EtherCAT Industrial Ethernet Protocol Overview

Industrial Ethernet Protocol

EtherCAT is the new open real-time Ethernet Master/Slave network developed by Beckhoff. Today, it is an open standard, managed by the EtherCAT technology group. EtherCAT sets new limits for real-time performance since it processes 1000 distributed I/O in 30 µs or 100 axis in 100 µs using twisted pair or fiber optic cable. Regarding topology, EtherCAT supports a simple low cost line structure, a tree structure, daisy chaining or drop lines - no expensive infrastructure components are required. Alternatively the classic more expensive switched star topology can be used.

Principle of operation
The EtherCAT transmission method is similar to the principles of Interbus. With EtherCAT, the Ethernet packet is no longer received, then interpreted and process data then copied at
every device. The EtherCAT slave devices read the data addressed to them while the frame passes through the device. node. Similarly, input data is inserted while the telegram
passes through. The frames are only delayed by a few nanoseconds. Since an EtherCAT frame comprises the data of many devices both in send and receive direction, the usable data rate increases to over 90%. The full-duplex features of 100BaseTX are fully utilized, so that effective data rates of > 100 Mbit/s (>90% of 2 x 100 Mbits/s) can be achieved.

The Ethernet protocol according to IEEE 802.3 remains intact right up to the individual terminals; no sub-bus is required. In order to meet the requirements of the electronic
terminal block, only the physical layer in the coupler is converted from twisted pair or optical fibre to LVDS. LVDS is a fast and cost-effective, alternative physical Ethernet layer. At the end of the modular device, the system is simply switched back to 100 BASE-TX.

The EtherCAT protocol is optimized for process data and is transported directly within the Ethernet frame thanks to a special Ethertype. 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. Direct Ethernet frame transfer is used in cases where maximum performance is required and the EtherCAT components are operated in the same subnet as the controller.

Internetworking with other Ethernet-TCP/IP protocols
However, EtherCAT applications are not limited to a subnet: EtherCAT UDP packages the EtherCAT protocol into UDP/IP datagrams. This enables any control with Ethernet protocol stack to address EtherCAT systems. Even communication across routers into other subnet's 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.

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.

EtherCAT topology: Line, Tree or Star

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. The Fast Ethernet physics (100BASE-TX) enables a cable length of 100m between two devices. Since up to 65535 devices can be connected, the size of the network is almost unlimited.

To deliver Real Time performance, the EtherCAT master encapsulates a number of Ethernet commands in a standard Ethernet telegram. EtherCAT can interoperate with normal TCP/IP-based networks through switches. Any Ethernet based protocol can co-exist provided an EtherCAT segment is located at one switch port, and any other Ethernet-based communication such as normal TCP/IP traffic is located at a different port so as not to interfere with EtherCAT operation.

ETHERCAT FACTS
Network Type:	Ethernet based fast Real-Time Master/Slave network
Topology:	Very flexible with Ethernet Star, Line or Tree topologies Switches are not required
Installation:	Ethernet 100 TX twisted pair cables with RJ45 connectors: 100m distance between two nodes in the network
Speed :	100 Mbit/s full duplex
max. Stations :	65535
Data:	up to 1.500 Byte per telegram frame
Network Features :	EtherCAT is optimized for fast Real-Time transmissions as it can can processes 1000 distributed I/O in 30 µs
User Organization:	EtherCAT Technology Group