Beckhoff EL1258, Документация

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Commissioning
EL125x, EL2258 134 Version: 2.5
The process image of the EtherCAT master now looks as shown in Fig. Extended process image of the
EtherCAT master :
Fig. 169: Extended process image of the EtherCAT master
Note
SYSTIME
The EtherCAT master value DcSysTime is to be used with care and should serve only as a
coarse indication as to which time zone (order of magnitude: 1-2 task cycles) the Distrib-
uted Clocks system is currently in.
This is due to the fact that the local time in the terminal is read into associated buffers sev-
eral µs before retrieving frame. The EtherCAT strand then requires some time for transport-
ing the process data to the master. Due to the character of the visualization, the online dis-
play in the TwinCAT System Manager is accurate at best to a three-figure millisecond
range. Within a PLC the SysTime process data may also be outdated to a certain degree:
depending on when the analysis takes place in the program code, the process data may be
several µs/ms older. If responses of an EtherCAT strand (e.g. an EL2258 output terminal)
are to be based on a SysTime read in this way, 2-3 PLC cycles should be taken into ac-
count as a buffer.
It is more appropriate to use an EL2258 with other EtherCAT slaves that generate a time-
stamp based on ambient influences, such as the EL1252.
Alternatively, functions can also be called at NC/PLC runtime that immediately return the
current DC time, e.g. F_GetCurDcTickTime. Refer to the notes on the TwinCAT time
sources in the
EtherCAT system documentation .