IEC 61158-4-2:2023 is one of a series produced to facilitate the interconnection of automation system components. It is related to other standards in the set as defined by the "three-layer" fieldbus reference model described in IEC 61158?1.The data-link protocol provides the data-link service by making use of the services available from the physical layer. The primary aim of this document is to provide a set of rules for communication expressed in terms of the procedures to be carried out by peer data-link entities (DLEs) at the time of communication. These rules for communication are intended to provide a sound basis for development in order to serve a variety of purposes:- as a guide for implementers and designers;- for use in the testing and procurement of equipment;- as part of an agreement for the admittance of systems into the open systems environment;- as a refinement to the understanding of time-critical communications within OSI.