Glossary · PLC and control programming
User-defined data type (UDT)
Also known as: UDT, Derived data type, DUT
German: Benutzerdefinierter Datentyp (UDT)
In PLC programming, a user-defined data type (UDT) is a data type created by the programmer from elementary and other data types, such as a structure, enumeration, array or subrange; IEC 61131-3 calls these derived data types.
- PLC programming
- Standards
In one sentence
A user-defined data type (UDT) is a programmer-defined type, such as a structure or enumeration, reused across a PLC project.
Example
The UDT typeValve defines the data of every valve (command, feedback open, feedback closed, fault), and all 40 valves of the plant use it.
How it applies
- Engineering: UDTs standardize data across a project: one change to the type updates every variable that uses it. Libraries of device types (motor, valve, analog value) are built on UDTs and function blocks together.
- Integration: HMI, SCADA and OPC UA systems can map UDTs to object types and faceplates. A consistent type library makes plant-wide engineering faster and more reliable.
- Change management: Changing a UDT affects every variable, block interface and external connection that uses it. Version the type library and treat type changes like interface changes, see Schema governance.
- Documentation: UDTs are natural units for reference documentation: one table per type with each element's name, type, unit and meaning. The same structure can feed interface specifications and data dictionaries.
UDT vs. derived data type
UDT is the term used in Siemens and several other environments; IEC 61131-3 speaks of derived data types, and some tools call them DUTs. The concept is the same.