Overview
IEC 61158-3-21:2010 is part of the IEC 61158 series of fieldbus specifications and defines the data‑link layer (DLL) service for Type 21 fieldbus elements. The standard describes, in an abstract and implementation‑independent way, the externally visible service offered by the Type 21 DLL: the primitive actions and events, their parameters and formats, and the valid sequences and interrelationships between them. It replaces IEC/PAS 62573 (2008) and aligns the Type 21 DLL service with the OSI Basic Reference Model and the IEC fieldbus three‑layer reference model.
Key Topics
- Time‑critical communications: Emphasis on prioritized, full‑duplex, collision‑free, time‑deterministic messaging suitable for control systems where timing guarantees matter.
- Service primitives and events: Definition of actions (primitives), events, and the associated parameters that make up the DLL/API contract between DLL and the upper application or management layers.
- Services covered:
- Data service (DL‑DATA) and sporadic data service (DL‑SPDATA)
- Network control message service (DL‑NCM)
- Data link management (DLM) services (reset, set/get values, SAP allocation/deallocation, diagnostics, events)
- MAC control and Ph‑control services
- Structural guidance: Sequence diagrams, tables of primitives/parameters and message types to support protocol design and conformance testing.
- Scope constraints: Conceptual service definition - intended as the basis for APIs and protocol implementations, but not a prescriptive programming interface.
Applications
- Design and specification of real‑time industrial networks and fieldbus implementations where deterministic delivery is required (e.g., PLC interconnects, motion control, process automation).
- Development of data‑link layer protocols, firmware, and device drivers that must interoperate with Type 21 fieldbus elements.
- System integrators and automation architects implementing safety‑critical or time‑sensitive control systems that need clear DLL service contracts.
- Basis for conformance testing, interoperability profiles, and migration paths from legacy industrial protocols.
Who should use this standard
- Industrial protocol engineers and embedded firmware developers
- Network architects and system integrators in manufacturing and process industries
- Test laboratories and certification bodies validating fieldbus DLL behavior
- Vendors creating fieldbus‑compliant devices (PLCs, I/O modules, gateways)
Related standards
- IEC 61158 series (Fieldbus specifications)
- IEC/TR 61158‑1 (Fieldbus reference model / three‑layer model)
- IEC/PAS 62573 (replaced by this edition)
Keywords: IEC 61158-3-21, data-link layer, Type 21, fieldbus, industrial communication networks, time-critical communications, DLL services, fieldbus specifications.