1.1 Overview
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 standard 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 14 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 Type 14 fieldbus application layer in terms of
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a) an abstract model for defining application resources (objects) capable of being manipulated
by users via the use of the FAL service,
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b) the primitive actions and events of the service;
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c) the parameters associated with each primitive action and event, and the form which
they take; and
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d) the interrelationship between these actions and events, and their valid sequences.
The purpose of this standard is to define the services provided to
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a) the FAL user at the boundary between the user and the application layer of the fieldbus
reference model, and
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b) Systems Management at the boundary between the application layer and Systems Management
of the fieldbus reference model.
This standard specifies the structure and services of the Type 14 fieldbus application
layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498‑1) and the OSI application layer structure (ISO/IEC 9545).
FAL services and protocols are provided by FAL application-entities (AE) contained within the application processes. The FAL AE is composed of a set of object-oriented application service 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 FAL classes.
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 object behavior. In addition to these services,
some supporting services are also defined in this standard to provide access to the
FAL to control certain aspects of its operation.