Step sequence with timers in PLC Programming

Introduction

A step sequence with timers helps control machines by moving through steps with delays, like waiting before starting the next action.

Starting a conveyor belt, then waiting before turning on a robot arm.

Turning on a pump, waiting for pressure to build, then opening a valve.

Lighting up warning signals one after another with pauses.

Automating a washing machine cycle with timed steps.

Syntax

PLC Programming

Step1:
  Start Timer1 (preset time)
  When Timer1 done, go to Step2
Step2:
  Start Timer2 (preset time)
  When Timer2 done, go to Step3
...
Repeat or stop

Each step starts a timer and waits for it to finish before moving on.

Timers count time in seconds or milliseconds depending on PLC settings.

Examples

This sequence waits 5 seconds, then 3 seconds, then stops.

PLC Programming

Step1:
  Start Timer1 (5 seconds)
  When Timer1 done, go to Step2
Step2:
  Start Timer2 (3 seconds)
  When Timer2 done, stop sequence

This sequence cycles through three steps with timers, then repeats.

PLC Programming

Step1:
  Start Timer1 (2 seconds)
  When Timer1 done, go to Step2
Step2:
  Start Timer2 (4 seconds)
  When Timer2 done, go to Step3
Step3:
  Start Timer3 (1 second)
  When Timer3 done, repeat Step1

Sample Program

This PLC program moves through three steps. Step 1 waits 5 seconds, Step 2 waits 3 seconds, then Step 3 ends the sequence.

PLC Programming

VAR
  Step: INT := 1;
  Timer1: TON;
  Timer2: TON;
END_VAR

// Main program loop
CASE Step OF
  1: // Step 1
    Timer1(IN := TRUE, PT := T#5S);
    IF Timer1.Q THEN
      Timer1(IN := FALSE, PT := T#5S);
      Step := 2;
    END_IF
  2: // Step 2
    Timer2(IN := TRUE, PT := T#3S);
    IF Timer2.Q THEN
      Timer2(IN := FALSE, PT := T#3S);
      Step := 3;
    END_IF
  3: // Step 3
    // Sequence finished
    // Could reset or stop here
    ;
END_CASE

Important Notes

Timers usually have inputs: IN (start), PT (preset time), and output Q (done).

Make sure to reset timers (IN := FALSE) after they finish to avoid restarting immediately.

Use a CASE or IF structure to control steps clearly.

Summary

Step sequences use timers to wait between actions.

Each step starts a timer and moves on when the timer finishes.

This method helps automate machines with timed steps.

Practice

1. What is the main purpose of using timers in a step sequence in PLC programming?

easy

A. To wait a set time before moving to the next step

B. To immediately jump to the last step

C. To reset the entire program

D. To skip steps randomly

Solution

1. Step 1: Understand the role of timers in step sequences

   Timers are used to create delays between steps, ensuring each step runs for a set time.

2. Step 2: Identify the correct timer function

   The timer waits before allowing the sequence to move to the next step, controlling timing precisely.

3. Final Answer:

   To wait a set time before moving to the next step -> Option A

4. Quick Check:

   Timers delay steps = A [OK]

Hint: Timers pause steps to control sequence timing [OK]

Common Mistakes:

• Thinking timers skip steps
• Assuming timers reset the program
• Believing timers jump to last step

2. Which of the following is the correct way to start a timer in a PLC step sequence?

easy

A. StartTimer(T1, 5s)

B. TimerStart T1 5

C. TON T1, 5s

D. T1 = Timer(5)

Solution

1. Step 1: Recognize standard PLC timer syntax

   In many PLC languages, the TON (Timer ON Delay) instruction is used with a timer name and preset time.

2. Step 2: Compare options to standard syntax

   TON T1, 5s uses 'TON T1, 5s' which matches common PLC timer start syntax.

3. Final Answer:

   TON T1, 5s -> Option C

4. Quick Check:

   TON is standard timer start = C [OK]

Hint: Look for TON keyword to start timers [OK]

Common Mistakes:

• Using incorrect function names
• Missing TON keyword
• Wrong timer parameter format

3. Given this step sequence code snippet:

Step 1: Start TON timer T1 for 3 seconds
Step 2: When T1 done, move to Step 2 and start TON timer T2 for 2 seconds
Step 3: When T2 done, move to Step 3

What is the total time before Step 3 starts?

medium

A. 5 seconds

B. 2 seconds

C. 3 seconds

D. Cannot be determined

Solution

1. Step 1: Add timer durations sequentially

   Step 1 waits 3 seconds, then Step 2 waits 2 seconds before Step 3.

2. Step 2: Calculate total wait time

   Total time = 3 seconds + 2 seconds = 5 seconds.

3. Final Answer:

   5 seconds -> Option A

4. Quick Check:

   3s + 2s = 5s total [OK]

Hint: Add timer durations in sequence for total delay [OK]

Common Mistakes:

• Only counting one timer duration
• Confusing timer start with timer done
• Assuming timers run in parallel

4. You have a step sequence with timers but the sequence moves to the next step immediately without waiting. What is the most likely error?

medium

A. Timer is counting up correctly

B. Timer preset value is set to zero

C. Step sequence is missing

D. PLC power is off

Solution

1. Step 1: Identify why timer causes no delay

   If the timer preset is zero, it finishes instantly, causing no wait.

2. Step 2: Check other options

   Counting up correctly means timer works; missing sequence or power off would stop all operation.

3. Final Answer:

   Timer preset value is set to zero -> Option B

4. Quick Check:

   Zero preset = no delay [OK]

Hint: Check timer preset is not zero for delay [OK]

Common Mistakes:

• Ignoring timer preset value
• Assuming power off causes immediate step
• Confusing timer counting with preset

5. You want a step sequence that:
1. Starts motor for 4 seconds,
2. Waits 3 seconds with motor off,
3. Then starts motor again.

Which timer setup correctly implements this sequence?

hard

A. Step 1: TON T1, 4s; Step 2: TON T2, 3s; Step 3: Motor ON

B. Step 1: Motor ON; Step 2: TON T1, 4s; Step 3: TON T2, 3s; Step 4: Motor ON

C. Step 1: TON T1, 3s; Step 2: Motor ON; Step 3: TON T2, 4s

D. Step 1: Motor ON and TON T1, 4s; Step 2: Motor OFF and TON T2, 3s; Step 3: Motor ON

Solution

1. Step 1: Match motor state with timers per step

   Step 1 must run motor ON while timer T1 counts 4 seconds; Step 2 motor OFF with timer T2 for 3 seconds; Step 3 motor ON again.

2. Step 2: Verify options

   Only Step 1: Motor ON and TON T1, 4s; Step 2: Motor OFF and TON T2, 3s; Step 3: Motor ON correctly pairs motor ON/OFF states with timers in the right steps.

3. Final Answer:

   Step 1: Motor ON and TON T1, 4s; Step 2: Motor OFF and TON T2, 3s; Step 3: Motor ON -> Option D

4. Quick Check:

   Motor state matches timers per step = D [OK]

Hint: Pair motor ON/OFF with timers in each step [OK]

Common Mistakes:

• Not turning motor OFF during wait
• Mixing timer durations
• Skipping motor control in steps