IEC 61158-4-21:20 23描述: 用于从一个数据链路用户实体到对等用户实体以及在形成分布式数据链路服务提供商的数据链路实体之间及时传送数据和控制信息的过程； 基于ISO/IEC/IEEE 8802提供通信机会的程序？3个MAC，具有在正常运行期间添加或删除节点的规定； 用于通过本文档的协议传输数据和控制信息的现场总线数据链路协议数据单元（DLPDU）的结构，以及它们作为物理接口数据单元的表示。

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.