ST Function Library

Complete reference for all built-in functions available in the UniLogic ST editor.

Functions are grouped by category. All functions accept a single parameter of any compatible primitive type unless otherwise noted.

I want to...	Use these functions
Convert a value from one type to another	TO_INT, INT(x), REAL_TO_INT, MOVE
Do math on numbers	ABS, SQRT, LN, LOG, EXP, SIN, COS, TAN, ASIN, ACOS, ATAN
Choose between values based on a condition	SEL, MAX, MIN, LIMIT, MUX
Work with text strings	LEN, LEFT, RIGHT, MID, FIND, CONCAT, INSERT, DELETE, REPLACE
Convert or read date and time values	DT_TO_UDINT, UDINT_TO_DT, TO_TIME, TO_DT
Do advanced math on numbers	SINH, COSH, TANH, DEG, RAD
Work with numeric tags, ranges, and structs	RESET_NUMERIC, IN_RANGE, COPY_STRUCT, COMPARE_STRUCT

Converting Values Between Types

Use type conversion when you need to move a value from one data type to another - for example, converting a REAL sensor reading to an INT for display, or converting a BOOL flag to a numeric value.

Three conversion styles are supported and produce identical results. Use whichever style you prefer or that matches your existing code:

Style	Example	When to use
*TO_ function**	TO_INT(myReal)	Original UniLogic style
IEC cast	INT(myReal)	Common in IEC environments
Source-to-target	REAL_TO_INT(myReal)	Explicit - makes the source type clear at a glance

REAL to INT conversion truncates toward zero in all three styles: INT(3.9)=3, not 4.
If you need the legacy rounding behavior, use TO_INT_UNI_.

Function	Returns	Notes
TO_REAL / REAL	REAL	Any numeric -> REAL
TO_DINT / DINT	DINT	Truncates toward zero
TO_INT / INT	INT	Truncates toward zero
TO_SINT / SINT	SINT	Truncates toward zero
TO_UDINT / UDINT	UDINT	Truncates toward zero
TO_UINT / UINT	UINT	Truncates toward zero
TO_USINT / USINT	USINT	Truncates toward zero
TO_BOOL / BOOL	BOOL	0 -> FALSE, anything else -> TRUE
TO_TIME / TIME	TIME	Compatible value -> TIME duration
TO_DT / DT	DATE_AND_TIME	Compatible value -> timestamp
MOVE (src)	Same as src	Copy with implicit conversion. Equivalent to the Store Ladder element

Math

All math functions accept any compatible numeric type (INT, DINT, REAL, etc.).

Function	Description
ABS	Absolute value
SQRT	Square root
LN	Natural logarithm
LOG	Base-10 logarithm
EXP	Exponential function (e^v)
SIN	Sine (radians)
COS	Cosine (radians)
TAN	Tangent (radians)
ASIN	Arc sine
ACOS	Arc cosine
ATAN	Arc tangent
SINH	Hyperbolic sine
TANH	Hyperbolic tangent
COSH	Hyperbolic cosine
DEG	Converts radians to degrees
RAD	Converts degrees to radians

Selection and Limiting

Function	Description
SEL (G, IN0, IN1)	Returns IN1 if G is TRUE, IN0 if FALSE. G is BOOL: IN0 and IN1 must be the same primitive type.
MAX (a, b)	Returns the larger of two values (same primitive type)
MIN (a, b)	Returns the smaller of two values (same primitive type)
LIMIT (MN, IN, MX)	Clamps IN between MN and MX. Equivalent to MIN(MAX(MN, IN),MX) All three must be the same primitive type
MUX (K, IN0, ..., INn)	Returns INK - the input at index K. Up to 32 inputs. See MUX topic for full reference.

Numeric and Struct Operations

These functions provide ST access to selected Ladder elements. They use ST function-call syntax and are compiled to the corresponding Ladder operation.

Function	Description
RESET_NUMERIC	Resets a numeric tag. Accepts the same operands as the Reset Numeric Ladder element.
IN_RANGE	Returns TRUE when value is within the specific range. Equivalent to the Within Range Ladder element.
COPY_STRUCT (src, dst)	Copies one struct to another. src and dst must be structs of the same type.
COMPARE_STRUCT (struct1, struct2)	Compares two structs of the same type and returns TRUE when they are equal. Can be used in Boolean expressions, including IF conditions.

Strings

String functions use 1-based indexing; position 1 is the first character.
The position (P) and length (L) parameters accept tag values, not just constants.

All string functions support mixed string types. You can freely mix STRING, STRING8, STRING16, STRING32, and WSTRING.

Function	What it does	Example
LEN (IN)	Returns the length of the string	LEN("hello")->5
LEFT (IN, L)	First L characters	LEFT ("hello", 3) -> "hel"
RIGHT (IN, L)	Last L characters	RIGHT ("hello", 3) -> "llo"
MID (IN, L, P)	L characters starting at position P	MID ("hello", 2, 2) -> "el"
FIND (IN1, IN2)	Position of IN2 in IN1. Returns -1 if not found.	FIND("hello", "ll") -> 3
CONCAT (IN1, IN2)	Join two strings. Also: IN1 + IN2	CONCAT("hi", "world") -> "hi world"
INSERT (IN1, IN2, P)	Inserts IN2 into IN1 at position P	INSERT("hllo", "e", "2")->"hello"
DELETE (IN, L, P)	Remove L characters starting at P.	DELETE (hello", 2, 2) -> "hlo"
REPLACE (IN1, IN2, L, P)	Replace L characters at P with IN2.	REPLACE("hello", "a", 1, 2)->"halo"

Mixing STRING with WSTRING

Conversion	Behavior
STRING -> WSTRING (widening)	No characters are lost
WSTRING -> STRING (narrowing)	Characters outside 0-255 become '?'

Date/Time Conversion

Use these functions to convert between the DATE_AND_TIME type and numeric values, or to convert between TIME and other types.

Function	Description
DT_TO_UDINT	Copies the UINT32 representation of a DATE_AND_TIME value
UDINT_TO_DT	Converts a UINT32 value to DATE_AND_TIME
TO_TIME	Converts a compatible value to TIME
TO_DT	Converts a compatible value to DATE_AND_TIME

String functions use 1-based indexing. Position 1 is the first character.