Attention

IronPLC implements many parts of the IEC 61131-3 standard and is working toward full Structured Text support. Key features still missing include arrays and structures. Try it out in the IronPLC Playground.

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 vendor 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 vendor extensions are enabled. This is the default when no dialect is specified.

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 vendor extensions.

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 plus all vendor extensions.

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:

  1. Open File ‣ Preferences ‣ Settings (or Code ‣ Preferences ‣ Settings on macOS).

  2. Search for ironplc.

  3. 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.

--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.

--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-ref-to

Allow REF_TO, REF(), and NULL syntax without enabling full 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-pointer-arithmetic

Allow arithmetic (+, -) and ordering comparisons (<, >, <=, >=) on REF_TO types. By default, only = and <> are permitted on references.

--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 vendor extension supported by CoDeSys, TwinCAT, RuSTy, and virtually every PLC runtime.

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.