Siemens SIMATIC NET PROFIBUS - Manual (Page 410)

14
3 Network Topologies
Version 1.0 8/00
3.3 Redundant optical ring
This network topology represents a special form of line topology. A high degree of network operating safety is
achieved by ”closing“ the optical line. A redundant optical ring can only be realized with modules with two optical
ports of the same type.
Ⅲ Monitoring mechanisms:
Send echo:
yes
Monitor echo: yes
Suppress echo: yes
Segmentation: yes
An interruption of one or both optical fibers between two modules is detected by the OLM and the ring is trans-
formed into an optical line.
If one module fails only those terminals connected to this module or the RS 485 segment are uncoupled from the
ring. The remainder of the network itself continues to function as a line. The error is indicated by the LEDs on the
two OLMs connected to the malfunctioning optical fiber and their signaling contacts. The segmentation is lifted auto-
matically as soon as both modules recognize that the segmented field bus network is functioning correctly with the
help of test frames. The line forms itself into a ring.
Please note:
The following ambient conditions must be fulfilled to ensure that the network configuration functions correctly:
Ⅲ
Only use this operating mode if you optically connect PROFIBUS OLMs of the same version.
Ⅲ
The operating mode ”Redundant optical ring“ must be set at both optical ports of all the PROFIBUS OLM.
Ⅲ All modules in a ring must be connected to one another by fiber lines. The ring may not include an RS 485
bus line.
Ⅲ
The parameter 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.
Ⅲ
If a redundancy case occurs (e.g. a line break), there is a switching time during which data cannot be correctly
transmitted. In order to ensure a smooth transition, it is recommended that the frame repeat setting (Retry) on
the PROFIBUS master be set to at least 3.
After the error has been corrected, no frames should be present in the network when the optical line is trans-
formed back into an optical ring to ensure that the process is completed smoothly. This condition can arise
when a master selects a device whose address has been configured, but which does not physically exist.
The master tries to address this device cyclically and waits for a reply only until the configured slot time has
been exceeded (”GAP request“). The OLM recognizes this condition and closes the optical line to an optical
ring in the middle of this request sequence.
This results in two configuration requirements for the redundant optical ring:
– The value of the parameter HSA (Highest Station Address) must be set at all terminals so that between the
bus address 0 and the value HSA at least one address in the network has not been assigned to a bus sub-
scriber, i.e. so that there is at least one address gap. This address gap can also be created by simply setting
the value of the parameter HSA so that it is at least one greater than the highest number of subscriber bus
addresses present in the network.
Attention: If this requirement is not or no longer fulfilled, the optical line will no longer be closed into a
redundant optical ring after segmentation. The error report (LED and signaling contact) of the two affected
OLMs is not cancelled even after the error has been corrected.
– The slot time must be set to approximately twice the value required in a non-redundant network. Further
information can be found in Chapter 6 ”Configuration“, p. 29.
Refer to the manufacturer’s documentation provided with the terminal or configuration software for details
about how to adjust the settings.