Data types: BOOL, INT, REAL, TIME and the rest

Why the type matters

In a PLC, memory is limited and time is counted, so every variable takes exactly its size. Choosing a type that is too small gives you a silent overflow: the counter reaches its limit and wraps to negative with no error at all.

Boolean and numeric types

Type	Size	Range	Used for
`BOOL`	1 bit	TRUE / FALSE	a contact, a valve, an alarm
`BYTE` `WORD` `DWORD`	8 / 16 / 32 bit	bit patterns	status words, masks
`SINT` `INT` `DINT`	8 / 16 / 32 bit	-128…127, -32768…32767, ±2.1 billion	counters, steps
`USINT` `UINT` `UDINT`	unsigned	0…255, 0…65535, 0…4.29 billion	raw counts
`REAL`	32 bit	±3.4×10³⁸, about 7 digits	measurements, maths
`LREAL`	64 bit	about 15 digits	precise maths
`TIME`	32 bit	a duration	`PT` on timers
`STRING`	by length	text	messages, batch names

Trap one: INT overflows

INT stops at 32767. A production counter of that type wraps to -32768 after 32767 parts. Use DINT for anything that accumulates.

Trap two: REAL is not exact

REAL is approximate: 0.1 + 0.2 is not exactly 0.3. So never compare two REALs for equality:

IF Level = 50.0 THEN   (* may never be true *)
IF ABS(Level - 50.0) < 0.1 THEN   (* do this instead *)

TIME

TIME is not an ordinary number; it is a duration with its own literal form:

T#500ms    T#2s    T#1m30s    T#2h15m

You cannot assign 5000 to a TIME variable; it has to be T#5s.

Choosing in practice

an analogue field signal → REAL
a state machine step → INT
a batch counter or running hours → DINT
anything on/off → BOOL

Try it in the lab

The lab runs in your browser: an ST editor, a virtual process that suits the lesson, the same program as a live ladder diagram, a watch table, and export to Siemens SCL.

What to try

Run the program, press START and watch Bar and Step in the watch table: Step keeps only the whole number of Bar.

Change Counter from DINT to INT and download again: it still works here, but on a real PLC it would wrap negative after 32767.

Try Delay : TIME := 5000; and the compiler refuses it: a duration is written T#5s.

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