EVS-EN 61158-5-20:2012

The fieldbus Application Layer (FAL) provides user programs with a means to access the fieldbus communication environment. In this respect, the FAL can be viewed as a “window between corresponding application programs.” This part of IEC 61158 provides common elements for basic time-critical and non-time-critical messaging communications between application programs in an automation environment and material specific to Type 20 fieldbus. 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 different Types of the fieldbus Application Layer in terms of a) an abstract model for defining application resources (objects) capable of being manipulated by users via the use of the FAL service, b) the primitive actions and events of the service; c) the parameters associated with each primitive action and event, and the form which they take; and d) the interrelationship between these actions and events, and their valid sequences. The purpose of this standard is to define the services provided to a) the FAL user at the boundary between the user and the Application Layer of the Fieldbus Reference Model, and b) Systems Management at the boundary between the Application Layer and Systems Management of the Fieldbus Reference Model.