Requirement 3: higher-level global time, absolute time

If the time base T_DC is to be adjusted based on a higher-level time base, the time base and the associated procedure must be selected. Generally common synchronization protocols are used for the synchronization. Samples for time sources and synchronization procedures are listed below.

Sources: UTC world time, network time, adjacent control system, radio clocks (in Central Europe: DCF77)
Procedures: GPS, radio clocks, NTP (NetworkTimeProtocol), SNTP (Simple NTP), PTP (IEEE1588), distributed clocks DC

The following synchronization precisions can be achieved (depending on the hardware)

NTP/SNTP: ms range
PTP: < 1 µs
DC: < 100 ns

The following two control aims must be achieved:

The frequency of the subordinate time base must be adjusted to the higher-level time base.
Any offset between two absolute times does not have necessarily to be controlled to 0. It is sufficient for it to be announced and kept constant. The maximum offset adjustment is ±½ cycle time.

	External EtherCAT synchronization External synchronization sources (e.g. EL6688, EL6692) can only be used from TwinCAT 2.11 used. In older versions of TwinCAT such EtherCAT slaves have no meaningful function.

If a higher-level master clock is integrated in an EtherCAT system, a special EtherCAT device is generally used for the physical connection. The device monitors both time bases and is therefore able to determine the time difference.

Please refer to www.beckhoff.de for suitable products currently available for this purpose.

Requirement 3: higher-level global time, absolute time 2: — EtherCAT topology with external reference clock

The different time bases can be arranged hierarchically, so that at the start of the respective system the current absolute time is taken from the subordinate system. If necessary top-down synchronization is used, if external time bases or DC components are present in the system.

Readjustment of local time vs. higher-level absolute time

For the purpose of synchronization the local DC time is not adjusted based on the higher-level absolute time, but only to a constant offset. This offset is made available to the user as a process data. The offset is corrected by ±½ cycle time to ensure both tasks run in phase.

When TwinCAT starts the EtherCAT master, the local DC system in the slaves is started and synchronized immediately.
However, an external reference slave such as EL6688 (IEEE1588 PTP) takes a few seconds before it can supply a reference time that is synchronized with the higher-level clock.
As soon as the external reference time is available, the offset to the local time is calculated and corrected by ±½ cycle time to ensure that both tasks run in phase, and the EtherCAT master Info Data are made available to the user for reconciliation with the local time values.
From this time the offset is kept constant, depending on the selected control direction.