1.1 General
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 26 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 service provided by the Type26 fieldbus application layer in terms of:
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• an abstract model for defining applicationresources (objects) capable of being manipulated by users via the use of the FALservice;
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• the primitive actions and events of the service;
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• the parameters associated with each primitive action and event, and the form which they take; and
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• the interrelationship between these actions and events, and their valid sequences.
The purpose of this document is to define the services provided to:
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• the FAL user at the boundary between the user and the Application Layer of the Fieldbus Reference Model, and
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• Systems Management at the boundary between the Application Layer and Systems Management of the Fieldbus Reference Model.
This document specifies the structure and services of the Type 26 fieldbus application layer, in conformance with the OSI Basic Reference Model (see ISO/IEC 7498‑1) and the OSIApplication Layer Structure (see ISO/IEC 9545).
FAL
services and protocols are provided by FALapplication-entities (AE) contained within the application processes. The FALAE is composed of a set of object-oriented ApplicationService Elements (ASEs) and a Layer Management Entity (LME) that manages the AE. The ASEs provide communication services that operate on a set of related application process object (APO) classes. One of the FAL ASEs is a management ASE that provides a common set of services for the management of the instances of FALclasses.
Although these services specify, from the perspective of applications, how request and responses are issued and delivered, they do not include a specification
of what the requesting and responding applications are to do with them. That is, the behavioral aspects of the applications are not specified; only a definition of what requests and responses they can send/receive
is specified. This permits greater flexibility to the FAL users in standardizing such objectbehavior. In addition to these services, some supporting services are also defined in this document to provide access to the FAL to control certain aspects of its operation.