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The communication is extremely important. If you just speak about the operation of let's say a drive and controller pair. But if you have all kinds of different applications, then additional aspects have to come into consideration.

* If you want to develop some feature that needs specification to be supported by several other vendors, then the democratic and continuously operation Technical Working Groups (TWG) will help you to realize this feature inside the next specification round.

* As soon as your planning the plant, you will think of how you build up you cabinets etc. Within a SERCOS plant you just need to think about the devices you have anyway. No switches required. Costs no money, needs no space, No reliability problem.

* When you start-up a plant, you will have some problems at one or the other corner (we all know that).

* Due to fast-ethernet your automation components will not suck up all the bandwidth and you will want to add standard Ethernet components. SERCOS has built-in 3-port switches that transfer the non-sercos Ethernet frames unchanged through the network, no more efficient way like this. There is no point in the network where standard-Ethernet devices can do any harm.

As far as I understand this, the network can be disturbed in EtherCAT, VARAN and Powerlink, if you just connect Ethernet devices at any open port.
* As soon as we come to operation you will think about cycle times. Due to the dedicated commitment to line and ring topology the commitment to sum frame telegram goes along. The later provides the most efficient data transfer ratio, since there is only 64 byte frame overhead per 1500 bytes of user data. With a certain bus configuration this allows the shortest cycle times that cannot be reached by VARAN, Profinet RT or Powerlink.

* SERCOS allows operation of devices based on Direct cross communication (DCC) within one cycle. This is not possible with EtherCAT. As far as I know it not possible with VARAN, too. Whether the link of Powerlinkd devices via a hub allows it, is beyond my knowledge, since I do not know whether it means one cycle of delay.

* Also during operation you may have the need to do some diagnostics. Due to the dedicated association of each byte in the master data telegram (MDT) or the acknowledge telegram (AT) you can do diagnostics at every place in the network. There are certain limitations for EtherCAT, Profinet IRT that have similar communications. It is not possible for the star topologies in VARAN and Profinet RT For Powerlink I am not sure, since the data could be made available via hub, but this can also interfere with the slot management.