Network Topologies
3.3 Ring Topology
SIMATIC NET PROFIBUS, Optical Link Module
Operating Instructions, 07/2008, A2B00065774O, Edition V1.5
13
Please note the following:
For correct operation, the following conditions must be met:
¾
Only use this mode, if you only connect PROFIBUS OLMs with at least version
V3 with 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 over fiber-optic cables. There must be no RS-485 bus cable within
the ring.
¾
The MIN TSDR parameter described in the PROFIBUS standard EN 50170 /2/
must be set to a value
≥
11 on all DTEs. This is usually the case but should be
checked if permanent communication problems occur.
¾
Choose bus node addresses as low as possible when configuring your network,
to reduce master timeouts that may occur due to disruptions.
¾
If there is a failover (for example due to a cable break), there is failover time
during which correct data transmission is not possible. To ensure bumpless
bridging for the application, it is advisable to set the frame retry number on the
PROFIBUS master to at least 3. To ensure a bumpless return from the optical
bus to the optical ring once a problem has been eliminated, there must be no
frame on the network at this time. This status occurs when the master sends a
GAP query to an address lower than HSA. The master tries to address the
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 to form an optical ring in the middle of this query sequence. This
results in the following two important configuration requirements for the
redundant optical ring:
The value of the parameter HSA (Highest Station Address) must be set on
all DTEs so that there is at least one address between bus address 0 and
the value HSA that is not occupied by a bus node, in other words there is at
least one address gap. You can create this address gap simply by setting
the HSA value one higher than the highest existing bus node address.
Notice
If this requirement is not or no longer met, the optical bus will no longer close to form a redundant
optical ring after segmentation. In this case the error message (LED and signaling contact) of the two
affected OLMs will not be cleared after eliminating the problem.
The slot time must be set to about twice the value compared with a
redundant network. See section 5.10 for further information. You will find
information on changing the setting in the documentation supplied with your
DTE or with the configuration software.
Notice
No glass fiber-optic cable may be connected to an OLM that uses plastic fiber-optic cable and vice
versa.
