This part of IEC PAS 62953 provides common elements for basic time-critical messaging communications between devices in an automation environment. The term “time-critical?? is used to represent the presence of a time-window, within which one or more specified actions are required to be completed with some defined level of certainty. Failure to complete specified actions within the time window risks failure of the applications requesting the actions, with attendant risk to equipment, plant and possibly human life. This standard defines in an abstract way the externally visible service provided by the Type 25 fieldbus data-link layer in terms of the primitive actions and events of the service; the parameters associated with each primitive action and event, and the form which they take; and the interrelationship between these actions and events, and their valid sequences. The purpose of this standard is to define the services provided to the Type 25 fieldbus application layer at the boundary between the application and datalink layers of the fieldbus reference model; systems management at the boundary between the data-link layer and systems management of the fieldbus reference model.