The Fieldbus Application Layer (FAL) is an Application Layer communication standard designed to support the conveyance of time-critical application requests and responses among 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 specifies interactions between remote applications in terms of? the encoding rules that are applied to all the Application Layer Protocol Data Units (APDUs); ? the formal Abstract Syntax definitions of such APDUs; ? the protocol state machine descriptions that handle the APDUs and the primitives in the correct sequences; ? the mappings of the APDUs to and from the Data Link Layer services defined in IEC 61158-3. The FAL encoding rules are designed assuming that both the encoder (sender) and the decoder (receiver) have the common knowledge of the abstract syntax. Wherever possible, data types identifiers are not encoded and transferred over the network.NOTE This is why the Abstract Syntax Notation One / Basic Encoding Rule is not practical for the FAL.The purpose of this part of this standard is to define the protocol provided a) to the Fieldbus Data Link Layer at the boundary between the Application and Data Link Layers of the Fieldbus Reference Model, and b) to the System Management at the boundary between the System Management and Application Layers of the Fieldbus Reference Model.

IEC 61158-6 Ed. 3.0