Arduino Stepper Motor Pulse Generator

We have a NEMA 34 closed loop stepper motor and controller that needs a pulse generator to trigger it. Controller is an HSS86 (pdf attached) We want to control the motor direction and speed with a 5k-ohm potentiometer (centre off) Functionality as follows: Pot in centre position = motor off Pot moves left = left direction selected and pulse generated to spin motor Further pot is moved = faster motor is pulsed Pot moves right = right direction selected and pulse generated to spin motor Further Pot is moved = faster motor is pulsed The motor controller uses simple opto-isolators to select direction. Logic: 5k pot with centre off use as a voltage divider. mid range voltage = motor off >midrange voltage = trigger direction "a" and start pulses. As voltage increases = increase pulse rate <midrange voltage = trigger direction "b" and start pulses. As voltage decreases = increase pulse rate