Up to 254 Sercos slave devices are connected to a single master via a fiber optic ring. Fiber optic cables were chosen to provide high immunity against electromagnetic interference. In industrial applications, unforeseen noise levels must be expected in cable ducts. Furthermore, HF interference resulting from power control by pulse-width modulation, and separately-sited AC line and ground connections on controllers and drives must be reliably eliminated. The ring structure has the smallest number of fiber optic components and requires no complicated T-branches. The length of each transmission section can be up to 50 meters with plastic fiber optic cables and up to 250 meters with glass fiber cables.