EtherCAT (Ethernet for Control Automation Technology) is a real-time fieldbus protocol used in industrial automation. It runs on standard Ethernet hardware but handles data exchange with a method different from classic Ethernet logic. It is especially preferred in multi-axis motion applications and in systems where a high number of I/O points must be read with short cycles.
Difference from classic Ethernet
In standard Ethernet, each device receives its own packet, processes it, and sends its response back in a separate packet. As the number of devices on the network increases, the number of packets and the latency increase. This structure is not a problem for office networks; however, it is inefficient for a control loop repeated tens of thousands of times per second.
EtherCAT solves this problem with the processing on the fly method. The master (usually a PLC or industrial PC) sends a single Ethernet frame. As the frame passes through each slave device in the chain, the device reads its allocated data area while the frame is in motion and writes its own data into the same frame. When the frame reaches the end of the chain, it returns and reaches the master.
Result: the data of all devices on the entire network is exchanged with a single frame round trip. There is no separate packet or separate wait per device.
Bandwidth efficiency
In the classic approach, a full Ethernet frame (with its minimum length requirement) is spent even for a few-bit digital input. In EtherCAT, each slave occupies only as much space as it needs within the frame. Thus the useful-data ratio of the frame becomes very high, and bandwidth is used efficiently in I/O-intensive systems that work with small data pieces.
Topology
EtherCAT logically works over a ring: the frame goes out and comes back. Physically, however, it is flexible:
- Line: The most common structure. Devices are connected one after another. No additional switch
- Tree / star: Sub-branches are created with branching modules.
- Redundant ring: Both ends of the chain return to the master. In the event of a cable break,
is required, and the cabling is simple.
communication continues without interruption — preferred in critical facilities.
A standard Ethernet switch is not used inside an EtherCAT segment; the slave devices themselves already act as pass-through.
Addressing and commissioning
EtherCAT slaves are not given an IP address. Devices are automatically addressed according to their physical order in the chain. This simplifies commissioning but leads to the following consequence: the physical order of the devices must match the configuration. If a slave is moved or a device is inserted in between, the configuration must be updated.
Device definitions are made with ESI (EtherCAT Slave Information) XML files. The manufacturer provides this file; when it is imported into the master software, the data objects and settings that the device supports are recognized.
CoE, FoE and other profiles
EtherCAT is a transport infrastructure; different application profiles run on top of it:
- CoE (CANopen over EtherCAT): The most commonly used profile. The CANopen
- FoE (File over EtherCAT): File transfer to the device — used for firmware updates.
- EoE (Ethernet over EtherCAT): Tunneling of standard Ethernet traffic (e.g. a
- SoE (Servo Drive Profile over EtherCAT): An alternative drive profile.
object dictionary and drive profile are carried over EtherCAT. It is the standard approach in servo drives.
updates.
device’s web interface) over the same line.
Diagnostics and fault finding
One of the most valuable features of EtherCAT in practice is its diagnostic capability. Faulty-frame counters are kept on each slave port. When a cable or connector begins to degrade, the error counters start to rise before communication is completely lost, and it can be determined on which port of which device the problem is. This significantly shortens fault-finding time on long machine lines.
Where it is preferred
- Multi-axis synchronous motion (electronic cam, gearing, cutting applications)
- Machines requiring a short control cycle
- A high number of distributed I/O points
- The desire for simple cabling on long machine lines
- Systems requiring a redundant structure against cable breaks
Points to watch in application
Industrial-type shielded Ethernet cable is used in the cabling; the shield grounding and the connector assembly quality directly affect communication stability. The device order must be documented — this document makes the job easier during later maintenance when the chain will be modified. On the master side, the control cycle time is determined by the processing time of the slowest slave.
Related product families: PLC, Servo Drive, AC Drive, Remote I/O, Communication Modules, Gateway
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