Industrial communication networks - Fieldbus specifications - Part 4-21: Data-link layer protocol specification - Type 21 elements
IEC 61158-4-21:2023 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;
procedures for giving communication opportunities based on ISO/IEC/IEEE 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 document, and their representation as physical interface data units.