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Distributed clocks ensure that digital outputs can be updated synchronously and that digital inputs can be recorded synchronously. They allow incoming events to be provided with a precise time stamp (latch signals), synchronous output signals (sync signals) and synchronous interrupts to be produced.

There are DC participants that have their own system time and that enable the full functionality that distributed clocks offer. There are also EtherCAT slave devices that have a local clock, but only support the measurement of runtime delays. Devices with three or more ports must have at least this reduced functionality built in. Finally, there are slaves that have no integrated DC functionality. They have a maximum of two ports and their propagation delay is treated like the delay on a simple electrical line.

The distributed clocks are synchronized between the EtherCAT participants. A reference clock is determined for the synchronization. Typically, the participant who is first behind the master and who is a DC participant manages the reference time. The EtherCAT master uses its clock to initialize the reference clock. As a result, however, the EtherCAT master is also synchronized according to this reference clock.

All local clocks initially run independently of the reference clock. The local clocks of the DC participants are synchronized with the reference clock in three steps so that each DC participant has the system time. First the propagation delay between the local clocks is measured. Next, the offset of the clocks of the DC subscribers is compensated in relation to the reference clock. Finally, the drift between the local system time and the reference clock is regularly corrected.

The master sends a frame to measure the propagation delay. With the help of the distributed local clocks, times are determined at which the transmitted frame reaches the ports of the slaves, and corresponding time stamps are assigned. In this way, runtime differences are measured between the ports of the slaves. The master reads all time stamps and calculates the delay times between all participants according to the topology of the EtherCAT network.

After the runtime delay in the EtherCAT network is known, the system time can be distributed to the DC participants. The local time of each DC participant is compared with the system time of the reference clock. The difference from this comparison is compensated by writing it individually to each DC participant. All DC participants get the same system time.

The drift between the reference clock and the local clock of a DC participant must be compensated regularly. For this purpose, time differences to the DC participants are measured and local clocks are reset.

As a default setting in a master-slave system, the first slave device that needs DC support is the reference clock. The external mode is used to synchronize two or more separate master-slave systems with one another. One possibility to set up the external mode is to use the EL6692 terminal. This terminal can be used in the sync master system, for example. Several sync slave systems can then be synchronized based on the sync master system. The sync master system calculates a common isochronous reference signal from the reference signals of the separate systems.

DC mode

Automatic DC Mode Selection: This check box is selected by default. The reference clock is selected automatically.

DC in use: If this check box has been selected, the reference clock and the synchronization direction can be selected manually. If there is only one EtherCAT device in the configuration and if DC slave devices are used, then "Independent DC Time (Master Mode)" must be used as the synchronization direction (exception: external synchronization).

Reference Clock - Select…: The first EtherCAT participant that supports Distributed Clocks (DC) functionality is usually entered in the text field. The "Select ..." button is currently without function. The slave device settings are decisive for the selection of the reference clock. Note the description for "Use as potential Reference Clock" in the slave device settings.

Independent DC Time (Master Mode): One of the EtherCAT participants, usually the first EtherCAT participant to support distributed clocks (DC), is the reference clock. All other DC participants are readjusted to this EtherCAT participant.

DC Time controlled by TwinCAT Time (slave mode): The DC reference clock is adjusted to the local TwinCAT time. This setting is used if several EtherCAT systems are operated on one controller, each with a distributed clock function. However, this tracking mode has decreased accuracy. If high accuracy is required, the external CU2508 EtherCAT distributor must be used.

DC Time controlled by External Sync Device (External Mode): If the EtherCAT system is to be readjusted according to a higher-level clock, an external clock can be selected under "External Sync Device - Select ...".

DC Time controlled by CCAT Time: The reference clock from the CCAT device is used to control the distributed clock time. The CCAT is an internal Beckhoff software interface and hardware interface for various bus systems. (The CCAT interface can address the E-bus or the K-bus, for example.)

Settings

Continuous runtime measurement: If this check box is selected, the time intervals between the participants are measured cyclically during the runtime. This process also takes place when the EtherCAT starts. It is recommended to disable this feature.

Sync window monitoring: If this check box is activated, bit 12 (0x1000) of the EtherCAT input variable "DevState" shows whether all DC participants keep their local clocks within the window specified in the "Sync window (µs)" text field. A cyclic BRD command on x092C (register in the EtherCAT slave. System time difference. Contains control errors in nanoseconds) is used to measure the time. The display "DC not in sync" can only be used if the first EtherCAT participant also contains the reference clock.

Sync window (µs): This editable text field contains the time window in which all DC participants must keep their local clocks in order to be "in sync". If the "Sync window monitoring" check box is activated, 2 µs are automatically entered in the text field here.

Show DC system time (64 bit): If this check box is activated, the input variable "DcSysTime" of type UDINTARR2 is added to the inputs of the EtherCAT master. It shows the current DC time as a copy from the master clock. Reading out the DC time consumes resources from the fieldbus transport. Alternatively, PLC modules can be used to find out the current DC system time.

Dc Sync Task: The task that is to regulate the distributed clocks can be selected from the drop-down list.

SYNC Shift Time (µs)

Percent of cycle time: In the example, the first summand for the outputs is around 30% of the cycle time. The frame lengths and the delays of the individual slaves are included in the shift time.

For outputs: The first summand contains the shift time automatically calculated by TwinCAT for all EtherCAT slave devices that are declared as output modules. With the second addend, the user can also intervene and shift the PDI pulses by positive or negative time values.

For inputs: The first summand contains the shift time automatically calculated by TwinCAT for all EtherCAT slave devices that are declared as input modules. With the second addend, the user can also intervene and shift the PDI pulses by positive or negative time values.