IEC 61158-6-21:2019

Overview - IEC 61158-6-21:2019 (Type 21 application layer protocol)

IEC 61158-6-21:2019 is an International Electrotechnical Commission standard that specifies the application layer protocol for Type 21 fieldbus within industrial communication networks. It provides the wire representation and externally visible behavior of service primitives used by application programs in automation environments, covering both time‑critical and non‑time‑critical messaging. The standard defines how remote applications interact, the structure of APDUs (Application Protocol Data Units), encoding rules, and protocol state machines in conformance with the OSI Basic Reference Model (ISO/IEC 7498‑1) and the OSI application layer structure (ISO/IEC 9545). Edition 2.0 (2019) adds the WriteAndRead service and includes editorial corrections.

Key topics and technical requirements

• Application layer semantics and syntax
  • Definitions of service primitives and their wire representation.
  • Abstract and transfer syntax for APDUs, including FalArHeader, InvokeID and Type fields.
• Support for time‑critical messaging
  • Concepts of a time‑window for actions and implications if deadlines are missed (risk to equipment, plant and people).
• Data encoding and transfer syntax
  • Encoding rules for basic data types (Boolean, integers, floating point, strings, time values) and bit sequences.
• Protocol state machines
  • State machine specifications for FAL, AP context, AP ASE, Service Data Object (SDOM), Process Data Object (PDOM), AR protocol machines (PTC‑AR, MSU‑AR, MTU‑AR) and DLL mapping.
• Primitives and primitives mapping
  • Primitive definitions exchanged between protocol layers (FAL-user, APAM, DMPM, DLL) and parameter descriptions.
• Conformance and interoperability
  • Requirements to ensure consistent externally visible behavior for interoperable implementations.
• Notable enhancement
  • Introduction of the WriteAndRead service in the second edition.

Practical applications and who uses this standard

• Fieldbus and control system vendors implementing Type 21 application layer stacks.
• System integrators and automation engineers designing real‑time control, distributed I/O, PLC-to-PLC and sensor/actuator communication.
• Safety‑critical process and manufacturing plants that require clearly defined time‑critical messaging behavior and deterministic interactions.
• Test laboratories and certification bodies validating compliance and interoperability of fieldbus devices.
• Developers of monitoring and SCADA gateways that map fieldbus application layer services to higher-level systems.

Related standards

• IEC 61158 family (fieldbus specifications)
• ISO/IEC 7498‑1 - OSI Basic Reference Model
• ISO/IEC 9545 - OSI application layer structure

Keywords: IEC 61158-6-21, Type 21, fieldbus, application layer protocol, industrial communication networks, time‑critical messaging, APDU, WriteAndRead, OSI model, automation.