Beckhoff EL6751 Documentation page 158

Parameterization and commissioning
The following flow chart shows the sequence of the CAN cycle in the Fast CAN Queue mode.
Fig. 133: Flow chart for CAN cycle in Fast CAN Queue mode
Synchronization with SM2 event
In the transmit direction, the sequence is identical to the Buffered CAN Queue mode. In the receive direction,
the copying of the received CAN messages from the local Rx queue to the EtherCAT data input is dispensed
with.
Synchronization with SYNC0/SYNC1 event
If distributed clocks are switched on, the CAN interface cycle would be started by the SYNC0 event. The
sending of the first CAN Tx message is delayed until the SYNC1 event occurs, so that the sending of the first
CAN Tx message takes place with a jitter of maximum 500 ns. The output delay time is the time between the
SYNC1 event and the start of the CAN transmission of the first CAN Tx message in the CAN controller. The
remaining sequence of the CAN interface cycle corresponds to that in the case of synchronization with SM2
event.
EtherCAT Update
In the EtherCAT Update it should be noted that the process data is usually transmitted with a LRW telegram.
As a result of this, two cycles elapse in the task with which the EtherCAT master cycle is synchronized until
the increment of the TxCounter is confirmed by the EL6751. This dead time can be avoided by selecting
'Separate input update' in the task, since in this case the EtherCAT output data are to be transmitted with a
158 Version: 3.5 EL6751