Retentive timer (RTO) behavior in PLC Programming

Introduction

A retentive timer (RTO) helps keep track of time even if the timer is stopped or paused. It remembers how much time has passed so you don't lose progress.

You want to count how long a machine has been running, even if it stops and starts again.

You need to pause a timer during a break and continue timing after the break.

You want to accumulate total run time over multiple sessions without resetting.

You want to trigger an action after a total time, even if interrupted.

You want to monitor downtime or delays that happen in parts.

Syntax

PLC Programming

RTO TimerName, PresetTime, AccumulatedTime, Enable

TimerName is the label for the timer.

PresetTime is the target time to reach.

AccumulatedTime keeps the current counted time and does not reset when Enable is false.

Enable starts or pauses the timer counting.

Examples

Timer1 counts up to 5000 ms when StartSignal is true. AccTime1 keeps the time counted so far.

PLC Programming

RTO Timer1, T#5000ms, AccTime1, StartSignal

MotorRunTimer counts while MotorRunning is true, remembering total run time in MotorRunAccum.

PLC Programming

RTO MotorRunTimer, T#10000ms, MotorRunAccum, MotorRunning

Sample Program

This simple program shows how a retentive timer adds time only when enabled (StartSignal true). When paused, it keeps the accumulated time. When the preset time (500ms) is reached, TimerDone becomes true. Then the timer resets for next use.

PLC Programming

(* Example of RTO timer behavior in structured text *)
VAR
  StartSignal : BOOL := FALSE;
  TimerDone : BOOL := FALSE;
  AccTime : TIME := T#0ms;
END_VAR

(* Simulate timer with RTO behavior *)
IF StartSignal THEN
  AccTime := AccTime + T#100ms; (* Add 100ms each cycle while enabled *)
  IF AccTime >= T#500ms THEN
    TimerDone := TRUE;
  END_IF
ELSE
  (* Timer paused, AccTime keeps value *)
END_IF

(* Reset logic for demonstration *)
IF TimerDone THEN
  AccTime := T#0ms;
  TimerDone := FALSE;
END_IF

(* Simulation steps *)
(* Step 1: StartSignal = TRUE, timer counts up *)
(* StartSignal := TRUE; *)
(* After 5 cycles, AccTime = 500ms, TimerDone = TRUE *)

(* Step 2: StartSignal = FALSE, timer pauses, AccTime keeps value *)
(* StartSignal := FALSE; *)
(* AccTime stays at 500ms *)

Important Notes

Retentive timers retain the accumulated time even if the enable signal goes off, unlike normal timers that reset.

You must reset the accumulated time manually when you want to start fresh.

Retentive timers are useful for tracking total time over multiple start-stop cycles.

Summary

Retentive timers remember elapsed time even when paused.

They are useful for tracking total run or wait time across sessions.

You control when to reset the timer manually.

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