EtherCAT
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EtherCAT is an open high performance Ethernet-based fieldbus system. The development goal of EtherCAT was to apply Ethernet to automation applications which require short data update times (also called cycle times) with low communication jitter (for synchronization purposes) and low hardware costs.
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[edit] Introduction
Typical automation networks are characterized by short data length per node, typically less than the minimum payload of an Ethernet frame. Using one frame per node per cycle therefore leads to low bandwidth utilization and thus to poor overall network performance. EtherCAT therefore takes a different approach, called "processing on the fly".
[edit] Functional principle
With EtherCAT, the Ethernet packet or frame is no longer received, then interpreted and copied as process data at every node. The EtherCAT slave devices read the data addressed to them while the telegram passes through the device. Similarly, input data are inserted while the telegram passes through. The frames are only delayed by a fraction of a microsecond in each node, and many nodes - typically the entire network - can be addressed with just one frame.
Image:EtherCAT Functional Principle.png
[edit] Protocol
The EtherCAT protocol is optimised for process data and is transported directly within the standard IEEE 802.3 Ethernet frame using Ethertype 0x88a4. It may consist of several sub-datagrams, each serving a particular memory area of the logical process images that can be up to 4 gigabytes in size. The data sequence is independent of the physical order of the nodes in the network; addressing can be in any order. Broadcast, Multicast and communication between slaves are possible. If IP routing is required, the EtherCAT protocol can be inserted into UDP/IP datagrams. This enables any control with Ethernet protocol stack to address EtherCAT systems.
[edit] Performance
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Process data exchange with 1000 distributed digital I/O only takes about 30µs.
[edit] Topology
Line, tree or star: EtherCAT supports almost any topology. The bus or line structure known from the fieldbusses thus also becomes available for Ethernet. Particularly useful for system wiring is the combination of line and branches or stubs: the required interfaces exist on many devices; no additional switches are required. The classic switch-based Ethernet star topology can also be used. 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, so the size of the network is almost unlimited.
[edit] Synchronization
For synchronization a distributed clock mechanism is applied, which leads to very low jitters of significantly less than 1 µs even if the communication cycle jitters. Therefore EtherCAT does not require a special hardware in the master device and can be implemented in software on any standard Ethernet MAC, even without dedicated communication coprocessor.
The typical process of establishing a distributed clock is initiated by the master by sending a broadcast to all slaves to a certain address. Upon reception of this message, all slaves will latch the value of their internal clock twice, once when the message is received and once when it returns (remember EtherCAT has a linear topology). The master can then read all latched values and calculate the delay for each slave. This process can be repeated as many times as required to reduce jitter and average out values. Total delays are calculated for each slave depending on their position in the slave-ring and will be uploaded to an offset register. Finally the master issues a broadcast readwrite on the system clock, which will make the first slave the reference clock and forcing all other slaves to set their internal clock appropriately with the now known offset.
To keep the clocks synchronised after initialisation, the master or slave must regularly send out the broadcast again to counter any effects of speed difference between the internal clocks of each slave. Each slave should adjust the speed of their internal clock or implement an internal correction mechanism whenever they have to adjust.
The system clock is specified as a 64bit counter with a base unit of 1 ns starting at January 1st 2000, 0:00.
[edit] Device profiles
The device profiles describe the application parameters and the functional behaviour of the devices including the device class-specific state machines. For many device classes, fieldbus technology already offers reliable device profiles, for example for I/O devices, drives or valves. EtherCAT supports both the CANopen device profile familiy as well as the drive profile standardized in IEC61491, which is known as the Sercos drive profile. Since the application view does not change when migrating from CANopen or Sercos, this greatly assists users and device manufacturers alike.
[edit] Gateways
For integration of existing fieldbus components (e.g. CANopen, DeviceNet, Profibus) into EtherCAT networks gateway devices are available. Also other Ethernet protocols can be used in conjunction with EtherCAT: The Ethernet frames are tunneled via the EtherCAT protocol, which is the standard approach for internet applications (e.g. VPN, PPPoE (DSL) etc.). The EtherCAT network is fully transparent for the Ethernet device, and the real-time characteristics are not impaired. All internet technologies can therefore also be used in the EtherCAT environment: integrated web server, e-mail, FTP transfer etc.
[edit] Implementation
Master can be implemented in software on any standard Ethernet MAC. Several vendors supply code for different operating systems. There are also several Open Source projects. For slave devices special EtherCAT slace controller chips are required in order to perform the "processing on the fly" principle. EtherCAT slave controllers are available as code for different FPGAs and will be available as ASIC implementation in mid 2006.
[edit] EtherCAT Technology Group
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The EtherCAT Technology Group promotes the philosophy that EtherCAT, a technology developed by Beckhoff, should support use and implementation by anyone. The ETG is a forum for end users from different sectors, and for machine manufacturers and suppliers of powerful control technology with the aim of supporting and promoting EtherCAT technology. The wide range of sectors that are represented ensures that EtherCAT is optimally prepared for a large number of applications. With their qualified feedback, the system partners ensure simple integration of the hardware and software components in all required device classes. ETG was established in November 2003. As of October 2006, it has 382 member companies from 28 countries.
[edit] International standardization
The EtherCAT Technology Group as an official liaison partner of the IEC (International Electrotechnical Commission) working groups for digital communication. The EtherCAT specification has been published as IEC/PAS62407. Integration into the international fieldbus standards IEC 61158 and IEC 61784-2 is under way, as well as the integration in the drive profile standard IEC61800-7. EtherCAT is also part of ISO15745-4, the standard for XML device description.
[edit] Applications
Typical application fields for EtherCAT are machine controls (e.g. semiconductor tools, metal forming, packaging, injection molding, assembly systems, printing machines, robotics).
