Enabling Dialects and Features¶

IronPLC aims to let you take code from another PLC environment and use it without changes. To support this, IronPLC uses dialects — named presets that select the IEC 61131-3 edition and a default set of extensions. Individual --allow-* flags provide fine-grained control on top of the selected dialect.

Supported Dialects¶

iec61131-3-ed2 (default)

Strict IEC 61131-3:2003 (Edition 2). No extensions are enabled. This is the default when no dialect is specified.

Enables: nothing beyond strict IEC 61131-3 (no extensions).

iec61131-3-ed3

Strict IEC 61131-3:2013 (Edition 3). Enables Edition 3 keywords including LTIME, LDATE, LTIME_OF_DAY, LDATE_AND_TIME, REF_TO, REF, and NULL. No extensions.

Enables: --allow-long-time-types and --allow-ref-to (the Edition 3 keywords), plus --allow-partial-access-syntax.

rusty

RuSTy-compatible dialect. Uses Edition 2 as a base (so Edition 3 type names like LDT remain available as identifiers) and enables REF_TO support together with the extensions that RuSTy accepts (listed below).

Enables: --allow-c-style-comments, --allow-missing-semicolon, --allow-top-level-var-global, --allow-constant-type-params, --allow-empty-var-blocks, --allow-time-as-function-name, --allow-ref-to, --allow-ref-arithmetic, --allow-ref-stack-variables, --allow-ref-type-punning, --allow-int-to-bool-initializer, --allow-sizeof, --allow-system-uptime-global, --allow-cross-family-widening, --allow-partial-access-syntax, --allow-pragmas, --allow-short-circuit-operators, --allow-mixed-located-var-declarations, --allow-constant-initializer-expressions, --allow-bit-string-case-labels, --allow-paren-string-length, --allow-struct-initializer-expressions, and --allow-fb-inheritance.

codesys

CODESYS-compatible dialect. Uses Edition 2 as a base and enables the long-time-type keywords (LTIME, LDT, etc.) and REF_TO together with the extensions that the CODESYS IDE accepts. The implicit __SYSTEM_UP_TIME globals are not pre-bound under this dialect, since they are an IronPLC runtime convention rather than a CODESYS feature.

Enables: --allow-c-style-comments, --allow-missing-semicolon, --allow-top-level-var-global, --allow-constant-type-params, --allow-empty-var-blocks, --allow-time-as-function-name, --allow-long-time-types, --allow-ref-to, --allow-reference-to, --allow-ref-arithmetic, --allow-ref-stack-variables, --allow-ref-type-punning, --allow-int-to-bool-initializer, --allow-sizeof, --allow-cross-family-widening, --allow-partial-access-syntax, --allow-pragmas, --allow-short-circuit-operators, --allow-mixed-located-var-declarations, --allow-constant-initializer-expressions, --allow-bit-string-case-labels, --allow-paren-string-length, --allow-struct-initializer-expressions, and --allow-fb-inheritance.

twincat

Beckhoff TwinCAT-compatible dialect. TwinCAT 3 is built on the CODESYS V3 runtime, so it uses an Edition 2 base and enables the long-time-type keywords (LTIME, LDT, etc.) along with the extensions TwinCAT shares with CODESYS, such as curly-brace pragmas, C-style comments, and the AND_THEN short-circuit operator. Unlike codesys, it does not enable the REF_TO / REF() / NULL reference extensions: TwinCAT spells references REFERENCE TO (bound with REF=), which this dialect enables instead via --allow-reference-to. (Pointer types — POINTER TO with ADR() — are not parsed yet.) As with codesys, the implicit __SYSTEM_UP_TIME globals are not pre-bound, since they are an IronPLC runtime convention rather than a TwinCAT feature.

Enables: --allow-c-style-comments, --allow-missing-semicolon, --allow-top-level-var-global, --allow-constant-type-params, --allow-empty-var-blocks, --allow-time-as-function-name, --allow-long-time-types, --allow-reference-to, --allow-int-to-bool-initializer, --allow-sizeof, --allow-cross-family-widening, --allow-partial-access-syntax, --allow-pragmas, --allow-short-circuit-operators, --allow-mixed-located-var-declarations, --allow-constant-initializer-expressions, --allow-bit-string-case-labels, --allow-paren-string-length, --allow-struct-initializer-expressions, and --allow-fb-inheritance.

Editions are additive — enabling a later edition includes all features from earlier editions.

See Edition Support for a complete list of features that require a specific edition.

Tip

Run ironplcc dialects to see which features each dialect enables.

How to Select a Dialect¶

Command Line¶

Pass the --dialect flag when running ironplcc:

ironplcc check --dialect rusty main.st

ironplcc check --dialect iec61131-3-ed3 main.st

See ironplcc for all compiler options.

Visual Studio Code¶

Set the ironplc.dialect setting:

Open File ‣ Preferences ‣ Settings (or Code ‣ Preferences ‣ Settings on macOS).
Search for ironplc.
Change Dialect to the desired value (e.g., rusty or iec61131-3-ed3).

Or add it directly to your settings.json:

{
  "ironplc.dialect": "rusty"
}

See Settings Reference for all extension settings.

Enabling Specific Features¶

Individual --allow-* flags can be combined with any dialect to enable additional features on top of the dialect’s defaults. Flags can only enable features — they never disable features that a dialect already includes. To see which flags a dialect already enables by default, see Supported Dialects.

--allow-c-style-comments: Allow C-style comments (// line comments and /* */ block comments). These are not part of the IEC 61131-3 standard but are supported by many PLC environments.
--allow-missing-semicolon: Allow missing semicolons after keyword statements like END_IF and END_STRUCT. Also allows a CASE branch with no statements at all (a label that falls straight through to the next label, ELSE, or END_CASE) – strict IEC 61131-3 only allows this via an explicit empty statement (5: ;); this fills in the dropped ;.
--allow-top-level-var-global: Allow VAR_GLOBAL declarations at the top level of a file, outside of a CONFIGURATION block. See Variable Scope.
--allow-constant-type-params: Allow constant references in type parameters such as array bounds and string lengths (e.g., ARRAY[1..MY_CONST] OF INT or STRING[MY_CONST]). See Array Types.
--allow-empty-var-blocks: Allow empty variable blocks (VAR END_VAR, VAR_INPUT END_VAR, etc.). Some PLC environments permit variable blocks with no declarations.
--allow-time-as-function-name: Allow TIME to be used as a function name (e.g., TIME()). Required for OSCAT compatibility where TIME() reads the PLC system clock.
--allow-long-time-types: Allow the IEC 61131-3:2013 long-time-type keywords LTIME, LDATE, LTIME_OF_DAY (LTOD), and LDATE_AND_TIME (LDT). Without this flag those words remain available as ordinary identifiers, so Edition 2 code may use them as names.
--allow-ref-to: Allow REF_TO, REF(), and NULL syntax (standardized in IEC 61131-3:2013) without enabling the rest of Edition 3. This is useful when you need references but want to keep Edition 2 keyword handling for the rest of your code. See Reference Types.
--allow-reference-to: Allow the Beckhoff TwinCAT / CODESYS REFERENCE TO reference type and the REF= binding operator. This is the TwinCAT/CODESYS-facing alternative to --allow-ref-to: the two describe the same underlying reference but with different surface syntax. The compiler does not restrict flag combinations, so --allow-ref-to and --allow-reference-to may be set at once. See Reference Types.
--allow-ref-arithmetic: Allow arithmetic (+, -) and ordering comparisons (<, >, <=, >=) on REF_TO types. By default, only = and <> are permitted on references.
--allow-ref-stack-variables: Allow REF() on stack-allocated variables (VAR_TEMP and function VAR_INPUT/VAR_OUTPUT). Required for OSCAT patterns where the reference does not escape the call.
--allow-ref-type-punning: Allow assigning between REF_TO types of different base types (type punning), such as reinterpreting the bits of a REAL through a REF_TO DWORD.
--allow-int-to-bool-initializer: Allow integer literals 0 and 1 as BOOL variable initializers (e.g., debug : BOOL := 0;). The compiler rewrites 0 to FALSE and 1 to TRUE. This is a universal extension supported by CoDeSys, TwinCAT, RuSTy, and virtually every PLC runtime.
--allow-sizeof: Allow the SIZEOF() operator that returns the size in bytes of a variable or type. This is an extension supported by CODESYS, TwinCAT, and RuSTy. See SIZEOF.
--allow-system-uptime-global: Expose __SYSTEM_UP_TIME (TIME) and __SYSTEM_UP_LTIME (LTIME) as implicit VAR_GLOBAL values holding the VM’s monotonic uptime. This is an IronPLC runtime convention.
--allow-cross-family-widening: Allow implicit widening between bit-string and integer type families. For example, passing a BYTE variable where an INT parameter is expected, or passing a bare integer literal 0 where a BYTE parameter is expected. This is an extension supported by CODESYS, TwinCAT, and RuSTy.
--allow-partial-access-syntax: Allow IEC 61131-3:2013 partial-access bit syntax .%Xn (e.g., myByte.%X3 to access bit 3 of a BYTE). Semantically equivalent to the short form .n. Byte/word/dword/lword partial access (.%Bn, .%Wn, .%Dn, .%Ln) is not yet supported.
--allow-pragmas: Allow curly-brace pragmas such as {attribute 'qualified_only'} and {attribute 'strict'}. These are CODESYS-core syntax (documented by CODESYS itself, and inherited by any IDE built on the CODESYS V3 runtime, including Beckhoff TwinCAT and Schneider Electric Machine Expert). A pragma is parsed and discarded like a comment — its contents are not yet interpreted. Pragmas do not nest; an unclosed { still produces a parse error.
--allow-short-circuit-operators: Allow the AND_THEN short-circuit boolean operator, a Beckhoff/CODESYS extension. Unlike plain AND (which always evaluates both operands), AND_THEN only evaluates its right operand when the left operand is TRUE — commonly used to guard a dereference (ptr <> 0 AND_THEN ptr^ = 99). ironplcc check fully supports AND_THEN (parsing, type-checking, and round-tripping through plc2plc with its spelling preserved — it is not normalized to AND, since the short-circuit behavior is a real, externally-visible difference in TwinCAT/CODESYS). Codegen (ironplcc compile) does not yet implement the short-circuit evaluation this operator requires and refuses to compile it (problem P9999) rather than silently emitting eager (behaviorally incorrect) bytecode.
--allow-mixed-located-var-declarations: Allow an AT-located variable (complete address like AT %IX0.0, or incomplete/wildcard address like AT %I*) inside an otherwise plain VAR/VAR_INPUT/VAR_OUTPUT block, instead of requiring located variables to live in their own dedicated block. The IEC 61131-3 standard requires located and plain variables to be declared in separate blocks; real CODESYS/TwinCAT code commonly mixes them. Without this flag, mixing produces problem P4036. A block containing only located variables is unaffected by this flag — it is standard syntax and always allowed.
--allow-constant-initializer-expressions: Allow a VAR initializer to be a constant expression — arithmetic between literals and/or references to declared CONSTANT variables (e.g. scaled : LREAL := SCALE*4.0;) — rather than only a bare literal. The IEC 61131-3 standard’s initializer grammar permits only literals in this position; this extension folds the expression to a literal at compile time. Using this form without the flag produces P4037; if the expression does not fully reduce to a constant (e.g. it references a non-CONSTANT variable), it produces P4038.
--allow-bit-string-case-labels: Allow a hex, binary, or octal bit-string literal (e.g. 16#D012, 2#1010, 8#17) as a CASE label. The IEC 61131-3 standard grammar for a case label permits only a subrange, a decimal signed_integer, or an enumerated value; radix-prefixed literals are separate productions the standard does not include here. Real TwinCAT/CODESYS code uses them. Without this flag, such a label produces P4041. A plain decimal label (5:) is standard syntax and is always allowed.
--allow-paren-string-length: Allow a string type’s maximum length to be delimited with parentheses (STRING(255), WSTRING(100)) in addition to the standard square brackets (STRING[255]). The IEC 61131-3 standard grammar declares a string length only with brackets; the parenthesis form is a dialect extension. Without this flag, the parenthesis form produces P4042. The bracket form is standard syntax and is always allowed, and the delimiters must match (STRING[255) is always a syntax error). The renderer normalizes the parenthesis form to brackets.
--allow-struct-initializer-expressions: Allow a general (non-constant) expression — such as a pointer dereference plus member access (pDevice^.Delta) — as the value in a structured or call-style initializer’s name := value pairs (e.g. tonDelta : TON := (PT := pDevice^.Delta);). The IEC 61131-3 standard grammar for a structured initializer value permits only a constant, enumerated value, array initializer, or nested structure initializer; a value computed at instantiation time is an extension used by TwinCAT/CODESYS. Without this flag, such a value produces P4043. A constant value is standard syntax and is always allowed.
--allow-fb-inheritance: Allow function-block inheritance syntax: EXTENDS/ IMPLEMENTS/ABSTRACT on FUNCTION_BLOCK declarations, and INTERFACE declarations. These are parsed and the EXTENDS/IMPLEMENTS/INTERFACE names are recognized as valid types, but inheritance, interface dispatch, method/property declarations, and abstract-instantiation checking are not yet semantically supported — using them produces problem P9999 rather than a parse error. Enabled by --dialect=rusty and --dialect=codesys.

Pass the flag when running ironplcc:

ironplcc check --allow-c-style-comments --allow-empty-var-blocks main.st

Or combine with a dialect:

ironplcc check --dialect iec61131-3-ed3 --allow-c-style-comments main.st

See ironplcc for all compiler options.