61158-4-21-2010 Réseaux de communication industriels – Spécifications des bus de terrain – Partie 4-21: Spécification du protocole de la couche de liaison de données – Eléments de Type 21 (Edition 1.0)
Industrial communication networks – Fieldbus specifications – Part 4-21: Data-link layer protocol specification – Type 21 elements (Edition 1.0)
Réseaux de communication industriels – Spécifications des bus de terrain – Partie 4-21: Spécification du protocole de la couche de liaison de données – Eléments de Type 21 (Edition 1.0) 是非强制性国家标准，您可以免费下载前三页
General The DLL provides basic time-critical data communications between devices in an automated environment. Type 21 provides priority-based cyclic and acyclic data communication using an internal collision-free@ full-duplex dual-port Ethernet switch technology. For wide application in various automation applications@ Type 21 does not restrict the cyclic/acyclic scheduling policy in the DLL. Specifications This standard describes: a) 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; b) 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; c) 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. Procedures The procedures are defined in terms of: a) the interactions between peer data link entities (DLEs) through the exchange of fieldbus DLPDUs; b) the interactions between a data link service (DLS) provider and a DLS-user in the same system through the exchange of DLS primitives; c) the interactions between a DLS-provider and a physical layer service provider in the same system through the exchange of Ph-service primitives. Applicability These procedures are applicable to instances of communication between systems that support time-critical communications services in the data link layer of the OSI or fieldbus reference models@ and that require the ability to interconnect in an open systems interconnection environment. Profiles provide a simple multi-attribute means of summarizing an implementation's capabilities@ and thus its applicability to various time-deterministic communications needs. Conformance This standard also specifies conformance requirements for systems implementing these procedures. This standard does not contain tests to demonstrate compliance with such requirements.