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Step/dir signals are usually related with stepper motors and not with DC servo motors. Usually DC servo motors are velocity controlled not position (step) controlled) in open loop. If you want to control positon with a DC servo motor you need a positon sensor feedback. Please try:
"how can i drive dc servo motor by using step/dir signals" <---- This is not enough information.
You want to drive what? The Velocity or the Position? All is Ok! Just use PID. It is simple and easy to use. Don't ask me what is PID because you can find it on this forum.