Quick Start

This guide walks you through writing and running your first Structured Text program.

1. Create a Program

Create a file called hello.st:

PROGRAM HelloWorld
VAR
    counter : INT := 0;
    running : BOOL := TRUE;
END_VAR
    IF running THEN
        counter := counter + 1;
        IF counter > 100 THEN
            running := FALSE;
        END_IF;
    END_IF;
END_PROGRAM

2. Check for Errors

st-cli check hello.st

Output:

hello.st: OK

If there are errors, you’ll see them with file, line, and column:

hello.st:5:10: error: undeclared variable 'x'
hello.st:8:8: error: condition must be BOOL, found 'INT'

3. Run the Program

# Run a single scan cycle
st-cli run hello.st

# Run 1000 scan cycles (like a real PLC)
st-cli run hello.st -n 1000

Output:

Executed 1000 cycle(s) in 1.74ms (avg 1.74µs/cycle, 16 instructions)

4. Write a Function

Functions compute and return a value:

FUNCTION Clamp : REAL
VAR_INPUT
    value : REAL;
    low   : REAL;
    high  : REAL;
END_VAR
    IF value < low THEN
        Clamp := low;
    ELSIF value > high THEN
        Clamp := high;
    ELSE
        Clamp := value;
    END_IF;
END_FUNCTION

PROGRAM Main
VAR
    sensor : REAL := 150.0;
    output : REAL := 0.0;
END_VAR
    output := Clamp(value := sensor, low := 0.0, high := 100.0);
END_PROGRAM

5. Write a Function Block

Function blocks maintain state between calls:

FUNCTION_BLOCK Counter
VAR_INPUT
    reset : BOOL;
END_VAR
VAR_OUTPUT
    count : INT;
END_VAR
VAR
    internal : INT := 0;
END_VAR
    IF reset THEN
        internal := 0;
    ELSE
        internal := internal + 1;
    END_IF;
    count := internal;
END_FUNCTION_BLOCK

PROGRAM Main
VAR
    cnt : Counter;
    value : INT := 0;
END_VAR
    cnt(reset := FALSE);
    value := cnt.count;
END_PROGRAM

6. Use Global Variables

Global variables persist across scan cycles:

VAR_GLOBAL
    total_cycles : INT;
END_VAR

PROGRAM Main
VAR
    local_var : INT := 0;
END_VAR
    total_cycles := total_cycles + 1;
    local_var := total_cycles;
END_PROGRAM

# After 1000 cycles, total_cycles = 1000
st-cli run program.st -n 1000

Next Steps

• VSCode Setup — Get editor support with diagnostics, hover, and completion
• Language Reference — Full language documentation
• CLI Reference — All available commands