Step motors come in many sizes and shapes. But they all share one item in common - each step motor type moves in discrete-degree steps. They react to a series of voltage pulses supplied by their basic controller, known as an "indexer." The step motor was the first to accept digital pulses, then move or rotate a prescribed amount without any feedback device. All step motors are position devices without the need to use any feedback devices. They operate primarily in an open-loop control scheme.