I think Rutex do a StepDir to analogue converter, also the top board from the Tek10 drives can do this. I have never used the Rutex or in fact know anyone who has one. I have partially tried out the Tek10 top board, when I say partially what I mean is it was in the early days of my Lathe retrofit, I got some AC motors with Analogue drives. As a test I hooked the analogue output of the top board off of a Tek10 drive to a meter and had an encoder mounted to the Tek10. As I moved the encoder the voltage in the drive increased, turned it the other way and the voltage increased in the negative direction. Obviously after turning so many counts the drive tripped out with an out of position error but it did show it was working correctly. Never got as far as actually hooking up the motor and analogue drive as some drives that accepted Step/Dir came along at the right price.
Hood