Glossary · PLC and control programming
Output variable
Also known as: VAR_OUTPUT, Output parameter
German: Ausgangsvariable
In IEC 61131-3 programming, an output variable is a parameter declared with VAR_OUTPUT through which a function block, function or program returns values to its caller.
- PLC programming
- Standards
In one sentence
An output variable (VAR_OUTPUT) returns values from a PLC function block or program to the caller, such as Running or Fault.
Example
FB_Pump provides the output variables Running, Fault and RunHours, which the HMI program reads for the pump faceplate.
How it applies
- Engineering: Output variables form the result side of a block interface. In function blocks, outputs stay in the instance and can be read after the call, also by other program parts. Coding guidelines usually prefer that callers read outputs rather than internal variables.
- Integration: Status outputs such as Running, Ready or Fault are often mapped to HMI faceplates and alarm systems. Their meaning must be exact: does Running mean commanded on or confirmed by a Feedback signal?
- Testing: Check outputs in every state of the block, including fault and startup states.
- Documentation: Document every output with type, unit and exact meaning in the block reference. The same wording can feed HMI help texts and service documentation.
Output variable vs. PLC output
An output variable is a software parameter of a block. A physical PLC output drives a device through an output module. A block's output can be connected to a physical output, but the concepts differ and deserve different names.