tc-troubleshooting

name	tc-troubleshooting
description	Use this skill ONLY when the user reports a build error, a compilation warning, or when `CheckPlcSyntaxAsync` fails. It contains specific fixes for TwinCAT IL Errors (4200-4213) and explanations for standard Compiler Warnings (1502-1990).
disallowedTools	["edit","write","replace_string_in_file","multi_replace_string_in_file","create_file","grep_search","file_search","list_dir"]

TwinCAT Troubleshooting — Compiler Errors & Warnings (Cap. 8, 9)

Compiler Warnings (1xxx)

Code	Warning Message	Cause / Solution
1100	Unknown function '<name>' in library	External library used. Check all functions defined in .hex are also in .lib.
1101	Unresolved symbol '<Symbol>'	Define a function/program with this name.
1200	Task '<name>', call of '<name>' Access variables not updated	Variables used only at FB call in task configuration won't appear in cross reference.
1300	File not found '<name>'	Global variable object points to nonexistent file. Check path.
1302	New externally referenced functions inserted. Online Change not possible	Download complete project instead of online change.
1400	Unknown Pragma '<name>' is ignored	Pragma not supported. Check pragma syntax.
1401	Struct '<name>' does not contain any elements	Empty struct still uses 1 byte.
1410	'RETAIN' and 'PERSISTENT' have no effect in functions	Remanent vars in functions behave as normal local variables.
1412	Unexpected token '<name>' in pragma	Pragma syntax error or used at wrong location.
1501	String constant passed as VAR_IN_OUT must not be overwritten	Constant may not be written inside POU.
1502	Variable '<name>' has same name as a POU	Variable is used, not the POU. Use different names.
1504	Statement may not be executed due to logical expression evaluation	Short-circuit: if first operand is FALSE, second is not evaluated.
1507	Instance '<name>' has same name as a function	The function will be called, not the instance. Use different names.
1550	Multiple calls of POU '<name>' in one network may lead to side effects	Check if multiple calls are necessary.
1850	Input variable at %IB used in task '<name>' but updated in another task	Check task assignments.
1990	No 'VAR_CONFIG' for '<name>'	Missing address configuration. Use 'Insert All instance paths'.

Code Generation Errors (3100–3200)

Code	Error	Solution
3101	Total data too large	Reduce data usage.
3117	Expression too complex. No more registers	Split expression using intermediate variables.
3121	POU too large. Max 64K	Split the POU.
3124	String constant too large (max 253 chars)	Reduce string constant length.
3130	User-Stack too small	Reduce POU call nesting depth or increase stack in target settings.
3202	Stack overrun with nested string/array/struct function calls	Split CONCAT(x, f(i)) into two expressions using intermediate variable.
3210	Function '<name>' not found	Define it or add library.

Declaration Errors (3600–3900)

Code	Error	Solution
3612	Maximum number of POUs exceeded	Modify maximum in Target Settings / Memory Layout.
3614	Project must contain a POU named '<name>' or a task configuration	Create main POU of type PROGRAM (e.g., PLC_PRG).
3615	<name> (main routine) must be of type program	Init POU must be PROGRAM.
3701	Name used in interface not identical with POU Name	Use 'Rename object' or change declaration name to match.
3703	Duplicate definition of identifier '<name>'	Rename the variable or object.
3704	Data recursion	An FB instance uses itself directly or indirectly.
3705	VAR_IN_OUT in Top-Level-POU not allowed without Task-Configuration	Create task configuration or remove VAR_IN_OUT from PLC_PRG.
3720	Address expected after 'AT'	Add valid address (%I*, %Q*, %M*) after AT.
3721	Only 'VAR' and 'VAR_GLOBAL' can be located to addresses	Move to VAR or VAR_GLOBAL.
3722	Only 'BOOL' variables allowed on bit addresses	Modify address or change variable type.
3726	Constants cannot be laid on direct addresses	Remove address from constant.
3727	No array declaration allowed on this address	Remove address from array.
3740	Invalid type '<name>'	Define type or check for typos. Ensure library is referenced.
3744	Enum constant '<name>' already defined	Enum values must be unique within enum and across global/local enums.
3745	Subranges are only allowed on Integers	Use integer base type.
3748	More than three dimensions not allowed for arrays	Use ARRAY OF ARRAY (max 9 via nesting).
3760	Error in initial value	Initial value doesn't match type definition.
3761	VAR_IN_OUT variables must not have initial value	Remove := value from VAR_IN_OUT.
3782	Unexpected end	Add END_VAR or terminating keyword (END_IF, etc.).
3802	Out of retain memory	Retain memory exhausted.
3820	VAR_OUTPUT and VAR_IN_OUT not allowed in functions	Functions cannot have output or in-out variables.
3821	At least one input required for functions	Add at least one VAR_INPUT.
3840	Unknown global variable '<name>'	VAR_EXTERNAL references nonexistent global.
3841	Declaration of '<name>' doesn't match global declaration	VAR_EXTERNAL type must match global declaration.
3850	Unpacked struct inside packed struct not allowed	Causes memory misalignment. Modify struct definition.
3902	Keywords must be uppercase	Use capitals.
3903	Invalid duration constant	Use correct format: T#100ms, T#2s.
3904	Overflow in duration constant	Value exceeds TIME max (~49 days).

Expression & Statement Errors (4000–4370)

Code	Error	Solution
4001	Variable '<name>' not declared	Declare variable locally or globally.
4010	Type mismatch: Cannot convert '<t1>' to '<t2>'	Add explicit conversion function.
4011	Type mismatch in parameter '<name>'	Use type conversion or correct variable.
4017	'MOD' is not defined for 'REAL'	Use MOD only with integer/bitstring types.
4021	No write access to variable '<name>'	Use writable variable.
4029	Nested calls of same function not possible	Use intermediate variable: nTemp := fun1(b,c); nResult := fun1(a, nTemp);
4030	Expressions and constants not allowed as operands of ADR	Use a variable or direct address.
4031	ADR not allowed on bits! Use BITADR	Replace ADR with BITADR.
4034	Division by 0	Division by zero in constant expression.
4050	POU '<name>' is not defined	Define the POU or correct the name.
4052	'<name>' must be a declared instance of FB '<name>'	Declare an instance of the correct FB type.
4060	VAR_IN_OUT parameter needs variable with write access	Provide writable variable.
4061	VAR_IN_OUT parameter must be used	Always provide a variable for VAR_IN_OUT parameters.
4070	POU contains too complex expression	Simplify using intermediate variables.
4110	'^' needs a pointer type	Declare variable as POINTER TO.
4111	'[index]' needs array variable	Indexing only works on ARRAY OF variables.
4114	Constant index not within array range	Fix index to be within declared bounds.
4120	'.' needs structure variable	Left side of . must be STRUCT, FUNCTION_BLOCK, FUNCTION, or PROGRAM.
4121	'<name>' is not a component of <object>	Component not found in struct/FB definition.
4250	Another ST statement or end of POU expected	Line doesn't start with valid ST instruction.
4251	Too many parameters in function '<name>'	More parameters than declared.
4252	Too few parameters in function '<name>'	Fewer parameters than declared.
4253	IF/ELSIF requires BOOL expression	Condition must evaluate to BOOL.
4257	FOR variable must be of type INT	Counter must be integer or bitstring type.
4258	FOR expression is no variable with write access	Counter must be writable.
4262	EXIT outside a loop	EXIT only inside FOR, WHILE, or REPEAT.
4264	CASE requires selector of integer type	Use integer/bitstring type for CASE selector.
4267	FB call requires function block instance	Declare an instance of the FB.
4270	CASE constant '<n>' already used	Each selector value used only once.
4271	Lower border greater than upper border	Fix range bounds.
4273	CASE range '<a>..<b>' already used in '<c>..<d>'	Ranges must not overlap.
4274	Multiple ELSE branch in CASE statement	Only one ELSE permitted.

Task Configuration Errors (3550–3575)

Code	Error	Solution
3550	Duplicate definition of identifier '<name>'	Rename one task.
3551	Task '<name>' must contain at least one program call	Insert a program call or delete the task.
3553	Event variable '<name>' must be of type BOOL	Use a BOOL variable for event trigger.
3557	Maximum number of Tasks exceeded	Reduce task count to target limit.
3570	Tasks '<a>' and '<b>' share the same priority	Each task must have a unique priority.
3575	Task '<name>': cycle time must be multiple of <n> µs	Adjust cycle time.

SFC Errors (4350–4369)

Code	Error	Solution
4353	Step name duplicated: '<name>'	Rename one of the duplicate steps.
4354	Jump to undefined Step: '<name>'	Define target step or use existing step name.
4355	Transition must not have side effects	Transition must be a pure boolean expression.
4358	Action not declared: '<name>'	Add action below SFC POU and set name in qualifier box.
4359	Invalid Qualifier: '<name>'	Use valid qualifier (N, R, S, L, D, P, SD, DS, SL).
4364	Transition must be a boolean expression	Result of transition must be BOOL.

IL Errors (4200–4213)

Code	Error	Solution
4201	IL Operator expected	Each IL instruction must start with operator or jump label.
4202	Unexpected end of text in brackets	Insert closing bracket.
4203	<name> in brackets not allowed	JMP, RET, CAL, LDN, LD, TIME not valid in bracket expressions.
4204	Closing bracket with no opening bracket	Insert opening bracket or remove closing one.
4207	'N' modifier requires BOOL, BYTE, WORD or DWORD	N modifier needs a type supporting boolean negation.
4208	Conditional operator requires type BOOL	Expression must yield boolean result.
4210	CAL, CALC, CALN require FB instance as operand	Declare an instance of the function block.
4213	S and R require BOOL operand	Use a boolean variable.

Diagnostic Workflow

1. Run plc_check_syntax first for immediate syntax feedback, then run session_build.
2. Treat session_build as the source of truth for the current project state: plc_check_syntax can still report stale errors until a new build refreshes the diagnostics.
3. Read the Error List — fix errors top-down (later errors are often cascades of the first).
4. For each error code: look it up in the tables above, identify the offending POU/line, apply the indicated fix.
5. Run session_build again after each fix — never batch multiple speculative fixes.
6. Never login to a PLC with unresolved errors (rule 0.5 in the master agent).

Common Pitfalls

• Cascade errors: a single missing END_VAR (3782) can produce dozens of subsequent errors — fix the first one and rebuild.
• Stale syntax diagnostics: if plc_check_syntax still shows old errors after a fix, run session_build before trusting the result list.
• Type mismatch in batch edits: when refactoring data types, use plc_grep_pou to find ALL references before editing.
• Online change blocked: warning 1302 means new external references — perform a full download instead.
• Retain memory exhausted (3802): an FB instance with even one VAR RETAIN member places the entire instance in retain area; split into separate FBs.
• Padding-induced size mismatch: structs sent over EtherCAT/ADS must use {attribute 'pack_mode' := '1'} to match external definitions.

References (Beckhoff InfoSys)

• Compiler Errors
• System Functions