Retentive timer (RTO) behavior in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the retentive timer when the input is true.

PLC Programming

IF input_signal THEN
    RTO_Timer.EN := [1];
END_IF

Drag options to blanks, or click blank then click option'

AFALSE

BRESET

CDONE

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Setting EN to FALSE stops the timer.

Using RESET or DONE as enable values is incorrect.

The retentive timer enable (EN) must be set to TRUE to start timing when the input signal is active.

2fill in blank

medium

Complete the code to reset the retentive timer when the reset button is pressed.

PLC Programming

IF reset_button THEN
    RTO_Timer.ACC := [1];
END_IF

Drag options to blanks, or click blank then click option'

ARTO_Timer.EN

B0

CRTO_Timer.PRE

DRTO_Timer.DN

Attempts:

3 left

💡 Hint

Common Mistakes

Setting ACC to PRE (preset) does not reset the timer.

Changing EN or DN does not reset the accumulated time.

To reset the accumulated time of the retentive timer, set ACC (accumulated value) to 0.

3fill in blank

hard

Fix the error in the code to correctly check if the retentive timer is done.

PLC Programming

IF RTO_Timer.[1] THEN
    output_signal := TRUE;
END_IF

Drag options to blanks, or click blank then click option'

ADN

BACC

CEN

DPRE

Attempts:

3 left

💡 Hint

Common Mistakes

Checking ACC instead of DN causes errors.

Using EN or PRE does not indicate timer completion.

The DN (done) bit indicates when the retentive timer has reached its preset time.

4fill in blank

hard

Fill both blanks to create a retentive timer that starts when start_button is pressed and resets when reset_button is pressed.

PLC Programming

RTO_Timer.EN := [1];
IF reset_button THEN
    RTO_Timer.ACC := [2];
END_IF

Drag options to blanks, or click blank then click option'

Astart_button

B0

CFALSE

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using TRUE or FALSE directly instead of the start button for enable.

Resetting ACC to TRUE or FALSE instead of zero.

The timer enable is controlled by the start button, and the accumulated time resets to zero when reset_button is pressed.

5fill in blank

hard

Fill all three blanks to create a retentive timer that increments when input_signal is true, resets on reset_signal, and sets done_flag when timer is done.

PLC Programming

RTO_Timer.EN := [1];
IF reset_signal THEN
    RTO_Timer.ACC := [2];
END_IF
IF RTO_Timer.[3] THEN
    done_flag := TRUE;
END_IF

Drag options to blanks, or click blank then click option'

Ainput_signal

B0

CDN

DFALSE

Attempts:

3 left

💡 Hint

Common Mistakes

Using FALSE instead of input_signal for enable.

Resetting ACC to FALSE instead of zero.

Checking EN or ACC instead of DN for done_flag.

The timer enables when input_signal is true, resets ACC to zero on reset_signal, and sets done_flag when DN (done) bit is true.

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