IEC 61158-4-21:2019 PDF

Overview

IEC 61158-4-21:2019 is the IEC data‑link layer specification for Type 21 fieldbus elements within the IEC 61158 family of industrial communication standards. It defines the procedures and message structures for timely transfer of data and control information between peer data‑link user entities and among distributed data‑link entities. The standard specifies how data‑link protocol data units (DLPDUs) are assembled and represented as physical interface data units, and it adopts provisions for medium access based on ISO/IEC 8802-3 (MAC) with support for adding or removing nodes during normal operation. Edition 2.0 (2019) is a technical revision that adds Frame Control Value, DLM functions and DLL constants, updates the DLM state table, and refines several network control message names.

Key Topics

• Data‑link layer procedures: primitives and state machines (DL‑protocol machine, DLM) that manage data exchange and link behavior.
• DLPDU and MAPDU structure: encoding, common fields (Version, Length, DST/SRC addresses, Frame Control), and representation as physical interface data units.
• MAC integration: medium access control based on ISO/IEC 8802-3, including common MAC frame formats and bit/byte ordering rules.
• Dynamic network management: procedures and network control messages (NCM_*) for topology updates, network integrity checks, and runtime node add/remove.
• DLL management (DLM): management interface, state tables, constants and error codes used for configuration and operational monitoring.
• Data types and local parameters: standardized types (unsigned/signed integers, octet/visible strings, time of day) and device/local variables supporting SAP and topology management.
• Diagnostics and constants: error codes, DLL constants and updated DLM state descriptions for robust implementation and testing.

Applications

IEC 61158-4-21 is intended for:

• Fieldbus and industrial Ethernet device manufacturers implementing Type 21 data‑link protocols in firmware or ASICs.
• Protocol stack developers building compliant DLL layers for controllers, I/O modules and gateways.
• System integrators and automation engineers designing networks requiring deterministic data‑link behavior and hot‑plug capabilities.
• Conformance and test laboratories validating DLPDU formats, MAC framing and DLM state behavior. Practical use cases include real‑time industrial control, distributed automation systems, and networks that require standardized link management and runtime device handling.

Related standards

• ISO/IEC 8802-3 (Ethernet MAC) - referenced for MAC services and frame formats.
• Other parts of IEC 61158 (fieldbus specifications) - for higher/lower layer interfaces and complementary protocol element definitions.

For implementation, obtain the full text from the IEC webstore to ensure correct adoption of DLPDU encodings, state tables and NCM procedures.