BECKHOFF DK9222-0211-0021 - Manual (Page 3)

I/O
XFC
Application Note DK9222-0211-0021
If the temporal assignment is to be more precise, only the cycle time can be reduced by using a 1 ms task instead of a 10 ms
one. A temporal classification that is more precise by a factor of 10 is thus possible. This leads to a higher load on the controller
and fieldbus and can impair system stability in the case of overloading. Low performance controllers in particular impose limits
on this approach. As a positive side effect, the possible minimum reaction time of the controller to external events is thereby
shortened, but that is not the main subject of this application example.
Reduction of the cycle time for maximized accuracy
PLC cycle time
Demanded
resolution
PLC cycle time
Physical limits of the
fieldbus and the
controller are reached.
Residual demand cannot
be met.
Before
Demanded
nominal
state
Fig. 3 Reduction of the cycle time
Time-stamped process data with an application-wide, uniform timebase
A further approach to the refinement of the temporal resolution is the use of time-stamped process data under EtherCAT with
XFC. The special “Timestamp” XFC terminals for the EtherCAT terminal system from Beckhoff pass not only the event on to the
controller via the fieldbus, but also add a time specification in the form of a time stamp. This time specification can be used in
order to establish the exact temporal relationship between event and reaction inside the controller. The basis for the use of the
time stamp is an application-wide, uniform timebase, which is guaranteed by the EtherCAT Distributed Clock mechanism.
Application-wide synchronicity with Distributed Clocks
Fig. 4 Application-wide synchronicity
The various segments and devices in an EtherCAT network are synchronized via Distributed Clocks. The micro-delays in the
protocol runtime are calculated and the system times of the individual devices are corrected accordingly. This approach results
New Automation Technology Beckhoff 3
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