Allen-Bradley ControlNet 1786-RPFS Installation Instructions Manual page 2

2 ControlNet Modular Repeater Short-distance Fiber Module
Use this module when a short-distance (distances of 300m/984ft) fiber
link is required between two ControlNet products. This fiber link
provides ground isolation between nodes and is less susceptible to noisy
environments.
The module provides:
two independent full duplex fiber channels
activity LED indicators for each fiber channel
The figure below identifies the components of the module: