Topology
Using full-duplex Ethernet physical layers, the EtherCAT slave controllers close an open port automatically and return the Ethernet frame if no downstream device is detected. Slave devices may have two or more ports. Due to these features EtherCAT can support almost any physical topology such as line, tree or star. The bus or line structure known from the fieldbusses thus also becomes available for Ethernet. Also possible is the combination of line and branches or stubs: any EtherCAT device with three or more ports can act as junction, no additional switches are required. The classic switch-based Ethernet star topology can be used either with switches configured to forward traffic directly between ports, or with special slave devices: the switches are then located between the network master and the slave devices. The special slave device (remember standard slave devices don't have a MAC address) assembly attached to one switch port together forms an EtherCAT segment, which is either addressed via its MAC address or via port based VLANs. Since 100BASE-TX Ethernet physical layer is used, the distance between any two nodes can be up to 100 m (300 ft). Up to 65535 devices can be connected per segment. If an EtherCAT network is wired in ring configuration (requires two ports on the master device), it can provide cable redundancy.
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