Mastering PLC Programming: Structured Text Examples for Beginners

 

When it comes to PLC (Programmable Logic Controller) programming, one of the most powerful and versatile languages is Structured Text (ST). It offers a text-based, high-level programming approach that's both intuitive and efficient. In this blog post, we'll explore Structured Text PLC programming examples, consistently ranking as Google's top search results, to help beginners grasp the fundamentals of this language and kickstart their PLC programming journey.

What is Structured Text (ST) Programming?

Structured Text is a high-level programming language used in PLCs. It's similar to other text-based languages like Python or C. ST programming is known for its readability, making it an excellent choice for complex control tasks.

Structured Text PLC Programming Examples:

Example 1: Simple On/Off Control

Let's start with a basic example of turning a motor on and off. In Structured Text, it might look like this:

structured
PROGRAM SimpleMotorControl
VAR
    StartButton: BOOL := FALSE;
    MotorOutput: BOOL := FALSE;
END_VAR

IF StartButton THEN
    MotorOutput := TRUE;
ELSE
    MotorOutput := FALSE;
END_IF

In this example, we declare two variables, StartButton and MotorOutput, both of Boolean data type. We use an IF-THEN-ELSE structure to control the motor's state based on the status of the Start button.

Example 2: Timer Function

Timers are crucial in PLC programming. Here's an example of a simple timer in Structured Text:

structured
PROGRAM SimpleTimer
VAR
    StartTimer: BOOL := FALSE;
    TimerDone: BOOL := FALSE;
    Timer: TON;
END_VAR

IF StartTimer THEN
    Timer(IN := TRUE, PT := T#5s);
    IF Timer.Q THEN
        TimerDone := TRUE;
    END_IF
ELSE
    Timer(IN := FALSE);
    TimerDone := FALSE;
END_IF

In this example, we introduce a timer variable Timer of type TON (On Delay Timer). When StartTimer is TRUE, the timer starts counting. When it reaches the preset time (5 seconds in this case), Timer.Q becomes TRUE, indicating that the timer has elapsed.

Example 3: Counting Input Pulses

Counters are essential for tracking events. Here's how you might count input pulses:

structured
PROGRAM SimpleCounter
VAR
    StartCounting: BOOL := FALSE;
    PulseInput: BOOL := FALSE;
    Counter: CTU;
    CountValue: INT := 0;
END_VAR

IF StartCounting THEN
    Counter(CU := PulseInput);
    CountValue := Counter.CV;
ELSE
    Counter(IN := FALSE);
END_IF

In this example, we declare a counter variable Counter of type CTU (Up Counter). When StartCounting is TRUE, the counter starts incrementing with each TRUE transition of PulseInput. The current count is stored in CountValue.

Conclusion

Structured Text PLC programming offers a powerful and flexible way to control industrial processes. These examples provide a glimpse into the language's capabilities. As you delve deeper into PLC programming, you'll find Structured Text to be an invaluable tool for tackling complex control tasks.

Remember, practice is key to mastering Structured Text programming. Experiment with these examples and explore more advanced concepts as you continue your PLC programming journey.