BECKHOFF EtherCAT Technology Section I - Manual (Page 65)

SyncManager
Slave Controller – Technology I-45
8.5 Single Byte Buffer Length / Watchdog Trigger for Digital Output PDI
If a SyncManager is configured for a length of 1 byte (or even 0), the buffer mechanism is disabled,
i.e., read/write accesses to the configured address will pass the SyncManager without interference.
The additional buffers 1 and 2 are not used in buffered mode, and alternation of write/read accesses is
not necessary for mailbox mode. Consistency is not an issue for a single byte buffer.
Nevertheless, watchdog generation is still possible if the buffer length is 1 byte (interrupt generation as
well).
NOTE: For some ESCs in Mailbox mode, watchdog and interrupt generation are depending on the alternation of
write and read accesses, although the write/read accesses itself are executed without interference. I.e., Buffered
mode should be used for single byte buffers for watchdog generation.
Watchdog trigger generation with single byte SyncManagers is used for Digital Outputs, because the
outputs are only driven with output data if the Process Data watchdog is triggered. One SyncManager
has to be configured for each byte of the Digital Output register (0x0F00:0x0F03) which is used for
outputs. The SyncManagers have to be configured like this:

Buffered Mode (otherwise the process data watchdog will not be wound up with some ESCs upon
the second and following writes, because the Digital I/O PDI does not read the addresses)

Length of 1 byte

EtherCAT write / PDI read

Watchdog Trigger enabled
For more details refer to the Digital I/O PDI description of Section III and the chapters about Watchdog
and Digital Output in this document.
NOTE: A SyncManager with length 0 behaves like a disabled SyncManager. It does not interfere accesses nor
generate interrupt or watchdog trigger signals.
8.6 Repeating Mailbox Communication
A lost datagram with mailbox data is handled by the application layer. The Repeat Request/Repeat
Acknowledge bits in the SyncManager Activation register (offset 0x06[1]) and the PDI Control register
(offset 0x07[1]) are used in mailbox mode for retransmissions of buffers from a slave to the master. If
a mailbox read frame gets lost/broken on the way back to the master, the master can toggle the
Repeat Request bit. The slave polls this bit or receives an interrupt (SyncManager activation register
changed, register 0x0220[4]) and writes the last buffer again to the SyncManager. Then the PDI
toggles the Repeat Acknowledge bit in the PDI Control register. The master will read out this bit and
read the buffer content. Communication resumes afterwards.
This mechanism is shown in Figure 26 for a mailbox write service. The Mailbox confirmation is lost on
its way from the slave to the master and has to be repeated again.