not sure it is really step/dir as such, think the PMDX or V4 (whatever that is) will convert the step pulses to 0-10v for the drive.
It will depend what Fwd and Rev signals your drive accepts as to how it will do the Fwd/Rev. It may not even be capable of doing Fwd/Rev, some spindle control drives can only do one direction, for example the Control Techniques Mentor drives come in 2 or 4 quadrent models, the 2 quadrent will not reverse direction, that may be your issue.
Hood