Fig. 12 Mapping Of Dc To The Topology - Beckhoff EL6692 Documentation

• Not all EtherCAT devices have to support this feature. If a slave does not support this concept, the
master will not include it in the synchronization. If the EtherCAT master does not support this feature,
DC is also ineffective in all slaves.
• such a clock also runs in the EtherCAT master, in this case software-based.
• In the system one of the existing clocks is selected as reference clock and used for synchronizing all
other clocks. This reference clock is usually one of the EtherCAT slave clocks, not the EtherCAT
master clock. The clock of the first EtherCAT slave in the topology that supports distributed clocks is
usually automatically selected as reference clock.
• A distinction is made between
- the EtherCAT master (the software that "manages" the EtherCAT slaves with Ethernet frames) and
the EtherCAT slaves managed by it.
- the reference clock, which is usually located in the first DC slave, and the slave clocks whose time is
based on it, including the clock in the EtherCAT master.
Master:
• During the system start phase the EtherCAT master must set the local time of the reference clock and
the other slave clocks to the current time and subsequently minimize deviations between the clocks
through cyclic synchronization datagrams.
• In the event of topology changes the EtherCAT master must re-synchronize the clocks accordingly.
• Not all EtherCAT masters support this procedure.
• The EtherCAT master in the Beckhoff TwinCAT automation suite fully supports distributed clocks.
Slave:
• Due to the high precision required this local clock is implemented in hardware (ASIC, FPGA).
• Distributed clocks are managed in the EtherCAT slave controller (ESC) in registers 0x0900 - 0x09FF.
Specifically, the local synchronized time runs in the 8 byte from 0x0910.
Fig. 12: mapping of DC to the topology
In Fig. Mapping of DC to the topology the 3
sample. The local time of this slave is now used to adjust all other distributed clocks, i.e. all other EtherCAT
slaves and the clock of the EtherCAT master. This is achieved through synchronization datagrams, which
the EtherCAT master sends cyclically.
This procedure ensures that all devices supporting DC always have local access to a time that is identical
(within the DC synchronization precision) in all devices.
The system now operates based on the timebase of the selected DC reference clock and its local clock
generator/quartz with T
DC
sidereal time/coordinated world time UTC T
exactly 1 ms T ≠ T
DC, DC UTC
is used for relative processes within the EtherCAT system, the deviation from the UTC is irrelevant.
However, if the DC time is to be used for data logging with a global timebase, for sample, the
must be synchronized with the
EL6692
rd
EtherCAT slave was selected as DC reference clock as a
. Due to production tolerances this timebase is rarely the same as the official
or another reference time. This means that 1 ms
UTC
. Over longer periods also drift processes may also change the ratio. As long as DC
timebase. This is described in section Requirement 3.
UTC
Version: 3.2
Product overview
is never
UTC
timebase
DC
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