CEI EN 61158-5-18

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 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 18 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 part of IEC 61158 defines in an abstract way the externally
visible service provided by the Type 18 fieldbus application layer
in terms of

a) an abstract model for defining application resources
(objects) capable of being manipulated by users via the use of the
FAL service,

b) the primitive actions and events of the service;

c) the parameters associated with each primitive action and
event, and the form which they take; and

d) the interrelationship between these actions and events, and
their valid sequences.

The purpose of this part of IEC 61158 is to define the services
provided to

a) the FAL user at the boundary between the user and the
Application Layer of the Fieldbus Reference Model, and

b) Systems Management at the boundary between the Application
Layer and Systems Management of the Fieldbus Reference Model.

This part of IEC 61158 specifies the structure and services of
the Type 18 IEC 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.