Enabling Dialects and Features¶

IronPLC lets 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, a default set of extensions, and the behavior policies the platform documents. Individual --allow-* flags and --policy-* selections 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).

Selects: the default of every behavior policy (--policy-string-to-num-non-numeric reject, --policy-string-to-num-failure trap).

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, NULL, and the object-oriented keywords (EXTENDS, IMPLEMENTS, ABSTRACT, INTERFACE, METHOD, THIS, and SUPER). No extensions.

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

Selects: the default of every behavior policy (--policy-string-to-num-non-numeric reject, --policy-string-to-num-failure trap).

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-cross-family-conversion, --allow-int-literal-to-bit-string, --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.

Selects: --policy-string-to-num-non-numeric reject and --policy-string-to-num-failure zero — RuSTy rejects a string with trailing characters and never faults.

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-pointer-to, --allow-adr, --allow-persistent-var, --allow-ref-arithmetic, --allow-ref-stack-variables, --allow-ref-type-punning, --allow-int-to-bool-initializer, --allow-sizeof, --allow-cross-family-widening, --allow-cross-family-conversion, --allow-int-literal-to-bit-string, --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.

Selects: --policy-string-to-num-non-numeric ignore-trailing and --policy-string-to-num-failure zero — CODESYS stops parsing at the first invalid character and returns 0 for a string that is not valid in the target type.

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 / OR_ELSE short-circuit operators. Unlike codesys, it does not enable the REF_TO / REF() / NULL reference extensions: TwinCAT spells references REFERENCE TO (bound with REF=) and pointers POINTER TO (bound with ADR()), which this dialect enables instead via --allow-reference-to, --allow-pointer-to, and --allow-adr. 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-pointer-to, --allow-adr, --allow-persistent-var, --allow-int-to-bool-initializer, --allow-sizeof, --allow-cross-family-widening, --allow-cross-family-conversion, --allow-int-literal-to-bit-string, --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.

Selects: --policy-string-to-num-non-numeric ignore-trailing and --policy-string-to-num-failure zero, as codesys.

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-pointer-to

Allow the Beckhoff TwinCAT / CODESYS POINTER TO pointer type. A POINTER TO variable behaves like a REF_TO reference: reading or writing the target requires the explicit dereference operator (^), unlike REFERENCE TO, which dereferences implicitly. Pointers are bound with the ADR() operator (--allow-adr) or the REF()/NULL forms from --allow-ref-to. See Reference Types.

--allow-adr

Allow the ADR() address-of operator, which returns a typed pointer to a variable for assignment to a POINTER TO variable (--allow-pointer-to). Unlike TwinCAT’s untyped PVOID result, ADR(x) in IronPLC has the type POINTER TO typeof(x) and is type-checked against the destination pointer’s target type. Addresses of sub-objects (array elements, structure fields) and pointer arithmetic are not supported and are rejected with a diagnostic. See ADR.

--allow-persistent-var

Allow the Beckhoff TwinCAT / CODESYS PERSISTENT variable qualifier (VAR PERSISTENT / VAR_GLOBAL PERSISTENT). This is an extension, not part of the IEC 61131-3 standard.

--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 from a bit-string type to a strictly wider integer type. For example, passing a BYTE variable where an INT parameter is expected. This is an extension supported by CODESYS, TwinCAT, and RuSTy.

--allow-cross-family-conversion

Allow implicit conversion between UDINT and DWORD, in both directions. The two types share a 32-bit slot, so this converts rather than widens, and it is the one case where an integer converts implicitly to a bit-string. See Type Conversions. This is an extension supported by CODESYS, TwinCAT, and RuSTy.

--allow-int-literal-to-bit-string

Allow a bare integer literal where a bit-string type is expected. For example, passing 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 syntax: the bit form .%Xn (e.g., myByte.%X3 to access bit 3 of a BYTE, equivalent to the short form .n) and the byte, word, double word, and long word forms .%Bn, .%Wn, .%Dn, and .%Ln (e.g., myDword.%B2 to access byte 2 of a DWORD). See Bit Access.

--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 and OR_ELSE short-circuit boolean operators, a Beckhoff/CODESYS extension. Unlike plain AND and OR (which always evaluate both operands), AND_THEN only evaluates its right operand when the left operand is TRUE, and OR_ELSE only when the left operand is FALSE — commonly used to guard a dereference (ptr <> 0 AND_THEN ptr^ = 99). The spelling is preserved end to end: the operators are not normalized to AND/OR, since the short-circuit behavior is a real, externally-visible difference in TwinCAT/CODESYS. Compiled code branches around the right operand, so the guarded expression above never dereferences a null pointer.

Short-circuiting applies to BOOL operands. AND and OR are also the bit-string operators, and skipping an operand has no meaning for a bit-string result, so AND_THEN/OR_ELSE on bit-string operands evaluate both operands and produce the same value AND and OR produce.

--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. The flag applies in every position a string type can appear, including as an array element type (ARRAY[1..10] OF STRING(255)), a function return type, and a TYPE alias.

--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. The pDevice^.Delta example spells its pointer with REF_TO, which is a separate extension needing --allow-ref-to; not every dialect that enables one enables the other, and some spell the same declaration POINTER TO FB_Device. The per-dialect lists above are the reference for which flags a dialect enables.

--allow-fb-inheritance

Allow the IEC 61131-3:2013 object-oriented syntax: EXTENDS/IMPLEMENTS/ABSTRACT on FUNCTION_BLOCK declarations, INTERFACE declarations, METHOD declarations, and THIS/SUPER. Support beyond parsing varies by keyword — see Object Oriented Programming for what each one analyzes and executes today; the parts that are parsed but not yet analyzed produce problem P9999 rather than a parse error. Enabled by --dialect=iec61131-3-ed3, --dialect=rusty, --dialect=codesys, and --dialect=twincat.

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.

Selecting Behavior Policies¶

An extension is syntax the parser accepts. A behavior policy is different: it selects what a standard operation does where IEC 61131-3 leaves the result to the implementer and real platforms disagree. Each policy has a small set of documented alternatives, the strict default is the standard’s result (or a runtime error where the standard says “error”), and each dialect selects the alternative its platform documents.

Unlike a flag, which can only enable, a --policy-* selection replaces the dialect’s. Policies compose freely with each other and with any dialect, and the selected alternative is compiled into the program, so a .iplc file behaves the same on every runtime that runs it.

--policy-string-to-num-non-numeric: What STRING_TO_<numeric> treats as convertible when the string has characters that are not part of a numeric literal: reject (default), ignore-trailing, or ignore-surrounding.
--policy-string-to-num-failure: What STRING_TO_<numeric> does when the string is not convertible: trap (default, runtime error V4006) or zero.

The alternatives, their results, and the dialects’ selections are on Type Conversions. To compile a CODESYS program with the strict failure behavior:

ironplcc compile --dialect codesys --policy-string-to-num-failure trap -o main.iplc main.st