Siemens SIMATIC NET PROFIBUS Owner's Manual page 457

Note
If the fiber protrudes beyond the surface of the connector, the connector must not
be inserted into the socket otherwise the optical components can be permanently
damaged.
Connecting the Electrical RS–485 Cable
Channel CH1 is used to connect a single PROFIBUS DP DTE. CH1 is designed as
an electrical RS 485 interface with a 9-pin sub D female connector. The maximum
cable length between the OBT and DTE is 100 m. Since this involves a
point-to-point connection, the terminating resistors in the PROFIBUS connectors
must be activated at both ends of the cable.
Use only SIMATIC NET shielded Twisted Pair cables as the RS 485 cable for
PROFIBUS.
Do not connect RS 485 cables when all or part of the cable is outside a building.
Lightning in the area can otherwise destroy the PROFIBUS OBTs. If connections
exit the building, use FO cables whenever possible!
Remove the RS 485 cable from the OBT if there is no node connected to the other
end of the cable. Noise can lead to problems on the PROFIBUS network.
PROFIBUS Optical Bus Terminal (OBT)
C79000-G8976-C122-02
Installation and Startup
5-7