Retentive timer (RTO) behavior in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Retentive Timer (RTO) in PLC programming?

A Retentive Timer (RTO) is a timer that keeps its accumulated time value even if the timer is turned off or the input condition is removed. It resumes counting from where it left off when re-enabled.

beginner

How does an RTO differ from a standard TON (On-Delay Timer)?

Unlike a TON timer that resets its accumulated time when the input is off, an RTO retains the accumulated time and continues from that value when the input turns on again.

beginner

What happens to the accumulated time of an RTO when the timer is reset?

When an RTO is reset, its accumulated time is cleared to zero, and the timer starts fresh when enabled again.

intermediate

Why is the retentive feature useful in industrial automation?

It allows processes to pause and resume timing without losing progress, useful in power interruptions or process stops where timing continuity is important.

beginner

In ladder logic, what input signals control an RTO timer?

An RTO timer typically has an enable input to start or pause timing and a reset input to clear the accumulated time.

What does an RTO timer do when its input turns off?

A Keeps the accumulated time and pauses counting

B Resets the accumulated time to zero

C Starts counting from zero again

D Stops and clears the timer output immediately

An RTO timer retains the accumulated time and pauses counting when the input is off.

How do you clear the accumulated time of an RTO timer?

A Turn off the enable input

B Disconnect power to the PLC

C Use the reset input signal

D Wait for the timer to finish

The reset input clears the accumulated time of an RTO timer.

Which timer type resumes timing from the last value after being paused?

A On-Delay Timer (TON)

B Retentive Timer (RTO)

C Off-Delay Timer (TOF)

D Pulse Timer (TP)

Only the Retentive Timer (RTO) keeps its accumulated time and resumes from there.

Why might you choose an RTO over a TON in a process?

A To keep timing progress through interruptions

B To avoid using reset signals

C To reset timing every time input changes

D To count down instead of up

RTOs keep timing progress even if the input is interrupted, useful for process continuity.

What happens if you power off the PLC with an active RTO timer?

A The accumulated time is lost

B The timer counts backwards

C The timer resets automatically

D The accumulated time is retained if memory is non-volatile

If the PLC has non-volatile memory, the RTO's accumulated time is retained through power loss.

Explain how a Retentive Timer (RTO) works and why it is useful in automation.

Think about what happens when the timer input turns off and on again.

Describe the difference between an RTO and a TON timer in simple terms.

Focus on what happens to the timer value when the input signal changes.

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