Glossary · PLC and control programming
Variable declaration
Also known as: Declaration section, VAR block
German: Variablendeklaration
In IEC 61131-3 programming, a variable declaration defines a variable's name, data type, optional initial value and qualifiers, within a section such as VAR, VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, VAR_GLOBAL or VAR_TEMP that determines its scope and role.
- PLC programming
- Standards
In one sentence
A variable declaration sets a PLC variable's name, data type, initial value and scope, for example in VAR_INPUT or VAR_GLOBAL.
Example
The declaration VAR_INPUT rSetpoint : REAL := 20.0; END_VAR defines an input with default 20.0 for a temperature control block.
How it applies
- Engineering: Declarations define the interface and internal data of every program unit. The section keyword determines scope: inputs, outputs and in-out parameters form the external interface; VAR and VAR_TEMP are internal; VAR_GLOBAL and VAR_EXTERNAL share data. Qualifiers such as RETAIN and CONSTANT add behavior.
- Quality: Naming conventions (prefixes for type or scope), meaningful initial values and comments at the declaration make code readable. Static analysis tools can check them.
- Maintenance: Changing declarations, especially of interfaces and retentive data, can affect online changes and data initialization.
- Documentation: Comments at declarations are often extracted into generated documentation. Write them for readers outside the programming team: what the value means, its unit and its valid range.
Declaration vs. definition of a type
A variable declaration creates a variable of a type. A type definition (TYPE … END_TYPE) creates a new data type, such as a User-defined data type (UDT), which declarations can then use.