Abstract: A shaft position, direction and timing assembly for a stepper motor in an ectrically controlled printing system operating in an environment subjected to relatively strong vibrational and shock levels is disclosed. A cylindrical coding member having one or more photo-optical coding tracks located adjacent one edge is mounted on the stepper motor shaft by being held in position by an inner support cylinder and an outer retaining collar. These three elements are commonly fastened to the stepper motor shaft by a pair of mutually opposing set screws. Photo emitter and receptor elements contained in a bifurcated subassembly are located at the edge of the cylindrical coding so as to oppose a coding pattern formed on the coding tracks for generating an electrical signal in response to the coding pattern acting to pass or interrupt light energy transmitted from the emitter to the receptor elements.