Overview
IEC 61158-5-21:2019 is an international standard developed by the International Electrotechnical Commission (IEC) that defines the application layer service for Type 21 fieldbus systems in industrial communication networks. Part of the comprehensive IEC 61158 series, this standard provides a structural and functional description of communication services that support real-time and non-real-time information interchange among devices in automation environments.
This document, aligned with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application Layer Structure (ISO/IEC 9545), details how application layer services facilitate seamless messaging, device coordination, and management of objects over industrial networks. The current edition introduces significant technical changes, notably the addition of Write and Read services, enhancing data access capabilities for fieldbus systems.
Key Topics
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Fieldbus Application Layer Structure
IEC 61158-5-21 describes the Fieldbus Application Layer (FAL) as an abstract interface offering messaging services between automation devices. FAL provides the building blocks for application programs to access and manipulate data on industrial networks.
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Service Definitions
The standard specifies a suite of application services, including primitive actions, events, and parameter definitions for both time-critical (real-time) and non-time-critical (background) communications. This ensures reliable control and monitoring in industrial settings.
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Object-Oriented Application Services
The FAL leverages object-oriented Application Service Elements (ASEs) for managing application process objects (APOs). These elements facilitate standardized operations like object creation, modification, deletion, data reading, and writing.
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OSI Model Conformance
Services defined are abstracted from implementation and reflect OSI layer principles, focusing on interoperability and uniformity across vendors and technologies.
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Enhancements in Edition 2.0
- Addition of Read and Write service operations
- Editorial corrections for improved clarity and usability
Applications
IEC 61158-5-21:2019 is widely applicable in the industrial automation domain. Typical uses include:
- Distributed Control Systems (DCS) and Programmable Logic Controllers (PLC): Supports real-time exchange of process signals and status updates between controllers and field devices.
- Process Automation: Facilitates communication for monitoring, control, and management tasks in industries such as chemical processing, oil and gas, and manufacturing.
- Factory Automation: Enables synchronization and data consistency between sensors, actuators, and central controllers over a robust fieldbus network.
- Systems Integration: Assures compatibility and interoperability when integrating different types of devices and subsystems on a unified industrial communication backbone.
By providing standardized communication protocols, the standard assists system integrators, equipment manufacturers, and plant operators in achieving reliable interaction among automation components.
Related Standards
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IEC 61158 Series
The IEC 61158 family is the foundation for fieldbus standards, covering components such as physical layer, data link layer, and application protocols.
- IEC 61158-3-21: Data-link layer service definition – Type 21 elements
- IEC 61158-4-21: Data-link layer protocol specification – Type 21 elements
- IEC 61158-6-21: Application layer protocol specification – Type 21 elements
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IEC 61784 Series
Provides profiles for fieldbus communications in industrial environments.
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ISO/IEC 7498
OSI Basic Reference Model, forming the conceptual basis for layered network standards.
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ISO/IEC 9545
Details application layer structure in open systems interconnection, complementing the functional definitions in IEC 61158.
Practical Value
Implementing IEC 61158-5-21:2019 ensures standardized, reliable, and interoperable communication solutions within industrial environments. It is instrumental in:
- Achieving vendor-agnostic integration of automation components
- Improving response times and data integrity for mission-critical applications
- Providing a foundation for scalable industrial automation solutions by defining clear application layer service interfaces
For organizations seeking to deploy robust industrial communication networks, compliance with IEC 61158-5-21:2019 assures compatibility, future-proofing, and alignment with international best practices.
Open preview (PDF)Industrial communication networks - Fieldbus specifications - Part 5-21: Application layer service definition - Type 21 elements
