Understanding EtherCAT

Synchronisation and Timing

Official Documentation

Distributed Clock (DC)

The DC is an option on some slaves. The Class B EtherCAT stack of Acontis does not suport DCs, but the Class A addon does („Synchronous with SYNC event“). DCs allow synchronized reading of inputs and synchronized setting of outputs in the nanosecond range.

Understanding synchronisation with DC

Synchronous with SM event

Without DC a synchronization of approximately some 10 to 100 microseconds is possible („Synchronous with SM event“).

Free Run

In „Free Run“ mode the slave measures as fast as it can. However, the time interval between two measured values is not always the same and can easily change by a factor of two.

ENI and ESI files


  • EtherCAT Slave Information
  • Describes the slave. Every slave type has its own ESI file.
  • Can be downloaded from the manufacturer website
    • Maxon MAXPOS 50/5 Downloads –> Handbücher –> English –> MAXPOS 50/5 ESI File (englisch, ZIP 48 KB)
    • Elmo:
      • Are installed with the 'EAS II' software from Elmo in Elmo Motion Control/Elmo Application Studio II/EtherCATSlaveLib
      • Can be downloaded here


  • EtherCAT Network Information
  • Describes the whole network.
    • Which slave is connected, to which port from which other slave or master.
    • Slave types
    • Cycle time
    • Which PDOs are transmitted in which order
  • Created with software EC-Engineer (one license is available at OST/Buchs→ Einar Nielsen)
    • After every change in the network (new slave or different connection sequence, different PDOs, different cycle time) the file has to be generated again.
    • An application which uses EtherCAT requres an eni-file for the current network