Distributed Clock Synchronization

Distributed Clock (DC) is one of the most important aspects of EtherCAT. It allows the MainDevice and SubDevices to be tightly synchronized in time with each other. This provides jitter of less than 1 µs, which is important for providing good feedback data to the RMC.

For best performance, feedback devices and drives on the EtherCAT network should support DC Sync. If the feedback does not support DC sync, especially for position sensors, the feedback may have jitter that adversely affects control.

EtherCAT devices that are not used for feedback need not necessarily support DC synchronization, such as valves, basic discrete I/O, temperature sensors, etc.

Configuring DC Synchronization

If a SubDevice supports DC Synchronization, make sure to configure it to use DC Synchronization, since it may not be on by default:

In the EtherCAT Editor, click the SubDevice in the list.

On the Distributed Clocks tab, in the Operation Mode box, choose the option that corresponds to DC Sync.

The terminology in the Operation Mode box varies between manufacturers. The table below lists terms that do or do not indicate DC Synchronization

Terms that indicate DC Synchronization	Terms that do NOT indicate DC Synchronization
DC for synchronization	Free run
Synchronized measurement for Sync 0	SM Synchronization
DC Synchronous	DC Unused
DC Sync	Sync Manager

Note: ‘Sync Manager’, also abbreviated ‘SM’, is NOT DC Synchronization.

Sync Unit Cycle

On the Distributed Clocks tab, under the Operation Mode box, is the Sync Unit Cycle. The Sync Unit Cycle defines the basic cycle of the SubDevice. The RMC200 automatically sets this to the same as the RMC’s loop time.