ABB UNS0863A-P HIEE305082R1 Excitation ontroller

Ethernet connections can be extended in a number of ways in order to build larger networks or travel greater distances. With the correct repeaters, it is possible to mix different transmission media on the same network. For example, the CI cannot communicate at 1000 Mbits/s or connect to fibre but it can be connected via an Ethernet switch to these networks.There are a number of issues involved in building multiple segment and mixed media networks that are beyond the scope of this document. Further information can be found in the Ethernet standards. Although Ethernet communications is ubiquitous in industrial and office networks, it is recommended that safety-related communications (and any industrial communications considered critical to production) are kept separate from general office communications because the load on the general network due to office communications will be unpredictable.

Network Reflections 

If two networks are connected together via a router or some other method which allows packets to transfer between the networks, it is possible for one network to pass the data to the second and then the second network to pass the data to the first network knowing that the first network contains that IP. If this happens, the same data will bounce back and forth between the networks. Most data packets have a transfer count limit (‘time to live’) of 32, 64 or 128 transfers. For example, if it is set to 32 it will only go through 32 routers and then the packet will be discarded. If each network has 1 router and the networks are connected together, every data packet sent to a particular CI module will arrive at the module 16 times and hence require 16 times the processing.

If is set to 128, it will give 64 times the loading. 

This problem is made worse when a switch or any other connection sends a broadcast message (e.g. updating its IP connection table). This will bounce to all nodes at least 16 times on top of the normally traffic x 16. The TTL (time to live) cannot be controlled by the application and it may even be a router broadcast packet that is causing the problem. If the above happens, the CIs will be overloaded and may crash on releases before 3.5. From release 3.5, although the CIs will continue to operate, peer to peer network timeouts are likely to occur instead, which will cause a trip in the application if safety related data is being carried.

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