Siemens ESM - Manual (Page 59)

Interfaces, Displays and Operator Controls
Industrial Ethernet OSM/ESM
C79000-Z8976-C068-10
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RM – Redundancy Manager (green LED)
Status Meaning
Lit The OSM/ESM is operating in the redundancy manager mode.
The ring is operating free of errors in other words the
redundancy manager does not allow traffic through but
monitors the ring.
Note: One OSM must operate in the redundancy manager
mode (and one only) in each OSM/ESM ring.
Not lit The OSM/ESM is not in the redundancy manager mode.
Flashes The OSM/ESM is in the redundancy manager mode and has
detected a break on the ring. The OSM/ESM makes the
connection between its two ring ports so that a functional bus
configuration is reestablished.