Overview
IEC 61158-5-21:2010 defines the Application Layer service model for the Type 21 fieldbus protocol within the IEC 61158 family. It specifies the structure and externally visible services of the Fieldbus Application Layer (FAL) in conformance with the OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application Layer Structure (ISO/IEC 9545). The standard replaces IEC/PAS 62573 (2008) and provides an abstract, object-oriented framework for time‑critical and non‑time‑critical messaging between automation application processes.
Key topics and technical requirements
- FAL architecture and conformance: Defines Application Entities (AEs), Application Service Elements (ASEs), and the Layer Management Entity (LME) that manage AE behavior and conformance requirements.
- Object model: Application Process Objects (APOs) and object classes are described as manipulable resources; the standard specifies an abstract model for object definition and access.
- Service primitives and parameters: Specifies primitive actions, events, parameters, sequencing and valid usage patterns for application-layer services (e.g., Identify, Status, Read, Write, TB-transfer, COS-transfer).
- Communication models: Covers publisher‑subscriber and client‑server models, including AREP/AR roles, APDUs, timeliness categories and interaction diagrams for pull and push communications.
- Data types and ASEs: Includes a Data Type ASE and formal data type specifications used by Type 21 ASEs; shows how ASEs convey services to application processes.
- Interaction with lower layers: Defines how FAL services map to the Data Link Layer and other lower-layer services in the fieldbus reference model.
- Time-critical behavior: Addresses services and constraints for applications requiring defined time windows to avoid functional failure or safety risk.
Applications and who uses this standard
IEC 61158-5-21 is intended for:
- Industrial automation system architects and integrators designing Type 21 fieldbus solutions
- Protocol implementers and device vendors building compliant application-layer stacks (ASE/AE/LME)
- Test laboratories and certification bodies performing conformance tests
- Control engineers specifying time‑critical messaging and object interfaces in distributed control and safety systems
Practical uses include defining application-layer messaging for control devices, implementing interoperable fieldbus components, and documenting application interfaces for system integration.
Related standards
- IEC 61158 series (fieldbus specifications)
- IEC/TR 61158-1 (fieldbus reference models)
- ISO/IEC 7498 (OSI Basic Reference Model)
- ISO/IEC 9545 (OSI Application Layer Structure)
Keywords: IEC 61158-5-21, Fieldbus Application Layer (FAL), Type 21, application layer service definition, industrial communication networks, ASE, APO, AE, OSI model.