General The data-link layer provides basic time-critical messaging communications between devices in an automation environment. This protocol provides communication opportunities to all participating data-link entities a) in a synchronously-starting cyclic manner@ according to a pre-established schedule@ and b) in a cyclic or acyclic asynchronous manner@ as requested each cycle by each of those data-link entities. Thus this protocol can be characterized as one which provides cyclic and acyclic access asynchronously but with a synchronous restart of each cycle. Specifications This standard specifies 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 datalink service provider; b) procedures for giving communications opportunities to all participating DL-entities@ sequentially and in a cyclic manner for deterministic and synchronized transfer at cyclic intervals up to one millisecond; c) procedures for giving communication opportunities available for time-critical data transmission together with non-time-critical data transmission without prejudice to the time-critical data transmission; d) procedures for giving cyclic and acyclic communication opportunities for time-critical data transmission with prioritized access; e) procedures for giving communication opportunities based on standard ISO/ IEC 8802-3 medium access control@ with provisions for nodes to be added or removed during normal operation; f) the structure of the fieldbus 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 DL-entities (DLEs) through the exchange of fieldbus DLPDUs; b) the interactions between a DL-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 Ph-service provider in the same system through the exchange of Ph-service primitives. Applicability These procedures are applicable to instances of communication between systems which support time-critical communications services within the data-link layer of the OSI or fieldbus reference models@ and which 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-critical 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.