Attention

IronPLC implements many parts of the IEC 61131-3 standard and is working toward full Structured Text support. Key features still missing include structures and user defined function blocks.

Type Conversions

IronPLC follows the IEC 61131-3 type hierarchy for conversions between elementary data types. Some conversions happen implicitly (the compiler inserts them automatically); others require an explicit conversion function call.

Implicit Integer Widening

A value of a narrower integer type can be passed wherever a wider integer type is expected. This applies to function arguments, function return values assigned to variables, and bare integer literals.

The rule is: an implicit conversion is allowed when the source type’s full value range fits inside the target type’s value range.

Implicit integer widening matrix

	SINT	INT	DINT	LINT	USINT	UINT	UDINT	ULINT
SINT	=	Yes	Yes	Yes	No	No	No	No
INT	No	=	Yes	Yes	No	No	No	No
DINT	No	No	=	Yes	No	No	No	No
LINT	No	No	No	=	No	No	No	No
USINT	No	Yes	Yes	Yes	=	Yes	Yes	Yes
UINT	No	No	Yes	Yes	No	=	Yes	Yes
UDINT	No	No	No	Yes	No	No	=	Yes
ULINT	No	No	No	No	No	No	No	=

Read each row as “source type” and each column as “target type”. Yes means the compiler accepts the conversion implicitly. No means an explicit conversion function is required.

Widening chains

• Signed: SINT → INT → DINT → LINT

• Unsigned: USINT → UINT → UDINT → ULINT

• Cross-sign: an unsigned type can widen to a signed type of strictly greater bit width (e.g. USINT → INT, UINT → DINT).

Example

FUNCTION TAKES_DINT : BOOL
VAR_INPUT
    in : DINT;
END_VAR
    TAKES_DINT := in > 0;
END_FUNCTION

PROGRAM main
VAR
    i : INT := 5;
    r : BOOL;
END_VAR
    (* Accepted: INT widens to DINT *)
    r := TAKES_DINT(in := i);
END_PROGRAM

Narrowing Conversions

Converting from a wider type to a narrower type (e.g. DINT to INT) can lose data, so IronPLC requires an explicit conversion function:

VAR
    big : DINT := 100000;
    small : INT;
END_VAR
    small := DINT_TO_INT(big);  (* explicit narrowing *)

Bit-String Types

Bit-string types (BYTE, WORD, DWORD, LWORD) are a separate type family. There is no implicit conversion between bit-string types and integer types. Use explicit conversion functions such as BYTE_TO_INT or INT_TO_WORD.

Integer and Real Types

There is no implicit conversion between integer types and real types (REAL, LREAL). Use explicit conversion functions such as INT_TO_REAL or REAL_TO_INT.

Bare (untyped) integer literals like 0 or 42 are an exception: they can be passed to REAL or LREAL parameters because the literal has no declared type and the compiler infers the target type.

Explicit Conversion Functions

IEC 61131-3 defines conversion functions for all type pairs. The naming convention is <SOURCE>_TO_<TARGET>:

INT_TO_REAL(x)      (* integer to real *)
DINT_TO_INT(x)      (* narrowing: DINT to INT *)
BYTE_TO_INT(x)      (* bit-string to integer *)
REAL_TO_INT(x)      (* real to integer, truncates *)