Hx58x EtherCAT®
5.1.9 Timing and Synchronization
The main feature of EtherCAT is its almost ideal representation of a real-time
system.
Hence the master has to synchronize all the slaves at the same time in order to
build a system where all nodes have the same reference time; this goal can be
achieved by using “distributed clocks”.
Master downloads its clock into one of the salves (customarily the first slave)
which becomes the reference clock for all the slaves in the network; so it has
the task of synchronizing the other slaves. Master controller periodically sends a
special synchronization-telegram where the reference slave writes its own
“current time”. This telegram is then sent to all the other slaves that, in this way,
provide for a new re-synchronization of their own clock in order to avoid
possible drifts.
This synchronization of the reference time is very important in order to have a
snapshot of the system and accordingly to take simultaneous actions in high
sensitive applications such as the coordination in axis control operations.
Besides, EtherCAT Slave Controller (ESC) is fitted with a capture/compare unit
that provides accurate synchronization signals (SYNC0 or interrupts): they are
sent to the local micro-controller so that it is able to synchronize its own clock
to slaves clock.
Sync Manager
Sync Manager has the task of synchronizing data transfer between master and
slave and prevents the same memory area from being written by different
events.
There are two synchronization modes:
•
3-Buffer Mode;
•
1-Buffer Mode.
Synchronisation mode is initialized through the XML file or by loading data
directly from EEPROM (SII).
Buffered Mode (3-Buffer Mode)
In this mode new data can be accessed at any time by both master EtherCAT
and ESC controllers; no timing restrictions are imposed.
Three buffers are necessary (three consecutive memory areas); one buffer is
always available to ESC controller for writing and one buffer always contains
updated data to be read by the master.
Customarily this is used for process data communication.
Mailbox Mode (1-Buffer Mode)
In this mode a “handshake” between master and slave must be used; in fact one
only memory buffer is available to both master and slave for writing and
reading; the master (or the slave) is enabled to write only when the buffer is
empty, that is when the slave (or the master) has finished reading the data
buffer. And vice versa: the master (or the slave) is enabled to read only when the
buffer is empty, that is when the slave (or the master) has finished writing the
data buffer.
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