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CODESYS vs TIA Portal: Structured Text Differences Every Engineer Should Know

Moving between CODESYS and TIA Portal? Learn the key Structured Text syntax differences to avoid common pitfalls.

· 9 min read

  • CODESYS
  • TIA Portal
  • Siemens
  • Platform Comparison

Why This Matters

If you work in industrial automation, you'll eventually encounter both CODESYS (used by Beckhoff, Wago, ABB, and many others) and Siemens TIA Portal (S7-1200/1500). While both follow IEC 61131-3 for Structured Text, there are important syntax and behavioral differences.

Key Differences

1. Program Structure

CODESYS uses standard IEC 61131-3 POUs: 

PROGRAM Main
VAR
    myVar : INT := 0;
END_VAR
// Logic here
END_PROGRAM

TIA Portal wraps code in Organization Blocks (OBs):

  • OB1 (Main scan), OB100 (Startup), FC/FB blocks
  • Variables declared in the block's interface, not inline VAR blocks
  • Uses Data Blocks (DBs) for persistent storage

    • 2. Timer Syntax

    CODESYS (IEC standard): 

    myTimer(IN := startCondition, PT := T#5s);
timerDone := myTimer.Q;
elapsed := myTimer.ET;

    TIA Portal (similar but with instance DB): 

    // Timer called as instance in TIA Portal
"IEC_Timer_DB".TON(
    IN := startCondition,
    PT := T#5s
);
timerDone := "IEC_Timer_DB".Q;

    3. String Handling

    CODESYS: 

    myString : STRING(80) := 'Hello';
length := LEN(myString);
sub := MID(myString, 3, 1);  // MID(string, length, position)

    TIA Portal: 

    // TIA uses slightly different function signatures
length := LEN("myString");
// String indexing starts from 1 in both platforms

    4. Data Type Differences

    FeatureCODESYSTIA Portal
    StringsSTRING(length)String[length]
    EnumsStandard IECLimited before V16
    PointersPOINTER TONot available in ST
    ReferencesREFERENCE TOInOut parameters
    ArraysARRAY[1..10] OFArray[1..10] of

    5. Commenting Styles

    Both support: 

    // Single-line comment
(* Multi-line
   comment *)

    TIA Portal also accepts: 

    / C-style comments /

    What Transfers Between Platforms

    The good news: core logic transfers well. These are identical on both platforms:

  • IF/ELSIF/ELSE/END_IF
  • FOR/WHILE/REPEAT loops
  • CASE statements
  • Boolean operators (AND, OR, NOT, XOR)
  • Arithmetic operators (+, -, *, /, MOD)
  • Comparison operators (=, <>, <, >, <=, >=)
  • Most IEC function blocks (TON, TOF, TP, CTU, CTD)

    • Best Practice: Write Portable Code

  • Stick to IEC 61131-3 standard syntax where possible
  • Avoid platform-specific pointer/reference types
  • Use standard timer and counter function blocks
  • Document platform-specific code with comments
  • Practice in a standard IEC simulator like CodeSchool ST to build transferable skills

    • Conclusion

    Learning Structured Text on a standard IEC 61131-3 platform gives you skills that transfer to both CODESYS and TIA Portal. The differences are mostly syntactic — the logic, control structures, and programming patterns are the same.

    Practice writing portable ST code in our free online editor to build skills that work everywhere.