Data types

Every IEC 61131-3 variable has a type. The type controls storage size, value range, allowed operations, and which conversion calls are needed when the value is used somewhere expecting a different type.

Sub-pages

Elementary types — BOOL, BYTE, INT, REAL, TIME, STRING and their cousins. The building blocks.
Generic types ANY, ANY_NUM, ANY_INT, ANY_BIT, ANY_REAL appear in standard-library function signatures (e.g. ABS(IN: ANY_NUM): ANY_NUM). They cannot be used in user variable declarations. (Stub.)
Derived types — STRUCT, ARRAY, enumerations, type-safe aliases (TYPE … : … END_TYPE). (Stub.)

Type-strict philosophy

IEC 61131-3 is strongly typed and matiec is among the strictest implementations. Two key consequences:

No implicit conversion across families. INT + REAL is a type error; you write INT_TO_REAL(i) + r. INT vs UINT is a comparison error; you write i = UINT_TO_INT(u).
No “truthy” coercion. IF iCount THEN … is illegal — the condition must be an explicit BOOL: IF iCount > 0 THEN.

These rules trade convenience at the call site for predictability at run-time. PLC code outlives its writer and runs unattended; the type-strictness pays back when the next maintainer reads the code.

Quick reference: type widths

Type	Width	Range / values
`BOOL`	1 bit	`FALSE`, `TRUE`
`BYTE`	8 bits	bit string, 0..255 unsigned
`WORD`	16 bits	bit string, 0..65535 unsigned
`DWORD`	32 bits	bit string, 0..2³²-1 unsigned
`LWORD`	64 bits	bit string, 0..2⁶⁴-1 unsigned
`SINT`	8 bits signed	-128..127
`INT`	16 bits signed	-32 768..32 767
`DINT`	32 bits signed	-2³¹..2³¹-1
`LINT`	64 bits signed	-2⁶³..2⁶³-1
`USINT`	8 bits unsigned	0..255
`UINT`	16 bits unsigned	0..65 535
`UDINT`	32 bits unsigned	0..2³²-1
`ULINT`	64 bits unsigned	0..2⁶⁴-1
`REAL`	32 bits IEEE-754	~7 significant decimal digits
`LREAL`	64 bits IEEE-754	~15 significant decimal digits
`TIME`	impl-defined	duration, e.g. `T#1h30m`
`DATE`	impl-defined	calendar date, e.g. `D#2026-05-14`
`TIME_OF_DAY` / `TOD`	impl-defined	wall-clock time, e.g. `TOD#14:30:00`
`DATE_AND_TIME` / `DT`	impl-defined	timestamp, `DT#2026-05-14-14:30:00`
`STRING`	impl-defined	variable-length character string

IEC reference

IEC 61131-3 third edition (2013), clause 6.4 — “Data types”.

ForgeIEC notes

The ForgeIEC pool stores every variable’s IEC type as a string field. The MCP tool project.list_pool_variables returns it in the iec_type field of each entry.
The ForgeIEC ST code generator emits matiec-compatible IEC types verbatim — no remapping. A variable declared INT in the editor is INT to matiec.