Fig. 13 Ethercat Topology With External Reference Clock - Beckhoff EL6692 Documentation

Product overview
Requirement 3: higher-level global time, absolute time
If the timebase T is to be adjusted based on a higher-level timebase, the timebase and the associated
DC
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 timebase must be adjusted to the higher-level timebase.
• 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 timebases and is therefore able to determine the
time difference.
Please refer to www.beckhoff.de for suitable products currently available for this purpose.
Fig. 13: EtherCAT topology with external reference clock
The different timebases 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 timebases 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.
18 Version: 3.2 EL6692