Variable Scope¶

IEC 61131-3 provides keywords to control the scope and direction of variables within program organization units.

IEC 61131-3	Section 2.4.3
Support	Partial

Scope Keywords¶

Keyword	Description	Status
`VAR`	Local variable	Supported
`VAR_INPUT`	Input parameter (read-only in callee)	Partial
`VAR_OUTPUT`	Output parameter (written by callee)	Partial
`VAR_IN_OUT`	In/out parameter (passed by reference)	Partial
`VAR_GLOBAL`	Global variable (accessible across POUs)	Supported
`VAR_EXTERNAL`	Reference to a global variable	Supported

Example¶

FUNCTION_BLOCK MotorControl
    VAR_INPUT
        start : BOOL;
        stop : BOOL;
    END_VAR
    VAR_OUTPUT
        running : BOOL;
    END_VAR
    VAR
        internal_state : INT;
    END_VAR

    IF start AND NOT stop THEN
        running := TRUE;
    ELSIF stop THEN
        running := FALSE;
    END_IF;
END_FUNCTION_BLOCK

Global Variables¶

Global variables are declared in a CONFIGURATION block using VAR_GLOBAL and accessed from programs using VAR_EXTERNAL. The VAR_EXTERNAL declaration must match the name and type of the global variable it references.

Top-Level Global Variables (Vendor Extension)¶

Note

This is a vendor extension not part of the IEC 61131-3 standard. See Enabling Dialects and Features for how to enable it.

Many PLC vendors allow VAR_GLOBAL blocks at the top level of a file, outside of a CONFIGURATION block. IronPLC supports this common extension to improve compatibility with code written for other PLC environments.

Enable with --allow-top-level-var-global or --allow-all on the command line.

Programs access top-level globals the same way as configuration globals — through VAR_EXTERNAL declarations that match the name and type.