Siemens SIMATIC NET PROFIBUS - Manual (Page 447)

4-1
PROFIBUS Optical Bus Terminal (OBT)
C79000-G8976-C122-02
Network Topology
4.1 Optical Bus
The OBT is operated in conjunction with other SIMATIC devices, for example the
IM 153-2 FO or IM 467 FO on the optical PROFIBUS DP in the form of an optical
bus.
Individual PROFIBUS DP nodes with an RS-485 interface are connected to
channel 1 of the OBT via a maximum 100 m long PROFIBUS cable with bus
connectors fitted at both ends. The terminating resistors on the bus connectors
must be activated. An active or passive PROFIBUS DP node can be connected.
The OBT can be included at any point in the optical bus. If it is included at the start
or end, the unused optical channel must be closed with the rubber plug supplied.
This prevents contamination of the optical elements and disturbances caused by
light entering the module.
The connection forming the optical bus is a two-fiber plastic FO cable (maximum
length 50 m) or PCF FO cable (maximum length 300 m). The fiber-optic cables
have two simplex connectors at each end.
The fiber-optic connection between two devices is established by connecting the
optical sender of one device with one fiber to the optical receiver of the other
device and the optical receiver of the one device to the optical sender of the other
device (cross-over connection).
If an OBT or a fiber-optic cable fails, the entire network becomes two subnets.
Depending on the location of the problem, individual devices may no longer be
accessible.
The OBT does not support the creation of single-fiber rings, of monitored optical
busses or redundant ring structures.
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