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Lexical elements

Whitespace

Spaces, tabs, line endings (CR, LF, CRLF) separate tokens. Multiple whitespace characters are equivalent to one. ST is not indentation-sensitive (unlike Python); use whatever indentation makes the code readable.

Identifiers

An identifier is a sequence of letters, digits and underscores that:

  • starts with a letter or underscore (not a digit);
  • doesn’t contain two consecutive underscores (__ is reserved by the standard for implementation-internal use);
  • doesn’t end with an underscore (style convention; matiec accepts it);
  • is not a reserved keyword.

Identifiers are case-insensitive per IEC 61131-3 §1.4.2. iCount, ICOUNT, icount and Icount all refer to the same variable. ForgeIEC normalises display to mixed-case but accepts any spelling in source.

Reserved keywords

The standard reserves all of:

ABS ACOS ACTION ADD AND ANDN ANY ANY_BIT ANY_DATE ANY_INT
ANY_NUM ANY_REAL ARRAY ASIN AT ATAN AUTHOR BCD_TO_INT
BCD_TO_DINT BCD_TO_LINT BOOL BY BYTE CAL CALC CALCN CASE
CD CONCAT CONFIGURATION CONSTANT COS CTD CTU CTUD D DATE
DATE_AND_TIME DELETE DINT DIV DO DT DWORD ELSE ELSIF
END_ACTION END_CASE END_CONFIGURATION END_FOR END_FUNCTION
END_FUNCTION_BLOCK END_IF END_PROGRAM END_REPEAT END_RESOURCE
END_STEP END_STRUCT END_TRANSITION END_TYPE END_VAR END_WHILE
EQ EXIT EXP F_EDGE F_TRIG FALSE FIND FOR FROM FUNCTION
FUNCTION_BLOCK GE GT IF IN INITIAL_STEP INSERT INT INTERVAL
JMP JMPC JMPCN LD LDN LE LEFT LEN LIMIT LINT LN LO LOG
LREAL LT LWORD MAX MID MIN MOD MUL MUX NE NOT NOW OF ON
OR ORN PRIORITY PROGRAM Q QU QD R R_EDGE R_TRIG READ_ONLY
READ_WRITE REAL REPEAT REPLACE RESOURCE RET RETC RETCN
RETAIN RETURN RIGHT ROL ROR RS S SD SDT SEL SHL SHR SIN
SINGLE SINT SR ST STN STEP STRING STRUCT SUB TAN T TASK
THEN TIME TIME_OF_DAY TO TOD TOF TON TP TRANSITION TRUE
TRUNC TYPE UDINT UINT ULINT UNTIL USINT VAR VAR_ACCESS
VAR_CONFIG VAR_EXTERNAL VAR_GLOBAL VAR_INPUT VAR_IN_OUT
VAR_OUTPUT VAR_TEMP WHILE WITH WORD WSTRING XOR

Plus the type-prefix forms (BYTE#, INT#, …) and the literal prefixes (T#, D#, TOD#, DT#, 2#, 8#, 16#).

Length

The standard recommends portable code keep identifiers under 24 characters; matiec accepts up to 256. ForgeIEC’s editor shows truncation warnings if an identifier exceeds the target-PLC’s limit (settable per-project).

Comments

Two forms accepted by IEC 61131-3 third edition:

  • Block comments: (* … *) — span any number of lines, cannot nest.
  • Line comments: // … — to end of line. Added in the third edition (2013); matiec rejects them. Use the block form for portability.
(* this is a block comment, the canonical form *)

(* multi-line block comments
   work too; the closing *)
   ends them *)

(* nesting (* doesn't work *) the inner *) closes outer *)

For inline notes that document a single line, write the block form on the same line:

iCount := iCount + 1;        (* increment per cycle *)

Pragmas

The standard does not define pragmas. ForgeIEC’s editor recognises a few non-standard pragma forms in comments ((*$ ForgeIEC: monitor=true *), etc.) for editor-specific metadata. These pragmas survive round-trip but matiec ignores them.

IEC reference

IEC 61131-3 third edition (2013), clause 6.1 — “Common elements” → “Use of printed characters”.

matiec conformance

  • Whitespace: per spec.
  • Identifiers: per spec, case-insensitive matching.
  • Block comments: per spec.
  • Line comments (//): NOT supported (see llm_signals).
  • Comment nesting: not supported (per spec — both implementations agree).