It’s celebration time. EtherCAT turns 15 years old this year!
It’s anything but a bratty teenager. EtherCAT is a mature, well-respected, hardworking technology that delivers day-in and day-out for its users. Over the last decade and a half, EtherCAT has extended its reach from Europe to Canada and the Americas to the Far East. It’s proved a formidable competitor to Profinet IRT, Sercos and CIP Sync. Users in automotive, food and beverage and many other industries are deploying EtherCAT applications to solve demanding applications that require redundancy, fast, highly accurate synchronization, pass-thru Ethernet and black channel safety.
Users are deploying EtherCAT in ever increasing numbers because of some significant advantages, the most interesting of which are:
- A system architecture based on a principle called “processing on the fly”
- A simplified master with no subsystem required
- Flexible media and topology
- A simple and straightforward data representation scheme
- High-speed, large bandwidth and highly synchronized clocks
Processing on the Fly
Ethernet application layers used in industrial and building automation systems are typically “low payload” application layers. These client /server systems (such as Modbus TCP, EtherNet/IP or Profinet IO) are typically bandwidth inefficient, since only small message packets with the I/O for one specific device are transmitted in an Ethernet frame.
In EtherCAT, the fundamental principle is pass-through reading, which means messages are not destined for a single node and consumed by that single node. Instead, messages are transmitted and processed by a node, byte by byte, as they arrive. A three-node system could have one node processing bytes in the leading portion of the frame while another node is processing bytes in the middle, while a third is processing bytes at the end of the packet. Input data to a node is read and output data bytes are inserted in the message as it passes through the node and continues to the next node. The packet never stops to be processed, hence the term, “processing on the fly”.
Unlike most other protocols, an EtherCAT master node issues a single message with data for all the nodes in a network. The message travels the network, turns around at the last node and returns to the master. When it arrives back at the master, every node in the network has received new input data and returned new output data to the master. This architecture is the reason for EtherCAT’s exceptional update rate and bandwidth utilization over other technologies.
A Simplified Master
EtherCAT master devices require no special hardware. Unlike most Ethernet master devices, the network interface is exceptionally uncomplicated. There is no I/O subsystem to map field data to process data. Instead, the EtherCAT master operates on the process data map; it simply transmits that entire process data map in every network scan. When the message returns to the master, outputs in the process data map have been set in the field devices and inputs in the map have been updated. There is no mapping in the master. Unlike most Ethernet protocols, data mapping to specific inputs and outputs happens in each EtherCAT device.
Flexible Media and Topology
EtherCAT uses the standard IEEE 802.3 physical layer. No special hardware or media is required to implement an EtherCAT master, but an ASIC is required to implement an EtherCAT slave.
External switches are not used in an EtherCAT network. Instead, each EtherCAT device embeds a switch (in the EtherCAT ASIC). Each device has two RJ45 ports. One RJ45 is connected to the previous node in the network and one is connected to the next.
Somewhat unique to Ethernet is the EtherCAT concept of self-terminating networks. Any node that does not detect a next node in the string automatically terminates the network at that point. Terminating nodes copy messages from the master’s transmit path to the masters receive path.
With this self-terminating feature, EtherCAT networks can be wired using any number of different topologies including star, line, or tree.
Data Representation and Data Mapping
Each individual EtherCAT slave device maps data from the process image to its physical inputs and outputs. The mapping from the process image to the physical I/O points happens during device configuration using an EtherCAT tool. A node can input with the EtherCAT master or with any other EtherCAT slave device.
High-Speed, Large Bandwidth and Highly Synchronized Clocks
Synchronization is now of increasing importance in the industrial networking industry. EtherCAT’s distributed clock mechanism offers a low jitter rate that meets the specifications of IEEE 1588 Precision Time Protocol standard without additional hardware.
This mechanism is possible because of the timestamps each node includes in the EtherCAT frame. Each node attaches a timestamp to the EtherCAT frame twice. First, the slave node adds a timestamp when receiving the message as it is sent through the network. Then, when the frame returns back through the nodes, each slave adds another timestamp. The master receives the frame with two timestamps per slave.
With the time information, the master calculates the delay for each node. The master repeats the calculation after every message frame it sends. As the network operates, the enormous sample size means the master has incredibly accurate data. The inherent ring topology creates an incredibly efficient clock mechanism that increases in accuracy with every message.
Summary
These are only the highlights of this impressive and very popular technology. There are many other EtherCAT features that space doesn’t permit describing, including: a large user group’ the EtherCAT Technology Group (ETG); black channel safety; fully redundant operation with exceptionally fast turnover; peer-to-peer slave communication; pass-through Ethernet; device profiling; and much more. EtherCAT is not just an alternative for highly deterministic motion applications, but an option for any application where performance, topology and overall deployment cost are driving factors.
