Ethernet protocol, EtherCAT, processes on the fly
Industrial Ethernet protocols typically require individual Ethernet frames for each device on the network. Because of the Ethernet frame’s minimum length requirements as defined by IEEE 802.3, small payloads of a few bytes of data per node (typical for automation devices) results in poor bandwidth utilization. This means much of the communication is spent on overhead for the Ethernet packet, resulting in performance comparable to traditional legacy fieldbus systems.
EtherCAT’s general operating principle of “processing on the fly” for Ethernet frames, along with mapping through a Fieldbus Memory Management Unit (FMMU), distinguishes it from other industrial Ethernet protocols. One EtherCAT frame holds data for many network devices. The frame is not received and interpreted with the process data copied at each individual device. Rather, each EtherCAT slave device reads the data specifically addressed to it from the frame and inserts data into the frame while that frame passes through the node at full speed. Also, the frame’s cyclic redundancy check (CRC) is updated to reflect the new contents of the frame.
Simple integration of traditional fieldbus devices to EtherCAT is assured, including such networks as DeviceNet, Profibus, SERCOS, CANopen, and others. All data from these networks can be transmitted in the EtherCAT frame over a standard Ethernet cable. In this way, EtherCAT users can implement a gradual migration path, if necessary, to advanced industrial Ethernet. Data from other major industrial Ethernet protocols also can be transmitted over EtherCAT, including EtherNet/IP and Profinet. Besides industrial networks, EtherCAT can transmit all Ethernet Protocols: SECS, GEM, HSMS, TCP/IP, and so on. This is useful for communicating to standard TCP/IP devices, such as barcode readers, label printers, or human machine interface (HMI) screens via the EtherCAT backbone.
Other EtherCAT advantages over fieldbus systems include:
- Very simple master and slave implementations – A synchronization method that sets the standard for determinism in industrial networks (distributed clocks)
- Ability to implement any topology, including redundant rings and hot connect/disconnect segments, without the need for switches or any other active infrastructure components
- Industry leading diagnostics with exact localization of faults for a variety of fault conditions
- Lower costs than other industrial networks due to less expensive slave implementations, no infrastructure requirements, and no special master cards required.
– Joey Stubbs, PE, PMP, is North American representative, EtherCAT Technology Group. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com.
Industrial Ethernet: Application Optimization – webcast
Friday, October 7, 2011 at 2 p.m. ET/1 p.m. CT/11 a.m. PT (… and archived after that for viewing.)