In network communications, as in many fields of engineering, it is a fact that “one size does
not fit all.” Engineering design is about making the right set of trade-offs, and these trade-offs
must balance conflicting requirements such as simplicity, generality, ease of use, richness of
features, performance, memory size and usage, scalability, determinism, and robustness.
These trade-offs must be made in light of the types of information flow (e.g. periodic, one-tomany,
request-reply, events), and the constraints imposed by the application and execution
platforms.
The Type 15 fieldbus provides two major communication mechanisms that complement each
others to satisfy communication requirements in the field of automation: the Client/Server and
the Publish/Subscribe paradigms. They can be used concurrently on the same device.
Type 15 Client/Server operates in a Client/Server relationship. Its application layer service
definitions and protocol specifications are independent of the underlying layers, and have
been implemented on a variety of stacks and communication media, including EIA/TIA-232,
EIA/TIA-422, EIA/TIA-425, HDLC (ISO 13239), fiber, TCP/IP, Wireless LANs and Radios.
Type 15 Publish/Subscribe operates in a Publish/Subscribe relationship. Its application layer
service definitions and protocol specifications are independent of the underlying layers and
can be configured to provide reliable behaviour and support determinism. The most common
stack is UDP/IP.
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 15 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 define in an abstract way the externally visible service provided by the
Type 15 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
1) the FAL user at the boundary between the user and the Application Layer of the Fieldbus
Reference Model, and 2) 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 15 IEC fieldbus
Application Layer, in conformance with the OSI Basic Reference Model (ISO/IEC 7498) 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.
BS EN 61158-5-15:2008 history
2012BS EN 61158-5-15:2012 Industrial communication networks. Fieldbus specifications. Application layer service definition. Type 15 elements
2009BS EN 61158-5-11:2008 Industrial communication networks — Fieldbus specifications — Part 5-11: Application layer service definition — Type 11 elements
2004BS EN 61158-5:2004 Digital data communications for measurement and control - Fieldbus for use in industrial control systems - Application layer service definition