Provides basic time-critical messaging communications between devices in an automation enviornment. Defines in an abstract way the externally visible service provided by the Fieldbus Data Link Layer in terms of

a) the primitive actions and events of the service;

b) the parameters associated with each primitive action and event, and the form which they take; and

c) the interrelationship between these actions and events, and their valid sequence. The services defined are a superset of those provided by OSI Data Link Protocols as specified in ISO/IEC 8886.