4/15/2003 - As its technological highlight at this year's Hanover Fair, Beckhoff will present the EtherCAT (Ethernet for control automation technology) system, which opens up new dimensions in industrial communications. With EtherCAT, the costly Ethernet star topology can be replaced with a simple line structure - no expensive infrastructure components are required. Optionally, EtherCAT may also be wired in the "classic" way using switches, in order to integrate further Ethernet devices. Where other real-time-Ethernet approaches require special connections in the controller, EtherCAT manages with very cost-effective standard Ethernet cards.
With EtherCAT technology, Beckhoff overcomes these system limitations of other Ethernet solutions: The Ethernet packet is no longer received, then interpreted and copied as process data at every connection. The newly developed FMMU (fieldbus memory management unit) in each I/O terminal reads the data addressed to it, while the telegram continues through the device. Similarly, input data are inserted while the telegram passes through. The telegrams are only delayed by a few nanoseconds.
Continuity up to the Ethernet terminal
On the hardware side, the new type ELxxxx Ethernet terminals are located in the proven modular Bus Terminal housing. In contrast to the Bus Terminals, there is no internal sub-bus; the Ethernet protocol remains intact right up to the individual Ethernet terminal. Only the transfer physics is converted in the coupler from twisted pair or optical fibre physics to E-bus without delay, in order to meet the requirements of the electronic terminal block.
On the control side, the Beckhoff TwinCAT network card driver complements the FMMU technology. This integrates transparently into the system, so that it appears as an operating system-compatible network driver, and additionally as a TwinCAT fieldbus card. An internal prioritisation system and buffer is provided at the transmitter end which always finds a free transmission channel for Ethernet frames from the real-time system that may be queuing. The operating system's Ethernet frames are only a later transmitted in the "gaps" if sufficient time is available. At the receiving end, all the Ethernet frames received are examined by the TwinCAT I/O system, and those with real-time relevance are filtered out. All other frames are passed on to the operating system after examination, outside the context of the real-time system.
Since the Ethernet functionality of the operating system is fully maintained, all operating system-compatible protocols can be operated in parallel on the same physical network. This not only includes standard IT protocols such as TCP/IP, HTTP, FTP or SOAP, but also practically all Industrial Ethernet protocols such as Modbus TCP, ProfiNet or EthernetIP.
The EtherCAT protocol is transported directly within the Ethernet frame. It 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. The data sequence is independent of the physical order of the Ethernet terminals in the network; addressing can be in any order. Broadcast, Multicast and communication between slaves are possible.
New dimensions in industrial communication
EtherCAT reaches new dimensions in network performance. Thanks to FMMU in the terminal and DMA access to the network card in the master, the complete protocol processing takes place within hardware and is thus fully independent of the run-time of protocol stacks, CPU performance or software implementation. The update time for 1000 digital I/Os distributed over any number of stations is only 30 µs – including terminal cycle time.
The communication with 100 servo axes only takes 100 µs. 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.
Open for Ethernet Standards
The EtherCAT technology is not only fully Ethernet-compatible, but also characterised by particular openness "by design": 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 strand 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. Finally, the intention is to disclose the technology once the development work is completed.
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