tc-syntax-control-flow

name	tc-syntax-control-flow
description	Use this skill to correctly format ST (Structured Text) logic, operators, and control flow statements. It includes precedence rules, IF/CASE/FOR/WHILE loops syntax, mathematical/bitwise operator rules (MOD, SHL, ADR, BITADR), and the list of reserved IEC keywords.
disallowedTools	["edit","write","replace_string_in_file","multi_replace_string_in_file","create_file","grep_search","file_search","list_dir"]

TwinCAT Syntax & Control Flow (Cap. 3, 4, 15)

Operator Precedence (strongest to weakest)

Precedence	Operation	Symbol
1	Parentheses	(expression)
2	Function call	FunctionName(params)
3	Exponentiation	EXPT
4	Negate / Complement	-, NOT
5	Multiply, Divide, Modulo	*, /, MOD
6	Add, Subtract	+, -
7	Comparison	<, >, <=, >=
8	Equality	=, <>
9	Boolean AND	AND
10	Boolean XOR	XOR
11	Boolean OR	OR

Equal-precedence operators are processed left to right.

All Operators

Operation	Operators	Example
Assignment	:=	bVar := TRUE;
Arithmetic	+, -, *, /, MOD, EXPT	nVal := (nA * nB) MOD 10;
Bitwise	AND, OR, XOR, NOT	nFlags := nInput AND 16#FF;
Bit Shift	SHL, SHR, ROL, ROR	nResult := SHL(nVal, 2);
Comparison	=, <>, >, <, >=, <=	IF nCnt >= 10 THEN ...
Selection	SEL, MAX, MIN, LIMIT, MUX	nVal := SEL(bSwitch, 0, 100);
Address	ADR, SIZEOF, XSIZEOF, BITADR	pData := ADR(stConfig);
Dynamic Memory	__NEW, __DELETE	pFB := __NEW(FB_Dyn); __DELETE(pFB);
Type Query	__QUERYINTERFACE, __QUERYPOINTER	Runtime interface/pointer conversion. Returns BOOL.
Validation	__ISVALIDREF	bOk := __ISVALIDREF(refVar);
Introspection	__VARINFO, __POUNAME, __POSITION	Runtime variable info, POU name, source position.

Operator Notes

• MOD is not defined for REAL types (error 4017).
• ADR is not allowed on bits — use BITADR instead (error 4031).
• ADR must not be applied to expressions or constants (error 4030).
• XSIZEOF returns ULINT on 64-bit, UDINT on 32-bit; use __UXINT for portability.
• __NEW allocates from router memory pool. FBs/DUTs require {attribute 'enable_dynamic_creation'}.

Control Flow

IF...THEN...ELSIF...ELSE...END_IF

IF fTemperature < 0.0 THEN
    eState := E_State.FROZEN;
ELSIF fTemperature = 0.0 THEN
    eState := E_State.FREEZING;
ELSIF fTemperature > 100.0 THEN
    eState := E_State.BOILING;
ELSE
    eState := E_State.NORMAL;
END_IF;

Condition must be BOOL expression (error 4253).

CASE...OF...ELSE...END_CASE

CASE nCommandID OF
    1:      bMotorOn := TRUE;
    2:      bMotorOn := FALSE;
    3, 4:   bAlarm := TRUE;
    10..20: nSpeed := nCommandID * 10;
ELSE
    bFault := TRUE;
END_CASE;

• Selector must be an integer type (error 4264).
• Each CASE constant may only be used once (error 4270).
• CASE ranges must not overlap (error 4273).
• Only one ELSE branch allowed (error 4274).

FOR...TO...BY...DO...END_FOR

FOR nIndex := 1 TO 100 BY 2 DO
    IF aBuffer[nIndex] = 70 THEN
        nFound := nIndex;
        EXIT;
    END_IF;
END_FOR;

• Counter must be an integer type (error 4257) with write access (error 4258).
• BY is optional (defaults to 1).

WHILE...DO...END_WHILE

nIdx := 0;
WHILE nIdx <= 100 AND aData[nIdx] <> 70 DO
    nIdx := nIdx + 2;
END_WHILE;

Condition must be BOOL (error 4254).

REPEAT...UNTIL...END_REPEAT

REPEAT
    nRetryCount := nRetryCount + 1;
    bSuccess := F_TryConnect();
UNTIL bSuccess OR nRetryCount >= 5
END_REPEAT;

Executes at least once. Condition must be BOOL (error 4255).

EXIT / RETURN

• EXIT — exits innermost loop. Error 4262 if outside a loop.
• RETURN — exits current POU immediately.

Variable Declaration Scopes

Scope	Description	Notes
VAR_INPUT	Input parameters	Read-only inside POU.
VAR_OUTPUT	Output parameters	Not allowed in functions (error 3820).
VAR_IN_OUT	Pass-by-reference	Cannot have initial values (error 3761). Must always be provided (error 4061).
VAR	Local variables	Static lifetime in FB, temporary in Function.
VAR_TEMP	Temporary variables	Stack memory, reset every cycle.
VAR RETAIN	Retained variables	Persisted across power cycles. No effect in functions (warning 1410).
VAR PERSISTENT	Persistent variables	Persisted across power cycles.
VAR CONSTANT	Constants	Read-only. Cannot be on direct addresses (error 3726).
VAR_EXTERNAL	External global ref	Must match a globally declared variable (error 3840).
VAR_STAT	Static variables	Only exist once even with multiple FB instances.
VAR_INST	Instance variables in methods	Retained between method calls. Only in methods of FBs.
VAR_GENERIC CONSTANT	Generic constant	Integer constant defined at instantiation. Requires Build 4026+.

VAR_INST Example

METHOD MethLast : INT
VAR_INPUT
    nVar  : INT;
END_VAR
VAR_INST
    nLast : INT := 0;
END_VAR
MethLast := nLast;
nLast    := nVar;

VAR_GENERIC CONSTANT Example

FUNCTION_BLOCK FB_Buffer
VAR_GENERIC CONSTANT
    nMaxLen : UDINT := 1;
END_VAR
VAR
    aData : ARRAY[0..nMaxLen-1] OF BYTE;
END_VAR

VAR
    fbBuf100 : FB_Buffer<100>;
END_VAR

SUPER and THIS

SUPER^();                  (* Call FB body of base class *)
SUPER^.METH_DoIt();        (* Call method of base class *)
THIS^.nVarB := 222;        (* Access FB variable shadowed by local *)
F_FunA(fbMyFb := THIS^);   (* Pass own instance to function *)

Only usable in methods and function block implementations.

Direct Addresses

VAR
    bInput1 AT %IX0.0 : BOOL;   (* Digital input, byte 0, bit 0 *)
    nAnalog AT %IW2   : INT;    (* Analog input, word 2 *)
    bOutput AT %QX0.0 : BOOL;   (* Digital output *)
    nMemory AT %MW10  : INT;    (* Memory word 10 *)
END_VAR

Prefix	Area
%I	Input
%Q	Output
%M	Memory/Marker
X	Bit

• Only BOOL variables allowed on bit addresses (error 3722).
• No array declaration allowed on direct addresses (error 3727).

Reserved Keywords (Cap. 15)

Keywords must be written in CAPITALS and cannot be used as variable names.

Control Flow: IF, THEN, ELSIF, ELSE, END_IF, CASE, OF, END_CASE, FOR, TO, BY, DO, END_FOR, WHILE, END_WHILE, REPEAT, UNTIL, END_REPEAT, EXIT, RETURN, CONTINUE

Declarations: VAR, VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, VAR_GLOBAL, VAR_TEMP, VAR_STAT, VAR_INST, VAR_EXTERNAL, VAR_CONFIG, VAR_GENERIC, END_VAR, CONSTANT, RETAIN, PERSISTENT, AT

POU Types: PROGRAM, END_PROGRAM, FUNCTION, END_FUNCTION, FUNCTION_BLOCK, END_FUNCTION_BLOCK, METHOD, END_METHOD, PROPERTY, END_PROPERTY, ACTION, END_ACTION, INTERFACE, END_INTERFACE

Special (ExST): __NEW, __DELETE, __ISVALIDREF, __QUERYINTERFACE, __QUERYPOINTER, __VARINFO, __POUNAME, __POSITION, __TRY, __CATCH, __FINALLY, __ENDTRY, AND_THEN, OR_ELSE

System Functions

• Timers: TON (On-Delay), TOF (Off-Delay), TP (Pulse).
• Triggers: R_TRIG (Rising Edge), F_TRIG (Falling Edge).
• Counters: CTU (Count Up), CTD (Count Down), CTUD (Count Up/Down).
• String: CONCAT, LEN, LEFT, RIGHT, MID, INSERT, DELETE, REPLACE, FIND.
• Memory: MEMCPY, MEMSET, MEMMOVE (in Tc2_System).
• Math: ABS, SQRT, SIN, COS, TAN, ASIN, ACOS, ATAN, EXP, LN, LOG.

VAR
    fbTonDelay : TON;
    fbTrigRise : R_TRIG;
END_VAR

fbTonDelay(IN := bStart, PT := T#5S);
IF fbTonDelay.Q THEN
    bDelayedStart := TRUE;
END_IF;

fbTrigRise(CLK := bButton);
IF fbTrigRise.Q THEN
    nPressCount := nPressCount + 1;
END_IF;

CONTINUE Statement

Skips the rest of the current iteration and proceeds with the next loop pass. Valid only inside FOR, WHILE, REPEAT.

FOR i := 1 TO 100 DO
    IF aData[i] = 0 THEN
        CONTINUE;   (* skip zero entries *)
    END_IF;
    nSum := nSum + aData[i];
END_FOR;

Short-Circuit Boolean Operators (ExST)

Operator	Behavior
AND_THEN	Right operand evaluated only if left operand is TRUE
OR_ELSE	Right operand evaluated only if left operand is FALSE

(* Safe pointer dereference using short-circuit *)
IF (pConfig <> 0) AND_THEN (pConfig^.bEnabled) THEN
    fbProcess();
END_IF;

Standard AND / OR always evaluate both sides — risky with side effects (warning 1504/1505).

CASE with Enumerations

{attribute 'qualified_only'}
{attribute 'strict'}
TYPE E_State : (eIdle, eRun, eStop) END_TYPE

CASE eMyState OF
    E_State.eIdle: ...
    E_State.eRun:  ...
    E_State.eStop: ...
END_CASE;

With {attribute 'strict'}, the compiler verifies all enum values are handled (or ELSE is provided).

Operands — AT Declarations

VAR_GLOBAL
    bEStop  AT %IX0.0  : BOOL;     (* Bit-aligned input *)
    nCount  AT %MW20   : INT;      (* Memory word *)
    aBuf    AT %MB100  : ARRAY [0..9] OF BYTE;  (* INVALID — error 3727 *)
END_VAR

• AT %I* and AT %Q* declarations are usually replaced in TwinCAT 3 by link mappings in the System Manager / .tsproj rather than fixed addresses.
• %M* (marker area) is preserved across cycles but not retained across power cycles unless declared RETAIN.

References (Beckhoff InfoSys)

• Operators
• Operands
• Keywords
• System Functions