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Peer-to-Peer Communications
To ensure the required determinism, most Real-time Ethernet standards enforce a master-to-slave-only method of communications. This can conflict with the need for a node in the system to exchange data efficiently with a node other than the network master. The conventional method for such a data exchange in a master-slave network is to pass data from one slave node to the master, where it is reissued to one or more different slaves. For example, if several servo drives on a network are to be synchronized to a signal from another drive on the network, the master must fetch the signal from this drive and reissue it to all other drives on the network. This induces delays due to the multiple cycles required; and the master’s processing load is increased as it must actively participate in the function, although it contributes nothing.
Since no data is destroyed in a Sercos III telegram, data to and from any slave can be accessed by another slave node on the network without any additional cycle delay or master intervention. Additionally, as telegrams pass each node twice in a cycle (for both topology types), a node can even have the opportunity to access data supplied by a subsequent node. Two peer-peer communication methods are defined in the Sercos III specification:
Controller to Controller (C2C) for multiple masters to communicate with one another, and Cross Communication (CC) among multiple slaves.
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