EtherCAT — telegram passes through, on-the-fly
EtherCAT (Ethernet for Control Automation Technology) takes a radically different approach: the master sends a single telegram that passes through every slave on-the-fly. Each slave reads and modifies its bytes as the frame transits — no stop, no store-and-forward. Watch the chip travel along the bus-trunk; the nearest slave lights up green as the telegram crosses it.
Animation
Engineering pitfalls
Slave processing time accumulates along the chain
Each slave adds ~100-300 ns to the telegram pass-through. A 100-slave chain adds 10-30 µs of cumulative delay. For sub-millisecond cycle times, audit your chain length against the slave processing-time specs.
Distributed Clocks (DC) compensation
EtherCAT's DC mechanism compensates for propagation delay. If a slave doesn't support DC and you have axes that need it, you'll see motion synchronisation errors. Verify DC capability of every slave on the bus.
Topology change mid-runtime
Hot-plugging slaves into a running EtherCAT bus requires Hot Connect or Cable Redundancy modes. Plugging without these = master stops the entire chain on slave-count mismatch. Plan for it explicitly.
Mailbox protocols (CoE / EoE / FoE) on the same chain
CANopen-over-EtherCAT (CoE), Ethernet-over-EtherCAT (EoE), File-over-EtherCAT (FoE) all coexist. Misallocating bandwidth (e.g. EoE flooding during cyclic operation) starves the cyclic channel. Configure mailbox slot timing carefully.
References
Primary sources
- IEC 61158-3-12 / 61158-4-12 / 61158-5-12 / 61158-6-12 — EtherCAT specification.
- EtherCAT Technology Group (ETG) — EtherCAT Specification.
- IEEE 802.1AS — Time-Sensitive Networking for Distributed Clocks.