Optical data links have been developed for use in multiplex wiring harnesses. The links consist of optical fiber cords, optical connectors, and optical transceiver modules. They can be operated under severe environmental conditions in motor vehicles and, because they have wide dynamic range, sensitivity adjustment is not required.A heat-resistant optical fiber cord made of plastic has been developed to enable use at temperatures up to 80°C. Mass-producible, low cost optical connectors have also been developed. In optical transceiver modules, custom designed hybrid ICs are adopted for size reduction.These optical data links are already in use in the Toyota Century.