Just about all digital motion controllers revert back to step and direction at there basic level. (How else could you digitally do it?) The fact that the loop is closed at the drives is no different than on a motion control board. (When it is done on the motion control card you can usually see following error at the control, with Rutex's new SPI interface this should also be possible.)
Darek
http://www.cnczone.com/forums/attach...achmentid=1712