ST Functions

Commercial

Included in the public demo Compare editions

Overview

Use built-in functions for calculations, limits, selection and explicit type conversion in a PLCProgram or user POU. A function returns a value; its local variables start fresh on every call. Use a function block when you need persistent state.

Parameters and results

Family	Functions	Parameters / result
Trigonometry	SIN, COS, TAN, ASIN, ACOS, ATAN	IN: numeric → REAL; angles in radians
Two-argument angle	ATAN2	IN1 (y), IN2 (x): numeric → REAL
Math	SQRT, EXP, LN, LOG	IN: numeric → REAL; LOG is base 10
Absolute value	ABS	IN: numeric → same type
Power	EXPT	IN1, IN2: numeric → widest input type
Extrema	MIN, MAX	IN1, IN2, …: numeric, at least two → widest type
Limit	LIMIT	MN, IN, MX: numeric → min(max(IN, MN), MX)
Selection	SEL	G: BOOL, IN0, IN1 → IN0 when FALSE, IN1 when TRUE
Multiplexer	MUX	K: INT, IN0, IN1, … → zero-based selected value
Truncation	TRUNC	IN: REAL → DINT, toward zero
Conversion	`<S>_TO_<T>`	All 25 pairs of BOOL, INT, DINT, REAL, TIME, including identity
Bits	SHL, SHR, ROL, ROR	IN: INT/DINT, N: INT/DINT → type of IN, 16/32-bit operation

Numeric inputs are INT, DINT or REAL; the widest type is REAL > DINT > INT. SEL/MUX values can also all be BOOL or all TIME. MUX requires at least two values, and K must be INT. TIME conversions use milliseconds.

Use either positional arguments or entirely named arguments: LIMIT(0.0, Raw, 1200.0) and LIMIT(MN := 0.0, IN := Raw, MX := 1200.0).

Conversions require the declared source type exactly: REAL_TO_INT(2.5) returns 3; REAL_TO_INT(-2.5) returns -3 (halves away from zero). Use TRUNC to truncate. BOOL targets use nonzero = TRUE, including NaN. There is no integer overflow wrap.

Quick start

Open a PLCProgram with Edit, enter this source, then Deploy and Run:

PROGRAM LimitDemand
VAR_EXTERNAL Raw : REAL; Demand : REAL; END_VAR
  Demand := LIMIT(MN := 0.0, IN := ABS(Raw), MX := 1200.0);
END_PROGRAM

Bind the external variables to model signals, using matching names or a signal pragma.

To define a reusable function, put this in a PLCLibrary:

FUNCTION Scale : REAL
VAR_INPUT value : REAL; factor : REAL; END_VAR
  Scale := value * factor;
  RETURN;
END_FUNCTION

Call Scale(value := Raw, factor := 2.0) in a program expression. All FUNCTION inputs are required each call. Assign the result to the function name; RETURN; exits with its current value. Only elementary parameters/results and local variables/constants are supported; a FUNCTION cannot instantiate FBs or use VAR_EXTERNAL, VAR_OUTPUT or VAR_IN_OUT.

Continuous simulation and DES

Built-in functions work in both simulation modes and return the same result for the same inputs. They have no state and no clock, so the DES check never flags a function call itself. What differs is how often the program that calls them scans:

	Continuous simulation	DES (event-driven)
Function available	All built-in and your own FUNCTIONs	All built-in and your own FUNCTIONs
When it is evaluated	On every scan, once per simulation tick	Only when the program scans: on start, when a signal it reads changes, when one of its timers runs out, or after its own change
Result	Same inputs give the same result	Same inputs give the same result
Between two evaluations	The result is at most one tick old	The result stays at its last value until the next event
DES check	Not applicable	A function call never triggers a finding; your own FUNCTION bodies are checked like program code

If a result must follow a value that changes between events, compute it from a signal that changes with it. A function does not wake a program in DES. For elapsed time, use TON, TOF or TP; see Writing ST for DES.

Use cases

Clamp an actuator demand with LIMIT.
Calculate geometry with SQRT, ATAN2 and trigonometric functions.
Select operating values with SEL/MUX.
Convert explicit numeric types before assigning a narrower value.