UNE-EN IEC 61158-6-26:2023

656 1.1 General

657 The Fieldbus Application Layer (FAL) provides user programs with a means to access the

658 fieldbus communication environment. In this respect, the FAL can be viewed as a window

659 between corresponding application programs.

660 This part of IEC 61158 provides common elements for basic time-critical and non-time-critical

661 messaging communications between application programs in an automation environment and

662 material specific to Type 2 fieldbus. The term time-critical is used to represent the presence

663 of a time-window, within which one or more specified actions are required to be completed

664 with some defined level of certainty. Failure to complete specified actions within the time

665 window risks failure of the applications requesting the actions, with attendant risk to

666 equipment, plant and possibly human life.

667 This International Standard specifies interactions between remote applications and defines

668 the externally visible behavior provided by the Type 2 fieldbus application layer in terms of

669 a) the formal abstract syntax defining the application layer protocol data units conveyed

670 between communicating application entities;

671 b) the transfer syntax defining encoding rules that are applied to the application layer

672 protocol data units;

673 c) the application context state machine defining the application service behavior visible

674 between communicating application entities;

675 d) the application relationship state machines defining the communication behavior visible

676 between communicating application entities.

677 The purpose of this document is to define the protocol provided to

678 a) define the wire-representation of the service primitives defined in IEC 61158-5-2, and

679 b) define the externally visible behavior associated with their transfer.

680 This document specifies the protocol of the Type 2 fieldbus application layer, in conformance

681 with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI application layer structure

682 (ISO/IEC 9545).

683 1.2 Specifications

684 The principal objective of this document is to specify the syntax and behavior of the

685 application layer protocol that conveys the application layer services defined in IEC 61158-5-2.

686 A secondary objective is to provide migration paths from previously-existing industrial

687 communications protocols.

20018 IEC CDV 61158-6-2 © IEC:2021

688 1.3 Conformance

689 This document does not specify individual implementations or products, nor does it constrain

690 the implementations of application layer entities within industrial automation systems.

691 Conformance is achieved through implementation of this application layer protocol

692 specification.