BOOL

**BOOL is a 1-bit data type** representing a logical TRUE or FALSE value. It is the most-used data type in PLC programming because every contact and coil in ladder logic, every comparison instruction result, and every digital I/O point is internally a BOOL.

**Range:** TRUE (1) or FALSE (0). Some IDEs accept the numeric values 0 and 1; others require the literal keywords. In Siemens TIA Portal: `TRUE`/`FALSE`. In Studio 5000 ladder: shown as 0 or 1 in monitor view.

**Memory:** 1 bit logically; physically packed into a byte (or larger word) by the compiler. You can rarely save memory by using multiple BOOLs vs a single BYTE — the compiler usually allocates the same.

**Common uses:** digital input states (e.g., `LimitSwitch_1`), digital output commands (`Motor1_Run`), permissives, interlock conditions, bits within status words.

**Syntax across major IDEs:** - Siemens SCL: `myFlag : BOOL := FALSE;` - Studio 5000 ST: `myFlag := 0;` - CODESYS / TwinCAT: `myFlag : BOOL := FALSE;`

**Common pitfalls:** comparing BOOL to integers (`IF myBool = 1` instead of `IF myBool`); reading output coils inside the same scan they were set (the bit reflects the previous scan value until end-of-scan); using BOOL where you needed an edge-triggered detection (use `R_TRIG`/`F_TRIG` instead).

**See also:** BYTE (8 BOOLs packed), R_TRIG (rising edge detection), DINT (integer counterpart for counters and math).