IEC 61158-4-21:2019 describes procedures for the timely transfer of data and control information from one data link user entity to a peer user entity, and among the data link entities forming the distributed data link service provider and procedures for giving communication opportunities based on standard ISO/IEC 8802-3 MAC, with provisions for nodes to be added or removed during normal operation; structure of the fieldbus data link protocol data units (DLPDUs) used for the transfer of data and control information by the protocol of this standard, and their representation as physical interface data units. This second edition cancels and replaces the first edition published in 2010. This edition constitutes a technical revision. The main changes are: •added Frame control Value, DLM function and DLL constants; •changed NCM_RETRY_RNMS to NCM_CHECK_NET_INTEGRITY_REQ; •updated DLM state table; •miscellaneous editorial corrections.