Overview
IEC 61158-6-2:2023 is an international standard that specifies the application layer protocol for Type 2 fieldbus communication in industrial automation. It defines common elements for both time‑critical and non‑time‑critical messaging between application programs in automation environments. The standard explains messaging semantics, protocol data units, encoding, state machines and mappings to data‑link layers, with emphasis on ensuring actions that must complete within defined time windows are handled deterministically and safely.
Key topics and technical requirements
- Application layer protocol specification: Abstract syntax, data dictionaries and encapsulation formats for Fieldbus Application Layer (FAL) PDUs.
- Transfer syntax and encoding rules: Compact encoding, transport format constraints and data type reporting for interoperable message handling.
- State machines and protocol machines: Detailed behaviour for AP‑Context, FAL Service Protocol Machine (FSPM), Application Relationship Protocol Machines (ARPMs such as UCMM and transport ARPMs), and Data‑Link Layer Mapping Protocol Machines (DMPM 1–3).
- Transport classes and connection management: Support for multiple transport classes (class 0–3), connection semantics, Forward_Open/Close handling and connection managers.
- Encapsulation and QoS for Ethernet: Guidance on encapsulation over TCP and UDP, IGMP usage for multicast, and Quality of Service mapping using DSCP and IEEE 802.1Q (VLAN/prioritization).
- Safety/time‑critical considerations: Definition of “time‑critical” as operations requiring completion within a time window; implications for system design to avoid equipment or human risk.
- Management and diagnostics: Error codes, service primitives, parameters and management UCMM sequences for configuration and troubleshooting.
Practical applications and who uses this standard
IEC 61158-6-2 is used by:
- Device and equipment manufacturers designing fieldbus‑compliant devices (controllers, I/O modules, drives).
- System integrators and automation engineers implementing interoperable, deterministic messaging in factory automation, process control, and safety‑related systems.
- Network architects specifying QoS and Ethernet encapsulation for industrial networks.
- Certifying bodies and test labs verifying conformance to application layer behaviors and state machines.
Typical applications include real‑time control loops, explicit messaging between controllers and remote I/O, deterministic alarm and event handling, and OT/IT converged systems requiring standardized transport and QoS.
Related standards
- Other parts of IEC 61158 (Fieldbus specifications) and subsystem parts referenced within this document.
- Relevant ISO/IEC documents cited in the standard (e.g., ISO/IEC 7498‑1, 9545, 8824‑1, 8825‑1) for terminology and ASN.1 encoding conventions.
Keywords: IEC 61158-6-2, fieldbus, industrial communication networks, application layer protocol, Type 2, time‑critical messaging, QoS, Ethernet encapsulation, transport classes, UCMM, DMPM.