EtherCAT

EtherCAT is the open real-time Ethernet network originally developed by Beckhoff. EtherCAT sets new standards for real-time performance and topology flexibility.

EtherCAT

STANDARDS BODY, TITAN AND PHILOSOPHY

The EtherCAT protocol was initially developed by Beckhoff, and the standard has now been handed off to the EtherCAT Technology Group (ETG). Real-time results have shown that EtherCAT delivers the most deterministic response of any industrial real-time Ethernet system available. Being able to process 1,000 I/Os in 32.5 μs or 100 axes in 125 μs offers machine builders the opportunity to deliver breakthrough in machine performance at a lower price.

ARCHITECTURAL PHILOSOPHY: OPEN SOFTWARE/MODIFIED ETHERNET

With EtherCAT, all devices are networked with the bus master in a ring formation. During each cycle, relevant output data is extracted by the devices from the Ethernet data packets sent by the bus master. Input data is also stuffed into packets “on the fly“; these packets arrive again at the bus master upon reaching the end of the ring. This system was designed for centralized controller architectures with simple field devices in a Master/Slave configuration.

To guarantee that devices from different manufactures can work with one another, ETG has run plug fests to make sure that different drives and IOs from different manufacturers can all work seamlessly on a single installation. The fact that more than 500 I/O and drive vendors have now adopted EtherCAT for their slave device configurations speaks volumes about the power and efficacy of the protocol. The adoption was driven by large vendors in the semiconductor and robotics industries who refused to buy a drive unless it supported EtherCAT.

EtherCAT uses the telegram structure of Ethernet, but with an entirely different basic mode of operation. Within a communication cycle, a telegram is not sent to each station separately as in other Ethernet approaches, but rather a single Ethernet telegram runs through all stations/slaves. The data area in the Ethernet telegram divides into sections for real-time and general data. In the real-time data area the headers and process data of all stations are defined in consecutive sub-telegrams, thereby increasing the user data rate in the protocol (in motion control applications the user data rate in the shortest Ethernet frame of 64 bytes is usually below 15%). The slaves feature special ASICS or FPGAs which convert the incoming Ethernet-framed data into an internal so-called E-bus. As the EtherCAT slaves are only able to interpret EtherCAT frames, general data is tunneled in EtherCAT frames in order to guide it through slaves.

If a general data packet is too large to be transferred in one cycle, it is distributed across multiple EtherCAT frames. Tunneling and de-tunneling takes place in the master (Virtual Ethernet Switch) or in slaves with gateway functionality. The entire protocol processing is hardware-based. The slaves do not treat incoming Ethernet telegrams in the normal fashion, interpreting the contents and then copying the process data for forwarding transmission. Instead, the EtherCAT slaves read and write their process data from and to the predetermined location in the telegram while the telegram is passing through the slave. The EtherCAT mechanisms permit extremely short cycle times to be implemented.

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