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 24 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 document defines in an abstract way the externally visible behavior provided by the Type 24 fieldbus application layer in terms of
• the abstract syntax defining the application layer protocol data units conveyed between communicating application entities,
• the transfer syntax defining the application layer protocol data units conveyed between communicating application entities,
• the application context state machines defining the application service behavior visibly between communicating application entities, and
• the application relationship state machines defining the communication behavior visibly between communicating application entities.
The purpose of this document is to define the protocol provided to
• define the representation-on-wire of the service primitives defined in IEC 61158-5-24, and
• define the externally visible behavior associated with their transfer.
This document specifies the protocol of the Type 24 fieldbus application layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI Application Layer Structure (ISO/IEC 9545).
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