Advantech PCIE-1203, Руководство пользователя

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PCIE-1203 User Manual 4
1.1.1.4 Synchronization
A Distributed Clock (DC) mechanism is used to provide highly precise time synchro -
nization between slaves in an EtherCAT network, which is equivalent to the IEEE
1588 Precision Time Protocol standard. By using distributed clocks, EtherCAT is able
to synchronize the time in all local bus devices within a very narrow tolerance range.
All EtherCAT slaves are provided with an internal clock which named as System
Time (t
Local Time
). One EtherCAT Slave, usually the first slave, will be used as a Ref -
erence Clock and cyclically distributes its Clock.
Possible misalignments between the reference clock and the clocks of the other
slaves are usually due to the following reason: when a slave is switched on, the inter -
nal free-running register that holds the current time is reset to zero. Unfortunately,
this action does not take place exactly at the same time in all the different slaves, and
this result in an initial offset (t
offset
) among clocks that has to be compensated.
Figure 1.4 EtherCAT Distributed Clock
Typically, the master sends a broadcast to all other slaves in the system. Once
receiving the message, slaves will latch the value of their internal clock. There are
two latch values, one is receiving and the other is returning back. Thus, the master
can read all latched values and calculate the delay for each slave (t
Propagation Delay
).
Delays will be stored into offset register. In the following, master will send a message
periodically to all other slaves in EtherCAT network to make the first slave the refer -
ence clock and forcing all other slaves to set their internal clock by the calculated off -
set.
∆t = (t
Local Time
+ t
Offset
- t
Propagation Delay
) - t
Received System Time