Siemens SIMATIC NET Operating Instructions Manual page 26

Operator control (hardware)
4.2 Setting the operating mode
Mode "redundant optical ring"
Note
Always set the "redundant optical ring" mode on both optical channels of a module.
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CH2 is set to this mode when S1 and S2 are in position 1.
CH3 is set to this mode when S3 and S4 are in position 1.
Operating Instructions, 01/2020, C79000-G8976-C270-06
Optical link module