BS EN IEC 61158-5-21:2019

What is BS EN IEC 61158-5-21 - Type 21 application layer about?  

The Fieldbus Application Layer (FAL) provides user programs with a means to access the Fieldbus communication environment. In this respect, the FAL can be considered a window between corresponding application programs. This part of IEC 61158 provides the common elements for basic time-critical and non-time-critical messaging communications between application programs in an automation environment as well as material specific to the Type 21 protocol. 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. 

BS EN IEC 61158-5-21 defines the externally visible service provided by the FAL in terms of: 

• An abstract model for defining application resources (objects) capable of being manipulated by users via the FAL service 
• The primitive actions and events of the service 
• The parameters associated with each primitive action and event and the form that they take 
• The interrelationship between these actions and events, and their valid sequences 

BS EN IEC 61158-5-21 defines the services provided to: 

• The FAL-user at the boundary between the user and the application layer of the Fieldbus Reference Model 
• Systems management at the boundary between the application layer and systems management of the Fieldbus Reference Model 

Note: BS EN IEC 61158-5-21  is not a formal programming interface, and any such interface must address implementation issues not covered by BS EN IEC 61158-5-21, including: 

• Sizes and octet ordering of various multi-octet service parameters 
• Correlation of paired primitives for request and confirmation, or indication and response 

Who is BS EN IEC 61158-5-21 - Type 21 application layer for? 

BS EN IEC 61158-5-21 on Type 21 application layer elements is useful for: 

• The IT industry / Software programmers   
• Internet engineering task forces    
• Internet service providers 
• Telecommunication companies 
• The automation industry  
• Networking sectors  

Why should you use BS EN IEC 61158-5-21 - Type 21 application layer?  

BS EN IEC 61158-5-21 is produced to facilitate the interconnection of automation system components. BS EN IEC 61158-5-21 is related to other standards in the set as defined by the “three-layer” fieldbus reference model described in IEC 61158-1. The application service is provided by the application protocol making use of the services available from the data-link or other immediately lower layer. BS EN IEC 61158-5-21 defines the application service characteristics that fieldbus applications and/or system management may exploit. Throughout the set of fieldbus standards, the term “service” refers to the abstract capability provided by one layer of the OSI Basic Reference Model to the layer immediately above. Thus, the application layer service defined in BS EN IEC 61158-5-21 is a conceptual architectural service, independent of administrative and implementation divisions. 

BS EN IEC 61158-5-21 specifies the characteristics of conceptual application layer services suitable for time-critical communications, and thus supplement the OSI Basic Reference Model in guiding the development of application layer protocols for time-critical communications. A secondary objective is to provide migration paths from previously existing industrial communications protocols. This latter objective gives rise to the diversity of services standardized as the various types of IEC 61158, and the corresponding protocols standardized in subparts of IEC 61158-6. BS EN IEC 61158-5-21 may be used as the basis for formal application programming interfaces. Using BS EN IEC 61158-5-21 guidelines, you are in a position to facilitate communication between applications and accelerate productivity. 

What’s changed since the last update?  

BS EN IEC 61158-5-21:2019 supersedes BS EN 61158-5-21:2012, which will be withdrawn on 15 May 2022. BS EN IEC 61158-5-21:2019 cancels and replaces the first edition published in 2010. The technical changes as compared to the first edition published in 2010 are: 

• Added Write and Read service
• Miscellaneous editorial corrections