Siemens SIMATIC NET PG PROFIBUS Operating Instructions Manual page 25

The fiber of the FO cable must be flush with the surface of the connector.
Note
If the fiber juts out beyond the surface of the connector, the connector must not be inserted
in the socket. Otherwise there is a risk of destroying the optical components.
Connecting the electrical RS-485 cable
Channel CH1 is used to connect a single PROFIBUS DP end device or an RS-485 segment.
CH1 is designed as an electrical RS-485 interface with a 9-pin D-sub female connector.
1. As the RS-485 bus cable, use only shielded and twisted SIMATIC NET two-wire cables
for PROFIBUS.
2. Do not connect any RS-485 bus cables that are laid entirely or partly outdoors.
Otherwise, lightning strikes in the surroundings could destroy the PROFIBUS OBTs. If
you have bus connections that leave the building, use fiber-optic cables whenever
possible.
3. Disconnect the RS-485 bus cable from the OBT if you have not connected a node to the
other end of the cable. Coupling in interference can cause disturbances on PROFIBUS.
PROFIBUS Optical Bus Terminal (OBT)
Operating Instructions, 04/2016, C79000-G8976-C122-07
Commissioning
5.2 Installation
25