IEC 61158-6-26:2019

Overview

IEC 61158-6-26:2019 is part of the IEC 61158 fieldbus family and specifies the application layer protocol for Type 26 fieldbus elements. The standard defines common elements for both basic time-critical and non-time-critical messaging between application programs in an automation environment, including the wire representation of service primitives and the externally visible behavior for their transfer. It is aligned with the OSI Basic Reference Model (ISO/IEC 7498-1) and the OSI application layer structure (ISO/IEC 9545).

Key Topics

• Fieldbus Application Layer (FAL) syntax - abstract and concrete syntax for Type 26 messages.
• PDU definitions - extensive PDUs including Token, Transparent-msg, Byte/Word block read/write, Network-parameter read/write, Cyclic-data, Trigger, Vendor-specific messages and more.
• Transfer syntax & encoding rules - rules for encoding octet-strings, SEQUENCE, bitstrings and fixed-length values for interoperable wire representation.
• Protocol behavior & state machines - FAL protocol state machine structure, timers, counters and variables (token holding, participation, watchdogs) that govern message sequencing and reliability.
• Network management and topology - logical ring maintenance, node addition/drop-out, successor node handling and node reset/configuration primitives.
• Time-critical messaging considerations - definitions and mechanisms to support operations that must complete within specified time windows to avoid application or safety failures.

Applications

IEC 61158-6-26 is targeted at professionals implementing or integrating industrial communication systems where a Type 2 fieldbus application layer is required:

• Device manufacturers building fieldbus-enabled I/O, controllers, and gateways who need to implement standardized PDUs and encoding for interoperability.
• System integrators and network architects designing industrial networks for factory automation, process control, or safety-critical installations that require deterministic and time-aware messaging.
• Control and safety engineers who must ensure that time-critical actions meet defined windows to reduce risk to equipment, plant operation and personnel.
• Test and certification laboratories performing conformance and interoperability testing against the application layer behavior and wire representation.

Practical benefits include consistent message semantics across vendors, deterministic cyclic-data exchange, robust node management, and a repeatable basis for conformance testing and troubleshooting in industrial automation networks.

Related Standards

• IEC 61158 (Fieldbus specifications) - other parts of the series for lower layers and other types.
• ISO/IEC 7498-1 (OSI Basic Reference Model) and ISO/IEC 9545 (OSI application layer structure) - normative references that shape the protocol structure and abstractions.

For official copies and amendments, obtain IEC 61158-6-26:2019 from the IEC Webstore.