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On the Data Link Layer process data is handled as an octet stream. On the the Application Layer this octet stream is parsed to/from objects of the Object Dictionary. There are PDO Mapping Objects that define what objects create a PDO. The PDO Assignment OBjects Objects define which PDO belongs to what Sync Manager. Thus the combination of PDO Mapping Objects and PDO Assignment Objects define the layout of the process data and the size of the Input and Output Sync Managers. For more details please read the corresponding section in the EtherCAT User Manual.