How to Write Structured Text Program for PLC - Simple Guide

Write a structured text program for PLC by using VAR to declare variables, IF statements for logic, and END_IF to close conditions; for example: IF Start AND NOT Stop THEN Motor := TRUE; ELSE Motor := FALSE; END_IF;.

Examples

InputStart = TRUE

OutputMotor = TRUE

InputStart = FALSE

OutputMotor = FALSE

InputStart = TRUE, Stop = TRUE

OutputMotor = FALSE

How to Think About It

To write a structured text program for a PLC, first decide what inputs and outputs you need. Use VAR to declare these variables. Then write simple logic using IF and ELSE statements to control outputs based on inputs. End each block with END_IF to keep the program clear and structured.

Algorithm

1

Declare input and output variables.

2

Check input conditions using IF statements.

3

Set output variables based on conditions.

4

Close each IF block with END_IF.

5

Repeat for all control logic needed.

Code

plc_programming

VAR
  Start : BOOL;
  Stop : BOOL;
  Motor : BOOL;
END_VAR

IF Start AND NOT Stop THEN
  Motor := TRUE;
ELSE
  Motor := FALSE;
END_IF;

// Output the Motor state
// (In real PLC, Motor controls a device)

Output

Motor = TRUE if Start=TRUE and Stop=FALSE; otherwise Motor = FALSE

Dry Run

Let's trace Start=TRUE and Stop=FALSE through the code

1

Check condition

Start=TRUE, Stop=FALSE, condition Start AND NOT Stop is TRUE

2

Set Motor

Motor := TRUE

3

Output result

Motor = TRUE

Start	Stop	Condition	Motor
TRUE	FALSE	TRUE	TRUE
FALSE	FALSE	FALSE	FALSE
TRUE	TRUE	FALSE	FALSE

Why This Works

Step 1: Variable Declaration

Use VAR block to declare all inputs and outputs so PLC knows what data it handles.

Step 2: Logic Condition

The IF statement checks if Start is TRUE and Stop is FALSE to decide Motor state.

Step 3: Output Assignment

Based on the condition, Motor is set to TRUE or FALSE, controlling the device connected.

Alternative Approaches

Using CASE statement

plc_programming

VAR
  Command : INT;
  Motor : BOOL;
END_VAR

CASE Command OF
  1: Motor := TRUE;
  2: Motor := FALSE;
ELSE
  Motor := FALSE;
END_CASE;

CASE is useful for multiple discrete commands but less flexible for complex conditions.

Using Boolean expressions directly

plc_programming

VAR
  Start : BOOL;
  Stop : BOOL;
  Motor : BOOL;
END_VAR

Motor := Start AND NOT Stop;

Direct assignment is simpler and efficient for straightforward logic without IF blocks.

Complexity: O(1) time, O(1) space

Time Complexity

The program runs in constant time because it only evaluates fixed logical conditions without loops.

Space Complexity

Uses constant space for declared variables; no dynamic memory allocation.

Which Approach is Fastest?

Direct Boolean assignment is fastest and simplest; IF statements add clarity but slightly more code.

Approach	Time	Space	Best For
IF statements	O(1)	O(1)	Clear logic with multiple conditions
CASE statement	O(1)	O(1)	Multiple discrete commands
Direct Boolean assignment	O(1)	O(1)	Simple true/false logic