EtherCAT Overview
Overview
1.1 Introduction to EtherCAT
EtherCAT (Ethernet for Control Automation Technology) is a real-time Ethernet technology for industrial automation. It uses the standard Ethernet physical layer, but slaves typically read output data and write input data directly as a frame passes through, reducing the latency introduced by forwarding frames from one slave to the next.
EtherCAT is commonly used in the following scenarios:
- Servo drives and multi-axis motion control.
- Industrial I/O, sensors, and actuators.
- Robots, CNC equipment, and automated production lines.
- Measurement and control systems that require distributed clock synchronization.
A typical topology is shown below:
Linux EtherCAT master
│
├── Slave 0: Servo drive / I/O
│
├── Slave 1: Servo drive / I/O
│
└── Slave N: Sensor / actuatorAn EtherCAT network should use a dedicated Ethernet interface. The master directly transmits and receives Layer 2 EtherCAT frames through this interface. The interface generally has no IP address and should not be managed simultaneously by NetworkManager, a DHCP client, or standard network services.
1.2 Introduction to the IgH EtherCAT Master
The IgH EtherCAT Master is an open-source Linux EtherCAT master implementation provided by the EtherLab project. Its core runs in kernel space, while user applications communicate with the master through a character device and libethercat.
The IgH master mainly consists of the following components:
| Component | Common file or interface | Function |
|---|---|---|
| Master kernel module | ec_master.ko | Manages the EtherCAT bus, slave states, process data, and clock synchronization |
| Network driver | ec_generic.ko or a dedicated driver | Connects the master to the physical Ethernet interface; dedicated drivers generally provide better real-time performance |
| Character device | /dev/EtherCAT0 | Interface between the kernel master and user-space tools and applications |
| User-space library | libethercat.so | Provides the ecrt_* application development API |
| Command-line tool | ethercat | Scans slaves, displays PDOs, reads and writes SDOs, changes states, and diagnoses the bus |
| Master application | User application | Performs cyclic communication, state-machine processing, and application control according to the device configuration |
EtherCAT application / ethercat tool
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libethercat.so
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/dev/EtherCAT0
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ec_master.ko
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Generic NIC driver or platform real-time NIC driver
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EtherCAT slavesOfficial IgH API documentation:
1.3 Basic Terminology
| Term | Meaning |
|---|---|
| Master | EtherCAT master responsible for discovering, configuring, and cyclically accessing slaves |
| Slave | EtherCAT slave, such as a servo drive, I/O module, or sensor |
| PDO | Process Data Object, real-time process data exchanged cyclically |
| SDO | Service Data Object, generally used for acyclic parameter access and device configuration |
| CoE | CAN application protocol over EtherCAT, a commonly used object dictionary protocol |
| DC | Distributed Clocks, the EtherCAT distributed clock synchronization mechanism |
| ESI | EtherCAT Slave Information, an XML description file provided by the slave vendor |
| Alias | Slave alias address used to identify a device consistently |
| Position | Physical position of a slave in the bus topology, starting at 0 |
EtherCAT slaves generally transition through the following states:
INIT → PREOP → SAFEOP → OPINIT: Initialization state; mailbox and process data communication have not started.PREOP: Mailbox communication and SDO configuration are available, but valid PDO data is not yet exchanged.SAFEOP: Input process data is valid, while outputs generally remain in a safe state.OP: Input and output process data is exchanged normally.