IEC 61158-3-21:2019

Overview

IEC 61158-3-21:2019 is part of the IEC 61158 fieldbus series and specifies, in an abstract and implementation‑independent way, the externally visible data-link layer service for Type 21 fieldbus elements. The standard defines the primitive actions and events exposed by the data‑link layer, the parameters associated with each primitive/event, valid parameter forms and value ranges, and the legal sequences and interactions between primitives. Edition 2 (2019) cancels and replaces the 2010 edition and introduces the Network Control Message (NCM) Type plus miscellaneous editorial corrections.

Key technical topics and requirements

• Service model and primitives: formal description of data‑link primitives (e.g., DL-DATA, DL-SPDATA, DL‑NCM_SND) and their parameters and sequencing.
• Data services: full specification of normal data exchange and sporadic data services, including transmit/receive sequencing and error/flow behavior.
• Network Control Message (NCM): new message type added in this edition to support control-plane messaging.
• Data-link management services (DLMS): management primitives such as DLM‑RESET, DLM‑SET_VALUE, DLM‑GET_VALUE, SAP‑ALLOCATION / SAP‑DEALLOCATION, Get‑diagnostic‑information, Event and Get‑path.
• MAC control services: MAC‑RESET and MAC‑FORWARD‑CONTROL primitives and interactions.
• Physical‑layer control (Ph‑control): Ph‑RESET, Ph‑GET_LINK_STATUS and Ph‑LINK_STATUS_CHANGE primitives for link monitoring and reset.
• Flow control & addressing: description of full‑duplex flow control, DLSAP/DLSAP‑address relations, group addresses and destination DL‑addressing rules.
• Conformance and terminology: normative definitions, symbols, and conventions to ensure interoperable implementations.

Practical applications and who uses it

• Protocol and firmware developers implementing Type 21 data‑link functionality for field devices, gateways, controllers and communication stacks.
• Device manufacturers building PLCs, I/O modules, sensors and actuators that must interoperate on Type 21 fieldbus segments.
• System integrators and OEMs designing industrial networks where explicit data‑link service behavior, management and diagnostics are required.
• Test labs and certification bodies that verify conformance to DL service semantics, primitive sequencing and management behaviors.
• Network architects designing robust control systems that rely on deterministic data‑link behavior, NCM handling and link management.

Related standards

• Other parts of the IEC 61158 series (additional Type-specific protocol parts) and higher-level fieldbus/profile standards that use the Type 21 data‑link definitions.

This standard is essential when you need a precise, interoperable specification of the Type 21 data‑link services - including primitives, parameters, management functions and the newly added Network Control Message Type - for industrial communication networks and fieldbus implementations. Keywords: IEC 61158-3-21, Type 21, data-link layer, fieldbus, industrial communication, DLMS, NCM, MAC control, Ph-control.