IEC 61158-4-21:2023

Overview

IEC 61158-4-21:2023 is the International Electrotechnical Commission specification for the data-link layer of Type 21 fieldbus elements. It defines procedures and message structures that ensure timely, reliable transfer of data and control information between data-link user entities and among distributed data-link service providers. The standard is part of the IEC 61158 family for industrial communication networks – fieldbus specifications (Edition 3.0, 2023).

Key topics

• Data-link service and procedures
  • Timely transfer of data and control primitives between peer user entities.
  • Distributed data-link service provider behavior and coordination.
• Medium access control (MAC) interaction
  • Procedures for allocating communication opportunities based on ISO/IEC/IEEE 8802‑3 (Ethernet) MAC.
  • Support for dynamic networks where nodes can be added or removed during normal operation.
• Frame and PDU structure
  • Definition of MAPDU/MAC frame format and common MAC frame elements.
  • Structure and encoding of Data Link Protocol Data Units (DLPDUs) and their physical representation.
  • Specific DLPDU types and control messages (examples in the standard include DT, SPDT, and network control messages such as NCM_LA, NCM_AT, NCM_LS, NCM_RS, NCM_AR).
• Protocol machines and management
  • DL-protocol machine (DLPM) state machine and primitives.
  • Data Link Layer Management (DLM) interface, state tables, variables, timers, queues and error codes.
• Data types and local parameters
  • Standardized data types (Boolean, unsigned/signed integers, octet/visible strings, time), SAP management, device and network information structures, path and topology management.
• Diagnostics and conformance
  • Constants, error codes and conformance rules for interoperable implementations.

Applications and who uses it

IEC 61158-4-21 is used by:

• Industrial automation equipment manufacturers implementing fieldbus/data-link firmware for PLCs, I/O modules, drives and controllers.
• Embedded systems developers building communication stacks that interoperate with Ethernet-based fieldbus MAC layers.
• System integrators and automation engineers designing deterministic, resilient industrial networks where nodes must be hot‑added or removed.
• Test labs and certification bodies verifying conformance to data-link behaviors, frame formats, and error handling. Practical applications include real‑time control networks, factory automation, process control systems and distributed I/O networks requiring deterministic message scheduling and robust device management.

Related standards

• IEC 61158 series (Fieldbus specifications)
• ISO/IEC/IEEE 8802‑3 (Ethernet / MAC layer) - referenced for MAC behavior and access control
• Other IEC data-link and application layer parts relevant to specific Type profiles

Keywords: IEC 61158-4-21, data-link layer, fieldbus, DLPDU, MAC, ISO/IEC/IEEE 8802-3, industrial communication networks, Type 21.