2 axes vs 6 on a parallel port

Carlos Guillermo wrote:

> I remember several people mentioning they're using old laptops to
> control their machines. I thought most step/dir systems needed one
> parallel port for every two axes. What do you do for three? Or, can
> you somehow reconfigure the output pins for another step/dir channel by
> forfeiting some of the extra I/O pins?

There are three main ways to drive steppers off a parallel port.

Keep in mind there are 12 outputs and 5 inputs in a standard
parallel port (although Expanded parallel ports EPP can have their
17 pins configured differently).

Step & Direction
There being 12 output bits, you can have a maximum of 6 axes
driven, each axes having one step, and one direction bit. Can be
unipolar or bipolar (bipolar is better IMHO).


Two bit Direction
Related to the above, each axis has two bits, but this time a
"move CW" bit and a "move CCW" bit. Obviously both
bits are never active at the same time, but can be inactive at
the same time. This also gives a possibility
of 6 axes from one parallel port. Can be unipolar or bipolar.


Direct Phase Drive (ie poor mans stepper driver IMHO)
The software drives 4 pins per axis, and cycles through
a move CW and move CCW sequence to usually drive
4 transistors in a unipolar arrangement (ie non-4 wire steppers).
I suppose this could drive 3 axes (using 12 bits), but I only
recall seeing 1 and 2 axis hardware.

- -

All this assumes stepper motor drive circuits. However, some
new servo designs (ie http://www.isomedia.com/homes/billmw/
& others) drive feedback and non-feedback ac/dc motors
from the parallel port. If there is feedback, it is usually to the
hardware to keep it tracking where it is supposed to be, not back
to the computer to tell it where it is (due to the limited number of
input pins back to the computer, usually reserved for Limit switches).

Doug Fortune
pentam@...