Commissioning and configuration
EP1518-0002
30
Version: 2.4
5.2
Distributed Clocks (DC)
EtherCAT System Documentation
A basic introduction to the topic of EtherCAT and Distributed Clocks is available in the
area on the Beckhoff homepage:
EtherCAT System Documentation.
The Box supports the distributed clocks functionality. In order for the Box to provide the current counter
reading before the arrival of the querying EtherCAT datagram in the specified process data, a corresponding
signal must be generated cyclically in the Box. This signal can be triggered in the Box by two events: the
SyncManager (SM) and the distributed clock (DC). Under operation mode selection the following options are
available (see Fig.
"DC" (Disributed Clocks) tab
)
•
SM-synchron
The SyncManager event occurs when an EtherCAT frame successfully exchanges process data with
the Box. Frame-triggered, the current counter value is thus cyclically determined, but with the low
temporal jitter of the Ethernet frame.
•
DC-synchron
In DC operation mode determination of the counter value is triggered cyclically at constant intervals
through the integrated DC unit, synchronous with the bus cycle as standard. More uniform polling
offers higher-quality position data for a higher-level control algorithm, for example.
The trigger for the counter reading is the SYNC0 signal.
Fig. 10: DC tab (Distributed Clocks)
When
DC-Synchron
operation mode is activated TwinCAT selects settings that ensure reliable operation of
the Box with current position data. This means that determination of the current counter value is triggered by
the SYNC0 signal at highly constant intervals and in good time (i.e. with an adequate safety buffer) before
retrieving EtherCAT datagram is started.
If necessary, the SYNC0 signal can be shifted along the time axis to the right/later or left/earlier in associated
dialogs by specifying a user-defined shift time, see Fig.
Advanced Distributed Clock (DC) settings
.
• A right-shift (positive shift value) will delay the counter value query, which means the position value
becomes more current from the PLC perspective. However, this increases the risk that the position
determination may not be finished in time before the arrival of EtherCAT frame, so that no current
position value is available in this cycle.
• A left-shift (negative shift value) means the counter value will be queried earlier, resulting in older
position values, with an associated increase in the safety buffer before the arrival of the EtherCAT
datagram. This setting may be useful in systems with high real-time jitter, if no Industrial PCs from
Beckhoff are used for control purposes, for example.
