Servo as a Spindle??

Peter, thanks for the info.
Can you share output pins with this spindle pulse train?
In other words having a 4th axis that is also the spindle motor?
What G-code activates this pulse train? How does the control know
the pulse train relationship to spindle speed and where is this set?
What happens if you give the axis a move command while the spindle is
running?

What are the realistic speed limitations?

I knew this was talked about in mach 2 but didn't realize it was
available in Mach 1
Thanks again
Joe V.