bus, and set the "here's a character" line "on".

Status information was printer oriented, ready-for-data, paper-out,
and maybe one or two other lines.

IBM implemented the port on the original PC, but made a mistake in the
technical documentation. The IBM printer ports simply would not work if
you programed a driver as described.

This caused problems because the compatible computers used the design
as specified and they would not work properly. 

This was resolved by the drivers, I expect originaly the clone makers
had their own versions included in MS/DOS, but eventually it became a moot
point as IBM PC's faded out and everyone used the new port design.

The first time I saw a printer port being used as a data port was as
a high speed (115k) serial port to transfer data from XT/AT computers
to PS/2 computers. Since PS/2's did not have floppy drives that were
"PC compatible" IBM came up with this method of transfer. 

Geoffrey S. Mendelson, Jerusalem, Israel gsm at mendelson.com  N3OWJ/4X1GM

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