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.