Siemens SIMATIC NET Operating Instructions Manual page 60

Planning/configuring
7.1 Network topologies
The problem is indicated by the LEDs of the two OLMs connected to the disrupted fiber-optic
link and by the signaling contacts of these OLMs. The segmentation is canceled
automatically as soon as both modules recognize that the segmented fieldbus (sub)network
is no longer disrupted based on test frames that they send out automatically. The bus is
once again closed to form a ring.
Requirements for correct operation
● Only use this mode, if you only connect PROFIBUS OLMs with at least version V3 to
each other. Both optical channels must be set to the "redundant optical ring" mode on all
PROFIBUS OLMs. All modules within a ring must be connected to each other via fiber-
optic cables. There must be no RS-485 bus cable within the ring.
● The MIN TSDR parameter described in the PROFIBUS standard EN 50170 /2/ must be
set to a value ≥ 11 on all end devices. This is usually the case but should be checked if
permanent communication problems occur.
● Choose bus node addresses as low as possible when configuring your network to reduce
master timeouts that may occur due to disruptions.
● If a fault occurs requiring a failover (for example a wire break), there is a failover time
during which correct data transmission is not possible. To ensure a "bumpless" failover
for the application, it is advisable to set the number of frame retries on the PROFIBUS
master to at least 3. To allow a "bumpless" return from the optical bus to the optical ring
after the fault has been eliminated, there must be no frame on the network at the failback
time. This status occurs when the master sends a GAP query to an address lower than
the HSA. The master tries to address this device cyclically and waits for a reply at the
longest until the configured slot time has elapsed ("GAP query"). The OLM recognizes
this status and closes the optical bus in the middle of this query sequence to form the
optical ring again. This results in the following two important configuration requirements
for the redundant optical ring:
– The value of the HSA (Highest Station Address) parameter must be set on all end
– The slot time must be set to about twice the value compared with a not redundant
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devices so that there is at least one address in the network between bus address 0
and the value of HSA that is not used by a node; in other words, there is at least one
address gap. You can create this address gap simply by increasing the value of the
HSA parameter to one higher than the highest node address in the network.
Note
If this condition is not or is no longer satisfied, the optical bus will no longer close to
form a redundant optical ring following segmentation. The disruption message (LED
and signaling contact) of the two OLMs affected is not canceled even after the
problem has been eliminated.
network. For more information, refer to the section "Configuration". You will find
information on changing the setting in the documentation supplied with your end
device or with the configuration software.
Operating Instructions, 01/2020, C79000-G8976-C270-06
Optical link module