4
EtherCAT interface
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Festo – GDCP-CMMP-M3/-M0-C-CO-EN – 1510b – English
4.13
Synchronisation (Distributed Clocks)
Time synchronisation is implemented via so-called “Distributed Clocks” in EtherCAT. Each EtherCAT
slave receives a real-time clock, which is synchronised in all slaves by the clock master during the ini
tialisation phase. The clocks in all slaves are then adjusted during operation. The clock master is the
first slave in the network.
This provides a uniform time base in the entire system with which the individual slaves can synchronise.
The Sync telegrams provided for this purpose under CANopen are unnecessary under CoE.
The FPGA ESC20 used in the motor controller CMMP-AS-...-M3 supports Distributed Clocks. This facilit
ates extremely precise time synchronisation. The cycle time of the EtherCAT Frame must exactly match
the cycle time tp of the controller-internal interpolator. If necessary, the interpolator time must be ad
justed via the object included in the device description file.
In the present implementation, synchronous transfer of PDO data and synchronisation of the controller-
internal PLL to the synchronous data framework of the EtherCAT Frame can be implemented even
without Distributed Clocks. For this purpose, the firmware uses the arrival of the EtherCAT Frame as a
time base.
The following restrictions apply:
–
The master must be able to send the EtherCAT frames with an extremely low jitter (< 70 % of the set
current controller cycle time).
–
The cycle time of the EtherCAT Frame must exactly match the cycle time tp of the controller-internal
interpolator.
–
The Ethernet must be available exclusively for the EtherCAT Frame. It may be necessary to synchron
ise other telegrams to the grid, as they may not block the bus.
