1.1 General The data-link layer provides basic time-critical messaging communications between devices in an automation environment. This protocol provides the data-link service by making use of the services available from the physical layer. The relationship between the International Standards for fieldbus data-link service, fieldbus data-link protocol, fieldbus physical service and systems management is described in IEC 61158-1. This protocol provides communication opportunities to all participating data-link entities a) in a cyclic asynchronous manner, sequentially to each of those data-link entities, and b) in a synchronous manner, either cyclically or acyclically, according to a pre-established schedule. The specified protocol also provides means of changing the set of participating data-link entities and of modifying the set of scheduled communications opportunities. When the set of scheduled communications opportunities is null, the distribution of communication opportunities to the participating data-link entities is completely asynchronous. Thus this protocol can be characterized as one which provides access asynchronously but with a synchronous overlay. 1.2 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) 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. NOTE In IEC 61158-4-1, gray boxes have been used in the tables to indicate that the specified field is not a conceptual part of the specific DLPDU 1.3 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. 1.4 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. 1.5 Conformance This standard also specifies conformance requirements for systems implementing these procedures. This standard does not contain tests to demonstrate compliance with such requirements.