Step sequence with timers in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple step sequence timer logic

Given the following PLC ladder logic pseudocode for a step sequence with a timer, what will be the value of the output coil Step1_Active after 5 seconds?

Step1_Timer: TON(IN=Step1_Start, PT=3s)
Step1_Active = Step1_Timer.Q
Step2_Start = Step1_Timer.Q
Step2_Timer: TON(IN=Step2_Start, PT=4s)
Step2_Active = Step2_Timer.Q

A False (Step1_Active is OFF)

B True (Step1_Active is ON)

C Timer error, no output

D Step1_Active toggles every second

💡 Hint

Think about how the timer output Q behaves after the preset time PT.

The Step1_Timer has a preset time of 3 seconds. After 3 seconds, its output Q becomes True and remains True as long as IN=Step1_Start is True. After 5 seconds, Step1_Active is True.

💻 Command Output

intermediate

Timer output after step transition

In a step sequence, a timer TON is started when Step1 is active. The timer preset is 10 seconds. If Step1 is active for 7 seconds and then deactivated, what is the value of the timer output Q immediately after Step1 deactivates?

A Timer output Q toggles

B True (Q remains ON until reset)

C False (Q resets immediately when input IN is False)

D Timer holds last value of Q

💡 Hint

Remember how TON timers behave when input IN goes False before PT is reached.

A TON timer output Q is True only when the input IN has been True continuously for the preset time PT. If IN goes False before PT, Q resets immediately to False.

🔧 Debug

advanced

Identify the error in step sequence timer logic

Consider this PLC structured text snippet for a step sequence with timers:

IF Step1_Active THEN
  Step1_Timer(IN := TRUE, PT := T#5s);
ELSE
  Step1_Timer(IN := FALSE);
END_IF;

IF Step1_Timer.Q THEN
  Step2_Active := TRUE;
END_IF;

What is the main issue with this code regarding the step sequence behavior?

A Step2_Active never resets to FALSE after Step1_Timer.Q becomes TRUE

B Step2_Active is set before Step1_Timer.Q is valid

C Step1_Timer is never started because IN is not set properly

D Timer preset PT is missing units

💡 Hint

Check what happens to Step2_Active when Step1_Timer.Q goes FALSE again.

The code sets Step2_Active to TRUE when Step1_Timer.Q is TRUE but never resets Step2_Active to FALSE when Step1_Timer.Q becomes FALSE. This causes Step2_Active to remain TRUE indefinitely.

🚀 Application

advanced

Design a step sequence with two timers

You want to create a step sequence with two steps. Step1 activates a timer TON1 with a preset of 3 seconds. When TON1.Q is TRUE, Step2 activates a timer TON2 with a preset of 5 seconds. After TON2.Q is TRUE, the sequence resets to Step1. Which of the following pseudocode snippets correctly implements this logic?

A

Step1_Active := TRUE;
TON1(IN := Step1_Active, PT := T#3s);
Step2_Active := TON1.Q;
TON2(IN := Step2_Active, PT := T#5s);
IF TON2.Q THEN Step1_Active := NOT Step1_Active;

B

Step1_Active := TRUE;
TON1(IN := Step1_Active, PT := T#3s);
Step2_Active := TON1.Q;
TON2(IN := Step2_Active, PT := T#5s);
IF TON2.Q THEN Step1_Active := FALSE;

C

Step1_Active := TRUE;
TON1(IN := Step1_Active, PT := T#3s);
Step2_Active := TON1.Q;
TON2(IN := Step2_Active, PT := T#5s);
IF TON2.Q THEN Step1_Active := TRUE;

D

Step1_Active := NOT Step2_Timer.Q;
TON1(IN := Step1_Active, PT := T#3s);
Step2_Active := TON1.Q;
TON2(IN := Step2_Active, PT := T#5s);
IF TON2.Q THEN Step1_Active := TRUE;

💡 Hint

Think about how to reset Step1_Active after Step2 timer finishes to restart the sequence.

Option A toggles Step1_Active when TON2.Q becomes TRUE, effectively restarting the sequence. Other options either do not reset Step1_Active properly or keep it always TRUE or FALSE.

🧠 Conceptual

expert

Behavior of overlapping timers in step sequences

In a complex step sequence, two timers TON_A and TON_B are started simultaneously with different preset times (TON_A: 4s, TON_B: 6s). Step transition depends on both timers being done (Q = TRUE). If the input to TON_A is interrupted at 3 seconds and then restarted immediately, what is the expected behavior of TON_A output Q compared to TON_B?

A TON_A output Q remains TRUE; TON_B output Q resets

B TON_A output Q holds last value; TON_B resets as well

C Both timers reset and start timing from zero

D TON_A output Q resets and starts timing from zero; TON_B continues uninterrupted

💡 Hint

Consider how TON timers behave when input IN goes FALSE and then TRUE again.

A TON timer resets its accumulated time and output Q immediately when input IN goes FALSE. When input IN is TRUE again, it starts timing from zero. TON_B is uninterrupted, so it continues timing normally.

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