tc-naming-conventions

TwinCAT Naming Conventions (Cap. 1, 14)

Strictly follow Hungarian notation prefixes and PascalCase for identifiers to maintain code standard compliance.

Prefixes for Object Types

Object Type	Prefix	Example Definition	Example Instance	Call Example
Function Block	FB_	FUNCTION_BLOCK FB_AxisControl	fbAxisControl	fbAxisControl();
Structure	ST_	TYPE ST_MachineStatus : STRUCT	stMachineStatus	stMachineStatus.nCounter := 5;
Enum	E_	TYPE E_OperationMode : (	eOpMode	eOpMode := E_STOP;
Program	P_	PROGRAM P_Main	-	-
Function	F_	FUNCTION F_Calculate : REAL	-	-
Interface	I_	INTERFACE I_Observable	iObservable	-
Global Variable List	GVL_	VAR_GLOBAL GVL_IO	-	-
Alias Type	T_	TYPE T_Nibble : ...	Nibble	Nibble := 1;

Prefixes for Data Types

Type	Prefix	Description	Example
BOOL	b	bit	bStart, bReady, bSwitch
INT, UINT, WORD, SINT, USINT, DINT, UDINT, BYTE, DWORD, LWORD	n or i	numeric / integer	nCount, nIndex, iErrorID
REAL, LREAL	f	float	fTemperature, fPosition, fValue
STRING, WSTRING	s	string	sMessage, sSerialNumber, sName
TIME	t	time	tDelayTime, tDelay
DATE	d	date	dMonday
DATE_AND_TIME (DT)	dt	date and time	dtTimestamp, dtNewYear
TIME_OF_DAY (TOD)	tod	time of day	todAlarm
ARRAY	a or arr	arrays	aDataBuffer, arrMessages
POINTER	p	pointer	pConfig, pData
REFERENCE	ref	reference	refAxis
ENUM	e	enum instance	eState
Count of bytes	cb	byte count	cbLength
Count of words	cw	word count	cwRead

Naming Rules

• Object name prefixes are always UPPER CASE with underscore _ as separator (e.g., FB_GetData).
• Variable and instance prefixes are always lower case. The first letter of the identifier is always upper case.
• If an identifier consists of several words, the first letter of each word is upper case (PascalCase). No separators between words (e.g., fbGetData, stBufferEntry).
• Identifiers should only contain: 0...9, A...Z, a...z, _.
• All identifiers should be specified in English.
• Multiple underlines in identifiers are not allowed (compiler error).
• Properties and Methods use plain PascalCase without Hungarian type prefixes (e.g., Position, IsReady, Execute, Reset).
• Backing fields for properties follow standard Hungarian notation. Do NOT use a leading underscore _ prefix. Example: for a property Name : STRING, the backing field is sName, not _sName.

Library Version Convention

Each library must contain a version function readable at runtime:

FUNCTION F_GetVersionMyLib : UINT
VAR_INPUT
    nVersionElement : INT;
END_VAR

CASE nVersionElement OF
    1: (* major *) F_GetVersionMyLib := 1;
    2: (* minor *) F_GetVersionMyLib := 0;
    3: (* revision *) F_GetVersionMyLib := 0;
ELSE
    F_GetVersionMyLib := 16#FFFF;
END_CASE

Shading Rules (Name Resolution — Cap. 14)

When the same identifier is used for different elements, the compiler searches in this order:

1. Local variables: (a) method locals → (b) FB/program/function locals + base FBs → (c) local methods of FB
2. Global variables: (a) project GVLs (if not qualified_only) → (b) library GVLs
3. Type names: (a) project types → (b) library types
4. Libraries: namespaces

Avoiding Name Conflicts

• Follow naming conventions (prefixes for variables vs. types).
• Use {attribute 'qualified_only'} on GVLs and enumerations.
• Use qualified library access: Tc2_System.ADSLOGSTR(...).
• Use static analysis rule SA0027 to detect duplicate names.
• Use THIS^.varName to access a shadowed FB variable from a method.

Identifier Rules

• Allowed characters: 0-9, A-Z, a-z, _. No spaces, no special characters, no umlauts.
• Identifiers are case-insensitive for the compiler (bMyVar and bmyvar collide), but always write them in PascalCase to keep readability.
• Cannot begin with a digit.
• Multiple consecutive underscores are not allowed (compiler error).
• Identifiers starting with __ (double underscore) are reserved for internal compiler use.
• A variable name must not equal a TwinCAT reserved keyword (see tc-syntax-control-flow).
• Action and step names follow the same rules as identifiers.

Library Naming

• Beckhoff-supplied libraries start with the prefix Tc (e.g., Tc2_System, Tc3_Module).
• Custom libraries should use a company namespace prefix (e.g., MyCorp_Motion).
• Always declare a version function F_GetVersion<LibName> returning UINT so the runtime version is queryable.

SUPER^ vs THIS^ — Quick Reference

Construct	Where Allowed	Use
THIS^	FB body, methods, properties	Access own FB instance, disambiguate shadowed locals
SUPER^	Derived FB body, methods, properties	Call base FB body or base method
SUPER^.FB_init	Forbidden in explicit FB_init	Base init runs implicitly
SUPER^.FB_reinit	Required in derived FB_reinit	Must be called explicitly

References (Beckhoff InfoSys)

• Programming Conventions (TC2 PLC Control)
• Identifier (TwinCAT 3)
• Shading Rules / Name Resolution