Abstract: A direct current servo motor operates through a toothed belt linkage to drive the pen carriage of a chart recorder. The position of the drive is monitored by a sectored encoder disc on the motor shaft. A pair of optical sensors is located relative to the encoder disc to provide alternately phased transition signals as corresponding edges of the encoder disc sectors pass the sensors when the motor rotates. A microprocessor interrupted by a transition signal goes to a sub-routine that determines which way the encoder disc is rotating and stores a count signal of the corresponding sign in a register. Further the microprocessor compares an input signal with the stored count signal and directs the motor to rotate in the direction that will reduce the difference or error signal to zero thus causing the pen to follow the magnitude of the input signal.