Enabling Distributed Clocks; Fig. 8 Example Structure Of An Ethercat Communication With Distributed Clocks; Fig. 9 Distributed Clocks Functionality Of An Ethercat Slave - Beckhoff FC1028 Manual

TwinCAT configuration
4.4

Enabling Distributed Clocks

Distributed clocks are a common part of EtherCAT communication. They enable local, absolute system
synchronization for CPU, I/O and drive units. This enables time-based and simultaneous data processing of
all distributed clocks capable EtherCAT devices.
Principle of operation
Without distributed clocks the devices would no longer work simultaneously in extended EtherCAT networks,
because EtherCAT works on the pass-through principle. In the pass-through principle, the first devices
receive the EtherCAT frame first and those behind significantly later. This would mean, for example, that not
all inputs could be read at the same time or all outputs could be set at the same time.
Fig. 8: Example structure of an EtherCAT communication with distributed clocks.
For this purpose a distributed clock is integrated in some specially designed EtherCAT devices. This
distributed clock is an electronic mechanism that runs in this device and is synchronized with other
participants with distributed clocks. Hierarchically, the structure looks like this: there is a master clock or a
reference clock and all other devices with distributed clocks support are then the slave clocks. The EtherCAT
master sends a special telegram in each I/O cycle through the EtherCAT network, which distributes the time
from the master clock to all following slave clocks. For this reason, the master clock must also be at the
beginning before all other slave clocks. By default TwinCAT therefore selects the first distributed clocks-
capable device as reference clock.
Fig. 9: Distributed clocks functionality of an EtherCAT slave.
Not all EtherCAT slaves support the distributed clocks method. An EtherCAT slave shows in TwinCAT its
distributed clocks functionality by having the DC tab.
Example structure with FC1028
The activation of the distributed clocks is shown on a small example structure, with an FC1028 as EtherCAT
master and an EK1100 EtherCAT coupler connected to it with EtherCAT Terminals, including two terminals
with distributed clocks functionality.
Activate the distributed clocks as follows:
FC1028 Version: 1.0 19