Why Step sequence with timers in PLC Programming? - Purpose & Use Cases

The Big Idea

What if your machine could run itself perfectly timed without you lifting a finger?

The Scenario

Imagine you are controlling a machine that needs to perform several actions one after another, each for a specific time. Doing this by pressing buttons manually or watching the clock is tiring and easy to mess up.

The Problem

Manually starting and stopping each step wastes time and can cause mistakes. You might forget to switch steps on time or press the wrong button, leading to machine errors or safety risks.

The Solution

Using a step sequence with timers lets the machine run each step automatically for the right amount of time. The program moves smoothly from one step to the next without needing you to watch or intervene.

Before vs After

✗ Before

IF button_pressed THEN start_step1
WAIT 10 seconds
stop_step1
start_step2
WAIT 5 seconds
stop_step2

✓ After

STEP 1: timer_10s -> STEP 2: timer_5s -> END

What It Enables

This lets machines run complex timed actions perfectly and safely, freeing you to focus on other tasks.

Real Life Example

In a bottling plant, the conveyor moves bottles, then a timer controls how long the filling valve stays open, then another timer controls the capping step, all automatically in sequence.

Key Takeaways

Manual timing is slow and error-prone.

Step sequences with timers automate timed actions.

This improves safety, accuracy, and efficiency.

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