Siemens SIMATIC NET PROFIBUS - Manual (Page 406)

10
3 Network Topologies
Version 1.0 8/00
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:
Ⅲ
The parameters MIN T
SDR
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.
Ⅲ
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.
Ⅲ
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.