Siemens ESM - Manual (Page 51)

Interfaces, Displays and Operator Controls
Industrial Ethernet OSM/ESM
C79000-Z8976-C068-10
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4.1.4 FO Ports
The FO ports have BFOC/2.5(ST) female connectors. They monitor the connected
cable for wire breaks complying with the IEEE 802.3 100 Base-FX standard. A
break on the FO cable is always signaled by the port status display of the two
attached OSMs (status LED of the port goes off).
Note
The FO ports operate at the fixed transmission rate of 100 Mbps. And optical link,
for example, to OLM (10 Mbps) is not possible.