A slot machine is disclosed having three coaxially mounted reels and a mechanism for simultaneously spinning the three reels when a starting handle is pulled, and stopping them at three new stopping positions in response to control signals from a microprocessors. Each reel has a sprocket with 32 stopping positions and a coding ring with a plurality of slots arranged on a circumferential path about the reel's axis, forming a single track of digital information which may be read by a single optic sensor as the reel rotates. The track has 32 equally spaced fields of optical information corresponding to the 32 stopping positions of the reels. Each field has one slot, the slot width alternating between wide and narrow arcuate sizes about the coding ring, with one sequence of three narrow slots marking a home position for its respective reel.
After passage of the home position is detected, a reel position counter associated with each reel counts the number of fields that pass the reel's sensor to monitor the rotational position of that reel relative to a fixed reference point. Each time a new field is detected for a given reel, the microprocessor determines whether the rotational direction, speed, and acceleration or deceleration of that reel are correct based on the information from its track, and an appropriate error signal is issued if the reel is not spinning properly.