Vacon optea, Installation Manual

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Layout and connections vacon • 19
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4.4.3 Topology: Ring
In some cases it is possible to use a ring topology. The ring topology gains the same reduced cabling
cost as the daisy chain topology, but decreases the damage caused by a single cable failure.
Both Ethernet boards can be used with the following media redundancy protocols:
NOTE! Ring network sizes should be limited from what is specified in the media redundancy
protocols (usually up to 50 nodes) depending on the network load. When low I/O data intervals are
used, we recommend that ring networks should be limited to fewer than 32 devices. High load on
network can cause recovery times to increase and in worst cases to trigger a watchdog failure.
4.4.3.1 Rapid Spanning Tree Protocol (RSTP)
To use the RSTP protocol, add a managed Ethernet switch that supports the RSTP protocol. If a
single link is broken, the RSTP switch will notice this and start sending data from the PLC to both
directions effectively creating two daisy chains. When the link has been repaired, the switch will
notice this too and reverts back to normal operating mode. Compared to the star topology, the ring
topology adds more network traffic to almost all drives. Damage to two cables will always create an
isolated subnetwork.
In the RSTP configuration, one of the ports in the switch is "Designated Port" (DP) and the other
"Alternative Port" (AP). When the network is functioning properly, the traffic flows through the
designated port. Only the BPDU (Bridge Protocol Data Unit) packets are transferred through the AP
port. The BPDU packets are used by the switch to determine if the network is working properly. If it
detects that the BPDU packets do not go through the ring, it will change the alternative port to a
second designated port. Now the switch will send packets to both directions in the broken ring (see
Figure 8).
Each designated port has a list of MAC addresses which are behind that port. Only frames directed
to the device in the MAC list are forwarded into that designated port. The broadcast and multicast
frames are sent to all designated ports.
Table 9. Supported media redundancy protocols
Protocol Active fieldbus protocol Recovery time
RSTP Any Seconds
MRP PROFINET
< 200 ms (typical)
< 50 or < 10 ms with Fast-MRP
DLR EtherNet/IP ~4 ms