mounted to a rack gear not shown as the treadmill deck moves up and down the grade pot 16 turns with it a voltage is applied across the grade pot 16 and the voltage at a tap point on the potentiometer is measured and received by the microprocessor module 20 as a tap signal 18 the grade of the treadmill deck is proportional to the voltage measured at the tap point and the microprocessor module 20 sends up or down pwm signals to the grade module 12 in response to the tap signal 18 to turn the grade motor 14 in the appropriate direction the variable speed drive system 10 of the present invention also includes the microprocessor module 20 having a microprocessor 22 and software as shown generally in fig4 and in more detail in fig8 the microprocessor 22 communicates with the external treadmill controller module 24 sends pulse width modulated signals to the predriver module 26 and monitors various system parameters as described more fully below in the preferred form of the present invention the microprocessor 22 includes a communications link to the treadmill controller module 24 which preferably includes an rs422 serial link in the preferred form of the present invention as described more fully below the microprocessor module 20 receives a signal shown as abs in fig4 from the adaptive bus scaling system.