To use a PC you generally need a motion card or other with analogue output. The loop is closed back to the motion card via the cards encoder input.
If you use a software based control like mach etc, the signal to the drives is step & direction through the parallel port, so if you have analogue drives you need a S/D to analogue conversion I believe if you search here for Pixie it will show up.
For an example of a closed Motion card look on the Galil Motion site.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.