Overview
IEC 62541-20:2025 - "OPC unified architecture - Part 20: File transfer" - defines an Information Model for file transfer in OPC Unified Architecture (OPC UA). The standard specifies object types and services to model files and file systems in the OPC UA AddressSpace, addressing limitations of modelling files as large ByteStrings (message size, resource and concurrency concerns). File transfer constructs previously in IEC 62541‑5:2020 (Annex C) are now defined as a separate part to provide a clear, reusable infrastructure for file access, directory operations and temporary transfer workflows.
Key Topics
- FileType object: represents a file in the AddressSpace with properties (Size, Writable, UserWritable, OpenCount, MimeType, MaxByteStringLength) and mandatory methods such as:
- Open, Close
- Read, Write
- GetPosition, SetPosition
- FileDirectoryType and FileSystem: models a directory hierarchy and provides methods to manage files and directories (CreateDirectory, CreateFile, Delete, MoveOrCopy). The root of a modeled file system uses the BrowseName FileSystem.
- TemporaryFileTransferType: supports transfer scenarios that require a handshake or commit step (typical for configuration files or firmware updates). Includes operations like GenerateFileForRead, GenerateFileForWrite, CloseAndCommit and a FileTransferStateMachineType to manage transfer states and reset behavior.
- State machine & sequences: File transfer workflows (read/write sequences) and states are formally defined so clients and servers can coordinate transfers reliably and handle concurrency and lifecycle differences between disk files and AddressSpace representations.
- Security & resource considerations: the model is designed to avoid large single-message transfers and concurrency issues by providing segmented read/write methods and transfer control.
Applications / Who Should Use It
- Industrial automation vendors and system integrators implementing OPC UA servers and clients that need robust file transfer (firmware updates, configuration distribution, log retrieval).
- Device manufacturers exposing file systems or files on field devices or controllers.
- Software developers and architects designing enterprise-to-edge integration where reliable, stateful file exchanges are required.
- Maintenance and operations teams who need standardized, auditable file transfer mechanisms within OPC UA ecosystems.
Practical benefits include safer firmware/configuration deployment, reduced denial-of-service risks from large messages, and clear concurrency handling.
Related standards
Keywords: IEC 62541-20:2025, OPC Unified Architecture, OPC UA file transfer, FileType, FileDirectoryType, TemporaryFileTransferType, file system modeling, industrial automation, firmware update.
