Beckhoff EP1518-0001, Документация

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Commissioning and configuration
EP1518-0002 44 Version: 2.1.0
4.1.3 Distributed Clocks (DC)
EtherCAT System Documentation
A basic introduction to the topic of EtherCAT and Distributed Clocks is available in the Download
area on the Beckhoff homepage: EtherCAT System Documentation.
The EtherCAT Box supports Distributed Clocks functionality. In order for the Box to be able to make the
current counter value available in the designated process data in time before the arrival of the querying
EtherCAT datagram, a suitable signal must be generated cyclically within the terminal. This signal can be
triggered in the Box through 2 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 SynManager event occurs when an EtherCAT frame successfully exchanges process data with
the EP1518. 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. In the EP1518 the
SYNC0 signal acts as trigger.
Fig. 41: 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.