ABB AC 800M User Manual page 48

Basics of fibre optics topology technologies
If a module fails, an optical fiber breaks or faults are determined on the optical
transfer link, the fiber link between the two optical link modules is interrupted. The
PROFIBUS network is divided into two partial networks, which remain functional
independently of one other. The segmentation is lifted automatically as soon as both
modules recognize that the field bus network is functioning correctly with the help
of test frames.
If single point-to-point connections are to be built up, this can be achieved using
two modules each with only one optical port.
The line topology can be realized with and without fiber link monitoring. If both
operating modes are used within an optical fiber line, the operating mode "Line
topology without fiber link monitoring" determines the availability of this fiber line.
It is recommended that fiber link monitoring be used in homogeneous fibre optic
networks.
If a module with two optical ports is used at the beginning or end of a line, the
optical port which is not assigned must be switched to the operating mode "Line
without fiber link monitoring", so that it does not signal a break in the fiber line.
Please note that optical ports which are not connected must always be fitted with
protective caps to guard against extraneous light and dirt.
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Section 2 Transmission Technology
3BDS009029R5001 B