Retentive Timer On (RTO) in PLC Programming

• A Retentive Timer On (RTO) is a crucial element in PLC programming used to track elapsed time across multiple cycles of a process, even when the process stops and starts intermittently. 
• Unlike non-retentive timers, RTO timers retain their accumulated value when the rung condition transitions from true to false. 
• This feature is particularly useful for applications where cumulative time tracking is necessary.

What is Retentive Timer On (RTO)?

An RTO (Retentive Timer On) timer in a PLC is a timer that retains its accumulated value during power interruptions or when the PLC stops. When re-enabled, it resumes counting from the last accumulated value rather than resetting to zero.

Components of an RTO Timer

1. Preset Value (PV): The target time value to which the timer counts. It is set by the programmer and represents the duration after which a specific action should occur.
2. Accumulated Value (AV): The current count of the timer. It increments as long as the rung condition is true and retains its value when the rung goes false.
3. Enable Bit (EN): Indicates whether the timer is enabled. This bit is set when the rung conditions are true.
4. Timer Timing Bit (TT): Indicates that the timer is currently counting. This bit is true when the accumulated value is less than the preset value and the rung conditions are true.
5. Done Bit (DN): Indicates that the timer has reached the preset value. This bit is set when the accumulated value equals the preset value.

How the RTO Works

1. Rung True: When the rung conditions (input conditions) are true, the timer starts incrementing its accumulated value.
2. Incrementing: The accumulated value continues to increase as long as the rung remains true.
3. Rung False: When the rung conditions go false, the timer stops incrementing, but the accumulated value is retained.
4. Restarting: If the rung conditions become true again, the timer continues incrementing from the retained accumulated value until it reaches the preset value.
5. Reset: The accumulated value can only be reset to zero through a specific reset instruction or by restarting the cycle by enabling the timer start bit after reaching the preset value.

Practical Example: Conveyor System Maintenance Inspection Alarm

• Consider a conveyor system that requires maintenance inspection after running for a total of 100 hours. 
• The RTO timer can be used to accumulate the runtime of the conveyor, even if the conveyor stops and starts multiple times.
• Once the accumulated runtime reaches 100 hours, an alarm is triggered to notify the maintenance team.

Requirements

• Start/Stop Conveyor(%I0.0)/(%I0.1): Button or signal to start and stop the conveyor.
• RTO Timer(%TM0): To track the cumulative runtime of the conveyor.
• Maintenance Alarm(%Q0.1): To alert when the conveyor has run for 100 hours.
• Reset(Start) Button(%I0.0): To reset the accumulated time after maintenance is performed.

Ladder Logic Diagram – Retentive Timer On (RTO)

The below ladder  logic created in Schneider EcoStruxure Machine Expert – Basic for a conveyor system maintenance inspection alarm that triggers after 100 hours of operation using a Retentive Timer On (RTO).

Click here for Step-by-Step Procedure for Creating a Ladder Diagram from Logic with Schneider Electric EcoStruxure Machine Expert Basic Software

Explanation

1. Conveyor Running(%Q0.0): This input condition represents the conveyor running status (true when the conveyor is running).
2. RTO Timer1:
   • Preset: Set to 6000 with base unit in minutes  (100 hours).
   • Accumulated Value: Starts from zero and increments while the conveyor is running.
   • Enable (EN) Bit(%Q0.0): Set when the conveyor is running.
   • Timer Timing (TT) Bit: Indicates the timer is active while accumulating time.
   • Done (DN) Bit(%TM0.Q): Set when the accumulated time reaches the preset value (100 hours).
3. Maintenance Alarm(%Q0.1): Activated when Timer(TM0) Done bit (%TM0.Q) is set, indicating the conveyor has run for 100 hours and maintenance is due.
4. Reset(Start) Button(%I0.0): When pressed, it resets Timer(TM0) accumulated value to zero, restarting the timing process after maintenance is completed.

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Practical Tips for Implementation – Retentive Timer On (RTO) in PLC Programming

• Ensure the reset button is used appropriately after maintenance to avoid missed inspections.
• Implement a clear procedure for responding to the maintenance alarm to ensure timely inspections.
• Ensure the PLC has sufficient memory to handle retentive timers, as they store values even when power is lost.
• Clearly document the ladder logic and the purpose of the RTO timer for maintenance staff and future programmers.

Click here for Designing 2 out of 4 Voting Logic in Control Systems: A Step-by-Step PLC Ladder Diagram Tutorial with Video

Retentive Timer On (RTO) is an essential tool in PLC programming for applications requiring cumulative time tracking across multiple cycles. The example of a conveyor system maintenance inspection alarm illustrates its practical use, ensuring timely maintenance and enhancing the reliability of the conveyor system.

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