Hi,
in order for a stepper to run it needs a stream of step pulses. If you could pulse an output pin of your controller
fast enough you could make the stepper move, faster the pulse rate the faster the stepper. You will not be able to
pulse an output pin anything like fast enough to make it useful using ordinary means.
If however you connected the step and direction pins (two outputs) of the A axis say, without coupling it to an axis
then the stepper would accelerate, run and stop on command and at commanded speed. If you wanted to run it
simultaneously with the X,Y Z axes then the feedrate limitation I mentioned comes into play, namely that the feedrate
will be determined by ALL the axes not just your free spinning stepper.
If you want a free spinning stepper that can run at a commanded speed that is set independently from the feedrate
that governs the coordinated motion axes then you need an out of band motor.
I'm not sure whether Mach3 has that functionality but Mach4 does, in fact you can have up to six out of band axes, one
is considered the spindle, so five independent step/direction controlled axes.
Mach3 development stopped six years ago, its so damned buggy, VB is so clunky and slow, the Modbus plugin is a nightmare....
get into Mach4.
Craig