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EtherCAT

In the EtherCAT Technology Group, OEM, End Users and Technology Providers join forces to support and promote the further technology development, representing a large variety of industries and vertical market segments. The EtherCAT Technology Group ensures the simple and cost efficient integration of EtherCAT interfaces to automation devices and their interoperability.


EtherCAT Technology Group
Ostendstra▀e 196
90482 Nuremberg
Germany
Tel: + 49 (0) 9 11 / 5 40 5620
Fax + 49 (0) 9 11 / 5 40 5629
info@ethercat.org
www.ethercat.org

EtherCAT - Technical overview
The EtherCAT technology overcomes the system limitations of other Ethernet solutions: The Ethernet packet is no longer received, then interpreted and copied as process data at every connection. Instead, the Ethernet frame is processed on the fly: the newly developed FMMU (fieldbus memory management unit) in each slave node reads the data addressed to it, while the telegram is forwarded to the next device. Similarly, input data is inserted while the telegram passes through. The telegrams are only delayed by a few nanoseconds.

On the master side, very inexpensive, commercially available standard network interface cards (NIC) or any on board Ethernet controller can be as hardware interface. The common feature of these interfaces is data transfer to the PC via DMA (direct memory access), i.e. no CPU capacity is taken up for the network access.

Protocol
The EtherCAT protocol uses an officially assigned Ether-type inside the Ethernet Frame. The use of this Ether-type allows transport of control data directly within the Ethernet frame without redefining the standard Ethernet frame. The frame may consist of several sub-telegrams, each serving a particular memory area of the logical process images that can be up to 4 gigabytes in size. Addressing of the Ethernet terminals can be in any order because the data sequence is independent of the physical order. Broadcast, Multicast and communication between slaves are possible.

Transfer directly in the Ethernet frame is used in cases where EtherCAT components are operated in the same subnet as the master controller and where the control software has direct access to the Ethernet controller.

However, EtherCAT applications are not limited such control systems: EtherCAT UDP packs the EtherCAT protocol into UDP/IP datagrams. This enables any control unit with Ethernet protocol stack to address EtherCAT systems. Even communication across routers into other subnets is possible. In this variant, system performance obviously depends on the real-time characteristics of the control and its Ethernet protocol implementation. The response times of the EtherCAT network itself are hardly restricted at all: The UDP datagram only has to be unpacked in the first station.

Performance
EtherCAT reaches new dimensions in network performance. Thanks to FMMU in the slave nodes and DMA access to the network card in the master, the complete protocol processing takes place within hardware and is thus independent of the run-time of protocol stacks, CPU performance or software implementation. The update time for 1000 distributed I/Os is only 30 Ás. Up to 1486 bytes of process data can be exchanged with a single Ethernet frame - this is equivalent to almost 12000 digital inputs and outputs. The transfer of this data quantity only takes 300 Ás.

The communication with 100 servo axes only takes 100 Ás. During this time, all axes are provided with set values and control data and report their actual position and status. The distributed clock technique enables the axes to be synchronised with a deviation of significantly less than 1 microsecond.

The extremely high performance of the EtherCAT technology enables control concepts that could not be realised with classic fieldbus systems. For example, the Ethernet system can now not only deal with velocity control, but also with the current control of distributed drives. The tremendous bandwidth enables status information to be transferred with each data item. With EtherCAT, a communication technology is available that matches the superior computing capacity of modern Industrial PCs. The bus system is no longer the "bottleneck" of the control concept. Distributed I/Os are recorded faster than is possible with most local I/O interfaces.

EtherCAT instead of PCI
The central PC becomes smaller and more cost effective because additional slots are not needed for interface cards since the onboard Ethernet port can be used. With increasing miniaturisation of the PC-components, the physical size of Industrial PCs is increasingly determined by the number of required slots. The bandwidth of Fast Ethernet, together with the data width of the EtherCAT communication hardware enables new directions: Interfaces that are conventionally located in the IPC are transferred to intelligent interface terminals at the EtherCAT. Apart from the decentralised I/Os, axes and control units, complex systems such as fieldbus masters, fast serial interfaces, gateways and other communication interfaces can be addressed. Ethernet devices without restriction on protocol variants can be connected via decentralised "hub terminals". The central IPC becomes smaller and therefore more cost-effective, one Ethernet interface is sufficient for the complete communication with the periphery.

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 the couplers; no additional switches are required. Naturally, the classic switch-based Ethernet star topology can also be used.

Wiring flexibility is further maximised through the choice of different cables. Flexible and inexpensive standard Ethernet patch cables transfer the signals optionally in Ethernet mode (100Base-TX) or in E-bus (LVDS) signal representation. Plastic fiber optics (PFO) can be used in special applications. The complete bandwidth of the Ethernet network - such as different fiber optic and copper cables - can be used in combination with switches or media converters.

Fast Ethernet or E-bus can be selected based on distance requirements. The Fast Ethernet physics enables a cable length of 100 m between devices while the E-bus line is intended for distances of up to 10m. The size of the network is almost unlimited since up to 65535 devices can be connected.

Distributed Clock
Accurate synchronisation is particularly important in cases where widely distributed processes require simultaneous actions. This may be the case, for example, in applications where several servo axes carry out coordinated movements simultaneously. The most powerful approach for synchronisation is the accurate alignment of distributed clocks, as described in the new IEEE 1588 standard. In contrast to fully synchronous communication, where synchronisation quality suffers immediately in the event of a communication fault, distributed aligned clocks have a high degree of tolerance from possible fault-related delays within the communication system.

With EtherCAT, the data exchange is completely hardware based on "mother" and "daughter" clocks. Each clock can simply and accurately determine the other clocks run-time offset because the communication utilises a logical and full-duplex Fast Ethernet physical ring structure. The distributed clocks are adjusted based on this value, which means that a very precise network-wide timebase with a jitter of significantly less then 1 microsecond is available.

However, high-resolution distributed clocks are not only used for synchronisation, but can also provide accurate information about the local timing of the data acquisition. For example, controls frequently calculate velocities from sequentially measured positions. Particularly with very short sampling times, even a small temporal jitter in the displacement measurement leads to large step changes in velocity. With EtherCAT new, expanded data types (timestamp data type, oversampling data type) are introduced. The local time is linked to the measured value with a resolution of up to 10 ns, which is made possible by the large bandwidth offered by Ethernet. The accuracy of a velocity calculation then no longer depends on the jitter of the communication system. It is orders of magnitude better than that of measuring techniques based on jitter-free communication.

Hot Connect
The Hot Connect function enables parts of the network to be linked and decoupled or reconfigured "on the fly". Many applications require a change in I/O configuration during operation. Examples are processing centres with changing, sensor-equipped tool systems or transfer devices with intelligent, flexible work piece carriers. The protocol structure of the EtherCAT system takes account of these changing configurations.

Diagnostic
Special attention was paid to exemplary diagnostic features during the development of EtherCAT, as availability and commissioning times crucially depend on the diagnostic capability. Only faults that are detected quickly and accurately and located unambiguously can be rectified quickly. During commissioning, the actual configuration of the nodes is checked for consistency with the specified configuration.

The topology should also match the configuration. I/O verification is possible during start-up and also via automatic configuration upload because of the built-in topology recognition. Bit faults during the data transfer are reliably detected through evaluation of the 32 bit CRC checksum, which has a minimum hamming distance of 4. The EtherCAT protocol, transfer physics and topology enables quality monitoring of each individual transmission segment. The automatic evaluation of the associated error counters enables precise localisation of critical network sections. Gradual or changing sources of error such as EMC influences, defective connectors or cable damage are detected and located, even if they do not yet overstrain the self-healing capacity of the network.

Openness
The EtherCAT technology is fully Ethernet-compatible and truly open. The protocol tolerates other Ethernet-based services and protocols on the same physical network - usually even with minimum loss of performance. There is no restriction on the type of Ethernet device that can be connected within the EtherCAT segment via a hub terminal. Devices with fieldbus interface are integrated via EtherCAT fieldbus master terminals. The UDP protocol variant can be implemented on each socket interface. The EtherCAT Technology Group ensures that each interested party can implement and use this network. The submission of EtherCAT for international standardisation is on its way.


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