EtherCAT for Servo Drives: A Practical Guide
What is EtherCAT?
EtherCAT (Ethernet for Control Automation Technology) is a real-time Ethernet fieldbus protocol developed by Beckhoff and established as an IEC standard (IEC 61158) since 2005. Its core idea: Ethernet frames are not buffered and forwarded at each node as usual, but are processed "on the fly" — each slave reads and writes its data while the frame passes through at line speed.
Why is this important?
For servo drives, deterministic communication is not an option, but a prerequisite. The control loop must exchange setpoints and actual values at precisely defined time intervals—jitter in the microsecond range can destroy the control quality.
EtherCAT delivers exactly that:
- Cycle times from 62.5 µs — fast enough for highly dynamic control loops with 4 kHz and above
- Jitter in the nanosecond range — measured values of 30–70 ns in real machine applications
- Multi-axis synchronization — Distributed Clocks synchronize all axes to an accuracy of less than 100 ns
- High bandwidth — 100 Mbit/s, no 8-byte limitation as with native CANopen; position, speed, torque, and additional data in every cycle
- Flexible topology — line, ring, star, or hybrid; up to 100 m between nodes with standard Cat5e cables
How does it work?
Processing on the Fly: A single Ethernet frame passes through all slaves in the network. Each EtherCAT Slave Controller (ESC) reads its input data and writes its output data to the frame — in hardware, without buffering the frame. The delay per node is approximately 1 µs. After the last slave, the frame returns to the master. A single frame cycle updates all I/O data throughout the network.
Distributed Clocks (DC): One slave is defined as the reference clock (typically the first DC-enabled slave). All other slaves synchronize their local clocks to this reference, with runtime delays automatically compensated. The result: SYNC0 signals are triggered simultaneously in all slaves—all drives scan sensors and switch outputs at the same moment.
PDO and SDO — Cyclic and acyclic data:
- PDO (Process Data Objects) — cyclic real-time data in each communication cycle: target position, actual position, target speed, status word, etc. PDO assignment is configured using mapping objects (0x1600–0x17FF for RxPDO, 0x1A00–0x1BFF for TxPDO).
- SDO (Service Data Objects) — acyclic parameter access via the mailbox mechanism: configuration, diagnostics, firmware updates. The SDO protocol is identical to CANopen.
CANopen over EtherCAT (CoE): CoE maps the entire CANopen profile family to EtherCAT — including CiA 301 (communication profile) and CiA 402 (drive profile with state machine and operating modes). Existing CANopen objects and profiles are retained in full, and the 8-byte payload limitation of native CANopen no longer applies.
How does SOMANET implement this?
All SOMANET servo drives (Node and Circulo) use EtherCAT with CoE as their primary communication interface. The internal sampling time is 250 µs (4 kHz), or 62.5 µs in high-speed configuration. Distributed clocks with SYNC0 synchronize multi-slave systems to less than 100 ns.
Up to 20 objects per PDO map and 4 PDO maps per direction allow flexible data assignments — a total of up to 80 cyclically transferred objects per direction. ESI files (EtherCAT Slave Information) for integration into EtherCAT masters such as Beckhoff TwinCAT are available at bundles.synapticon.com.
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