SA0121: Missing VAR_EXTERNAL declarations

Function

Determines the use of a global variable in the function block, without it being declared as VAR_EXTERNAL (required according to the standard).

Reason

According to the IEC 61131-3 standard, access to global variables is only permitted via an explicit import through a VAR_EXTERNAL declaration.

Importance

Low

PLCopen rule

CP18

SA0121: Missing VAR_EXTERNAL declarations 1:

In TwinCAT 3 PLC it is not necessary for variables to be declared as external. The keyword exists in order to maintain compatibility with IEC 61131-3.

Sample:

Global variables:

VAR_GLOBAL
    nGlobal : INT; 
END_VAR

Program Prog1:

PROGRAM Prog1
VAR
    nVar    : INT;
END_VAR
nVar := nGlobal;                 // => SA0121

Program Prog2:

PROGRAM Prog2
VAR
    nVar    : INT;
END_VAR
VAR_EXTERNAL
    nGlobal : INT;
END_VAR
nVar := nGlobal;                 // no error