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Network topology
5.1
Optical bus
Along with other SIMATIC devices such as the IM 153–2 FO or IM 467 FO, OBTs are connected
to optical PROFIBUS DP to form an optical bus.
PROFIBUS DP bus nodes with an RS-485 interface are connected to channel 1 of the OBT via bus
connectors and a PROFIBUS bus cable. The bus segment must be terminated at both ends.
The OBT can be used at any point on the optical bus. If it is used at the start or end, the unused
optical channel must remain closed with the rubber plug. This prevents contamination of the
optical elements and disturbances caused by incident light.
The connection on an optical bus is made using two-fiber plastic FO cable (max length 50 m) or
PCF FO cable (max. length 300 m). The FO cables have two simplex connectors fitted at both
ends.
The FO connection between two devices is established by the optical transmitter of one device
being connected to the optical receiver of the other device via a fiber, while the optical receiver
of the first device is connected to the optical transmitter of the other (crossover cable).
If an OBT or a fiber-optic cable fails, the network is split into two separate networks. Depending
on the location of the problem, devices are no longer accessible.
Note
If problems are detected on the electrical interface, there is a segmentation, in other words, the
frames are not forwarded. The segmentation is canceled when the problem has been
eliminated. In this case, it is assumed that a minimum number of error-free frames were received
on the electrical channel. The segmentation is also canceled when no frames are received for at
least 1 second. In this case, it is assumed that unplugging the PROFIBUS connector caused
problems and that there are no more nodes on the electrical channel.
The OBT does not support the establishment of single-fiber rings or redundant ring structures.
PROFIBUS Optical Bus Terminal (OBT)
Operating Instructions, 06/2022, C79000-G8976-C122-08
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