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.
Open preview (PDF)Industrial communication networks - Fieldbus specifications - Part 4-21: Data-link layer protocol specification - Type 21 elements
