Retentive timer (RTO) behavior in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

RTO Timer Accumulated Value After Pause

Consider a retentive timer (RTO) in a PLC program. The timer preset is set to 10 seconds. The timer input is ON for 6 seconds, then OFF for 4 seconds, and then ON again. What will be the accumulated time value when the timer input is ON again?

A 4 seconds

B 0 seconds

C 6 seconds

D 10 seconds

💡 Hint

Remember that a retentive timer keeps its accumulated value when the input goes OFF.

A retentive timer (RTO) keeps the accumulated time when the input goes OFF. So after 6 seconds ON, the accumulated value is 6 seconds. When the input goes OFF for 4 seconds, the timer does not reset. When the input goes ON again, the accumulated value remains 6 seconds and continues counting up.

🧠 Conceptual

intermediate

Difference Between TON and RTO Timers

Which statement correctly describes the main difference between a TON (Timer On Delay) and an RTO (Retentive Timer On) in PLC programming?

A TON retains accumulated time when input goes OFF; RTO resets accumulated time when input goes OFF.

B Both TON and RTO retain accumulated time when input goes OFF.

C Both TON and RTO reset accumulated time when input goes OFF.

D TON resets accumulated time when input goes OFF; RTO retains accumulated time when input goes OFF.

💡 Hint

Think about what happens to the timer's accumulated value when the input signal turns OFF.

A TON timer resets its accumulated time to zero when the input goes OFF. An RTO timer keeps (retains) the accumulated time even if the input goes OFF, allowing it to continue timing from where it left off when the input turns ON again.

🔧 Debug

advanced

Why Does the RTO Timer Not Reset?

A PLC program uses an RTO timer with a preset of 15 seconds. The timer input is turned ON for 10 seconds, then OFF for 5 seconds, then ON again. The timer never resets to zero. Which of the following is the most likely reason?

A The RTO timer retains accumulated time and does not reset automatically when input goes OFF.

B The timer preset is too high, so it never resets.

C The timer input signal is not connected properly.

D The timer resets automatically after 5 seconds OFF.

💡 Hint

Think about the behavior of retentive timers when the input signal changes.

An RTO timer is designed to keep its accumulated time when the input goes OFF. It only resets when a specific reset instruction is given. So the timer not resetting after the input goes OFF is expected behavior.

🚀 Application

advanced

Using RTO Timer to Measure Total ON Time

You want to measure the total time a machine has been running, even if it stops and starts multiple times. Which timer type and approach should you use in your PLC program?

A Use a TON timer and reset it every time the machine stops.

B Use an RTO timer and reset it only when total time measurement is complete.

C Use a TON timer without resetting it.

D Use a delay timer that resets automatically.

💡 Hint

Think about which timer keeps counting time across multiple ON/OFF cycles.

An RTO timer retains its accumulated time when the input goes OFF, so it can measure total run time across multiple start/stop cycles. Resetting it only when measurement is complete ensures accurate total time.

💻 Command Output

expert

RTO Timer Accumulated Value After Multiple Cycles

An RTO timer has a preset of 20 seconds. The timer input is ON for 7 seconds, OFF for 3 seconds, ON for 5 seconds, OFF for 2 seconds, then ON for 6 seconds. What is the accumulated time value at the end of the last ON period?

A 18 seconds

B 6 seconds

C 20 seconds

D 0 seconds

💡 Hint

Add all ON periods because RTO retains accumulated time during OFF periods.

The accumulated time is the sum of all ON periods: 7 + 5 + 6 = 18 seconds. The OFF periods do not reset the timer, so the accumulated value continues increasing.

Practice

1. What is the main feature of a Retentive Timer (RTO) in PLC programming?

easy

A. It counts down from a preset time to zero.

B. It resets automatically when the timer input turns off.

C. It remembers elapsed time even if the timer is paused or stopped.

D. It only works for on-delay timing.

Solution

1. Step 1: Understand Retentive Timer behavior

   Retentive timers keep track of elapsed time even when the timer is not running.

2. Step 2: Compare options with this behavior

   Only It remembers elapsed time even if the timer is paused or stopped. describes this feature correctly; others describe non-retentive timers or different timer types.

3. Final Answer:

   It remembers elapsed time even if the timer is paused or stopped. -> Option C

4. Quick Check:

   Retentive timer remembers time = A [OK]

Hint: Retentive timers keep time even when paused [OK]

Common Mistakes:

• Thinking RTO resets automatically
• Confusing RTO with countdown timers
• Assuming RTO only works when running

2. Which of the following is the correct syntax to reset a Retentive Timer (RTO) in a typical PLC program?

easy

A. RTO.ACC = 0

B. RTO.Reset()

C. RTO.EN = FALSE

D. RTO.TimerReset = TRUE

Solution

1. Step 1: Identify how to reset RTO timers

   Retentive timers are reset by setting their accumulated time (ACC) to zero manually.

2. Step 2: Match syntax options

   Only RTO.ACC = 0 correctly shows resetting ACC to zero; others are invalid or incorrect syntax.

3. Final Answer:

   RTO.ACC = 0 -> Option A

4. Quick Check:

   Reset RTO by ACC = 0 [OK]

Hint: Reset RTO by setting ACC to zero manually [OK]

Common Mistakes:

• Trying to reset with a method call
• Setting enable (EN) to false to reset
• Using non-existent properties like TimerReset

3. Given the following PLC code snippet using an RTO timer:

IF Start THEN
  RTO.EN := TRUE;
ELSE
  RTO.EN := FALSE;
END_IF;

// After 5 seconds running, Start goes FALSE for 3 seconds, then TRUE again

// What will be the accumulated time (RTO.ACC) after 10 seconds total?

medium

A. 5 seconds

B. 7 seconds

C. 10 seconds

D. 3 seconds

Solution

1. Step 1: Analyze timer enable periods

   The timer runs for 5 seconds (Start TRUE), then pauses for 3 seconds (Start FALSE), then runs again for 2 seconds (to reach 10 seconds total).

2. Step 2: Calculate accumulated time

   Since RTO is retentive, it keeps the 5 seconds from first run, then adds 2 seconds after restart, totaling 7 seconds.

3. Final Answer:

   7 seconds -> Option B

4. Quick Check:

   Elapsed + resumed time = 7 seconds [OK]

Hint: Add only running time intervals for RTO ACC [OK]

Common Mistakes:

• Assuming timer resets when stopped
• Counting total elapsed time instead of run time
• Ignoring retentive behavior

4. You have this RTO timer code snippet:

IF Start THEN
  RTO.EN := TRUE;
ELSE
  RTO.EN := FALSE;
  RTO.ACC := 0;
END_IF;

What is wrong with this code regarding RTO behavior?

medium

A. RTO.EN cannot be assigned directly; it causes syntax error.

B. EN should be set to FALSE only after ACC is reset.

C. The timer will never start because ACC is reset too late.

D. Resetting ACC inside ELSE resets timer every time Start is FALSE, losing retentive behavior.

Solution

1. Step 1: Understand retentive timer reset rules

   Retentive timers keep accumulated time until manually reset; resetting ACC on every ELSE disables retention.

2. Step 2: Analyze code effect

   Resetting ACC whenever Start is FALSE clears accumulated time, so timer never retains elapsed time.

3. Final Answer:

   Resetting ACC inside ELSE resets timer every time Start is FALSE, losing retentive behavior. -> Option D

4. Quick Check:

   Reset ACC only when needed to keep retention [OK]

Hint: Don't reset ACC on every disable to keep retention [OK]

Common Mistakes:

• Resetting ACC too often
• Confusing EN and ACC roles
• Assuming EN assignment causes errors

5. You want to track total machine run time over multiple start/stop cycles using an RTO timer. Which approach correctly implements this behavior?

hard

A. Enable RTO when machine runs, disable when stopped, reset ACC only during maintenance.

B. Enable RTO only once at program start, never disable or reset ACC.

C. Reset ACC every time machine stops to start fresh timing.

D. Use a non-retentive timer and add its time to a separate counter.

Solution

1. Step 1: Understand total run time tracking

   To track total run time, timer must accumulate elapsed time across multiple runs without resetting.

2. Step 2: Evaluate options for correct retention

   Enable RTO when machine runs, disable when stopped, reset ACC only during maintenance. enables timer only when running and resets ACC only manually (e.g., maintenance), preserving total time.

3. Step 3: Why others are incorrect

   Enable RTO only once at program start, never disable or reset ACC. never disables timer, which is impractical; C resets ACC losing total time; D uses wrong timer type.

4. Final Answer:

   Enable RTO when machine runs, disable when stopped, reset ACC only during maintenance. -> Option A

5. Quick Check:

   Manual ACC reset preserves total run time [OK]

Hint: Reset ACC only when you want to clear total time [OK]

Common Mistakes:

• Resetting ACC on every stop
• Leaving timer always enabled
• Using non-retentive timers for total time