Abstract: A communications system with a local network having two metallic cables providing redundant channels between stations of a group with redundant optical rings forming a highway interconnecting those networks. An optical-electrical interface connects the networks and the ring.This interface includes an optical detector producing a corresponding electrical signal, an optical transmitter which repeats the detected message, and a control element connecting the output of the detector with the transmitter. A control signal supplied over one metallic line from the station which is transmitting to its interface causes the control element to disconnect the detector and transmitter to prevent operation of the combination as a repeater. The transmission is then supplied over another metallic line from the station to the transmitter and thence around the ring until it is converted to an electrical signal at the corresponding detector.