Siemens SIMATIC NET PROFIBUS manual  Monitoring mechanisms

3 Network Topologies 3.1 Line topology

In a line structure, the individual PROFIBUS OLMs are connected together by dual-fiber optical fibers. Modules with one optical port are sufficient at the beginning and end of a line, between which modules with two optical ports are necessary.

If single point-to-point connections are to be built up, this can be achieved using two modules each with 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 OLM networks (default factory setting).

Please note that the following ambient conditions must be fulfilled to ensure that network configuration functions correctly:

The parameters MIN TSDR described in the PROFIBUS standard EN 50170 must be set to a value ≥ 11 on all terminals. This is usually the case, but the setting should be checked if communication malfunctions continuously arise.

When configuring your network, select low bus subscriber addresses wherever possible. This ensures that master timeout times which may arise are kept as short as possible in the event of a malfunction.

Refer to the manufacturer’s manual of the terminal concerned for details about how to alter the settings.

3.1.1 Line topology with optical fiber link monitoring and segmentation

This operating mode should preferably be used if an interrupted fiber segment is to be separated from the rest of the network.

Only use this operating mode if you have just connected PROFIBUS OLMs together of the same version.

Monitoring mechanisms:

Send echo:	yes
Monitor echo:	yes
Suppress echo:	yes
Monitor:	yes
Segmentation:	yes

In this operating mode the individual fiber links are monitored by the two connected modules.

If a module fails, an optical fiber breaks or faults are determined on the optical transfer link, the fiber link between the two OLMs is interrupted (segmented). The PROFIBUS network is divided into two partial networks, which remain functional independently of one other.

The malfunction is indicated at the two OLMs connected to the malfunctioning fiber link by the port LEDs switching to red and by activation of the signaling contacts. 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.

Please note that in the case of networks with several active bus subscribers, two logical token rings are formed in the event of an error. Every time the partial networks are switched together, network malfunctions may arise due to the double tokens or frame collisions.

Note:

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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