The functional, electrical, optical, and mechanical characteristics of a fibre optic interface for connecting a 4- or 16-Mbit/s token ring station to the trunk coupling unit of a token ring are specified. This Technical Report is based upon a fibre optic transmitter, a channel of multimode graded-index fibres, and a direct-detection photodiode receiver; and includes signaling provisions for bypass of the station from the ring and self-test capabilities.