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 International Standard defines in an abstract way the externally visible service provided 
by the different Types of 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 document 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 document specifies the structure and services of the IEC Fieldbus Application Layer, in 
conformance with the OSI Basic Reference Model (see ISO/IEC 7498-1) and the OSI 
Application Layer Structure (see ISO/IEC 9545).dition 1.0 2019-04 INTERNATIONAL STANDARD Industrial communication networks – Fieldbus specifications – Part 5-26: Application layer service definition – Type 26 elements IEC 61158-5-26:2019-04(en) THIS PUBLICATION IS COPYRIGHT PROTECTED Copyright ？ 2019 IEC, Geneva, Switzerland All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form