Due to its stochastic access method, classic Ethernet has certain limitations with regard to real-time capability. With EtherCAT (Ethernet for Control Automation Technology), a development by Beckhoff, this limitation is put into perspective, since EtherCAT, a fieldbus, is characterised by extremely short and predictable delay and cycle times. EtherCAT is therefore of particular interest as a real-time Ethernet for the fields of automation, production and manufacturing.
With the EtherCAT protocol, the reduction in cycle times is achieved by the fact that the telegrams, i.e. the datagrams known from data communication, are no longer interpreted and copied after receipt, but are processed on-the-fly; i.e. virtually as they fly past. In this process, the UDP telegrams addressed to a Fieldbus Memory Management Unit (FMMU) are read by this unit while the telegram is forwarded to the next station or control unit. Similarly, input data is inserted while the telegram is passing through the station. As a result, the delay times of the data telegrams are only a few nanoseconds (ns).
Ethernet with real-time propertiesEtherCAT operates in master-slave mode; commercially available network interface cards (NIC) or on-board controllers are used on the master side in the EtherCAT topology. Due to the FMMUs in the slave nodes and the direct memory access (DMA) via the network cards, the complete process is hardware-oriented and independent of runtimes due to protocol stacks, the computing power of the central processing unit (CPU) or software implementations.
Thus, the update time of 1,000 distributed I/Os is only 30 µs. Within one Ethernet telegram, up to 1,486 bytes of process data can be transferred in a data transfer time of 300 µs.
The prerequisite for these short transfer rates is an exact synchronization of the distributed processes. This is achieved by the exact alignment of the distributed clock signal, as described in the IEEE 1588standard for the Precision Time Protocol (PTP).