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 2 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 specifies interactions between remote applications and defines 
the externally visible behavior provided by the Type 2 fieldbus application layer in terms of 
a) the formal abstract syntax defining the application layer protocol data units conveyed 
between communicating application entities; 
b) the transfer syntax defining encoding rules that are applied to the application layer 
protocol data units; 
c) the application context state machine defining the application service behavior visible 
between communicating application entities; 
d) the application relationship state machines defining the communication behavior visible 
between communicating application entities. 
The purpose of this document is to define the protocol provided to 
a) define the wire-representation of the service primitives defined in IEC 61158-5-2, and 
b) define the externally visible behavior associated with their transfer. 
This document specifies the protocol of the Type 2 fieldbus application layer, in conformance 
with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI application layer structure 
(ISO/IEC 9545). 
1.2 Specifications 
The principal objective of this document is to specify the syntax and behavior of the 
application layer protocol that conveys the application layer services defined in IEC 61158-5-2. 
A secondary objective is to provide migration paths from previously-existing industrial 
communications protocols.
 1.3 Conformance This document does not specify individual implementations or products, nor does it constrain the implementations of application layer entities within industrial automation systems. Conformance is achieved through implementation of this application layer protocol specification.dition 4.0 2019-06 INTERNATIONAL STANDARD Industrial communication networks – Fieldbus specifications – Part 6-2: Application layer protocol specification – Type 2 elements IEC 61158-6-2:2019-06(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