The Sync message is as follows :-
|Can Header /COB ID|
And thats it, no data, is a very simple high priority message that does the following things for any PDOs configured as Synchronous :-
Any analog and digital input nodes must send their data after the sync node is received. They also sample the next data point with the reception of the sync node.
Any output nodes update their output state with the sync message, providing they have received a RX PDO before the SYNC message has been received.
These two facts enable the entire network to be synchronized and ensure that all inputs and outputs , are sampled/update their state at the same time.