FreertosHow-ToBeginner · 4 min read

How to Create Sequence Using Timer in PLC Programming

To create a sequence using a timer in a PLC, use timer instructions like TON (Timer On Delay) to delay transitions between steps. Each timer triggers the next step in the sequence after its preset time, enabling controlled automation steps.

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Syntax

In PLC programming, the common timer instruction for sequences is TON (Timer On Delay). It has these parts:

Timer Name: Identifier for the timer.
Input (IN): Boolean signal to start the timer.
Preset Time (PT): Duration to wait before timer finishes.
Elapsed Time (ET): Time counted so far (read-only).
Output (Q): Becomes true when elapsed time reaches preset.

structured_text

TON TimerName(IN:=StartSignal, PT:=T#5s, Q=>TimerDone, ET=>ElapsedTime);

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Example

This example shows a simple 3-step sequence using timers. Each step turns on an output for 3 seconds, then moves to the next step automatically.

structured_text

VAR
  Step1Timer : TON;
  Step2Timer : TON;
  Step3Timer : TON;
  Step : INT := 0;
  Output1, Output2, Output3 : BOOL := FALSE;
END_VAR

// Step 1: Start timer and output
IF Step = 0 THEN
  Step1Timer(IN := TRUE, PT := T#3s);
  Output1 := TRUE;
  IF Step1Timer.Q THEN
    Step := 1;
    Step1Timer(IN := FALSE, PT := T#3s);
    Output1 := FALSE;
  END_IF
END_IF

// Step 2: Start timer and output
IF Step = 1 THEN
  Step2Timer(IN := TRUE, PT := T#3s);
  Output2 := TRUE;
  IF Step2Timer.Q THEN
    Step := 2;
    Step2Timer(IN := FALSE, PT := T#3s);
    Output2 := FALSE;
  END_IF
END_IF

// Step 3: Start timer and output
IF Step = 2 THEN
  Step3Timer(IN := TRUE, PT := T#3s);
  Output3 := TRUE;
  IF Step3Timer.Q THEN
    Step := 0; // Loop back to step 0
    Step3Timer(IN := FALSE, PT := T#3s);
    Output3 := FALSE;
  END_IF
END_IF

Output

Output1 ON for 3 seconds, then OFF; Output2 ON for 3 seconds, then OFF; Output3 ON for 3 seconds, then OFF; sequence repeats.

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Common Pitfalls

Not resetting the timer input (IN) after the timer finishes causes the timer to keep running.
Forgetting to advance the step variable leads to the sequence freezing on one step.
Using timers without checking their Q output can cause premature or delayed step changes.

structured_text

(* Wrong: Timer input never reset, sequence stuck *)
Step1Timer(IN := TRUE, PT := T#3s);
IF Step1Timer.Q THEN
  Step := 1; // Step changes but timer keeps running
END_IF

(* Correct: Reset timer input to stop timing after done *)
Step1Timer(IN := Step = 0, PT := T#3s);
IF Step1Timer.Q THEN
  Step := 1;
END_IF

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Quick Reference

Use these tips for timer-based sequences:

Use TON timers to delay steps.
Control sequence steps with a step counter variable.
Reset timer inputs after completion.
Check timer Q output to move to next step.
Loop or stop sequence as needed.

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Key Takeaways

Use TON timers to create delays between sequence steps in PLC programs.

Control the sequence flow with a step variable and advance it when timers finish.

Always reset timer inputs after the timer completes to avoid stuck timers.

Check the timer's Q output to know when to move to the next step.

Loop sequences by resetting the step counter or stop as per your process needs.