Abstract: The print wheel and carriage position control signal generators each employ an LED light source for illuminating light sensitive detectors associated with the timing track of an encoder disc rotatably mounted on the print wheel or carriage motor shaft. The detectors are arranged to generate a pair of signal trains having a constant phase displacement in response to rotation of the associated disc. These signal trains are shaped and processed to provide position reference signals used to control an associated position counter unit and an electronic servo control system for driving the associated motor, the servo control system having both a position and a rate mode of operation. A separate light sensor is arranged to be constantly illuminated by the LED source to provide intensity monitoring signals which are compared with a reference voltage in a feedback circuit so that any variation in the light intensity is automatically corrected.