Mitsubishi PLC Self-Resetting Timer Tutorial for Beginners

In this tutorial, you'll learn how to create and use a self-resetting timer in a Mitsubishi PLC. We'll use the MELSEC series PLCs and the GX Works2/GX Developer programming environment. Self-resetting timers are useful in automation systems for creating repetitive operations such as blinking lights, periodic alarms, or process controls.

Step 1: Understanding Timers in Mitsubishi PLC

Mitsubishi PLCs provide On-Delay Timers (T), which turn ON after a preset time. The basic format of a timer is:

Txxx Kyyy

• Txxx: Timer number (e.g., T0, T1, etc.)
• Kyyy: Preset value in milliseconds (e.g., K1000 = 1 second).

Step 2: Goal of Self-Resetting Timer

A self-resetting timer automatically resets itself after reaching its preset time, starting the timing process again. This is achieved using:

1. A timer to count the preset time.
2. A Reset (R) command or logic that automatically resets the timer.

Step 3: Writing the Ladder Logic

1. Open GX Works2 or GX Developer:
• Create a new project and select your Mitsubishi PLC model.
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2. Define the Timer Circuit:
• Add a timer instruction that turns ON after a specific time interval.
• Use its output to reset itself, creating a loop.
3. Ladder Diagram Example:

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|------[X0]--------------(T0 K1000)------|

|       (Timer ON Delay, 1 sec)          |

|                                        |

|------[T0]--------------[/R T0]---------|

|       (Timer Output Resets T0)         |

Explanation:

• X0: Input switch to start the process.
• T0: Timer with a preset value of 1 second (K1000).
• [/R T0]: Reset instruction that resets the timer when it reaches the preset value.
3. Optional Enhancements:
• Add an output device (e.g., Y0) that activates every time the timer resets.

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|------[T0]--------------(Y0)------------|

|       (Output turns ON with T0)        |

Step 4: Testing the Program

1. Download the Program:
• Connect your PLC via USB/Ethernet.
• Download the program into the PLC.
2. Run the PLC:
• Switch the PLC to RUN mode.
3. Test the Logic:
• Activate the input (X0).
• Observe the output (Y0) blinking every second, indicating the timer resets and restarts.

Step 5: Debugging and Adjustments

• Change Timing: Modify the preset value (e.g., K2000 for 2 seconds).
• Add Conditions: Use additional logic to control when the timer operates.

Practical Application Example: Blinking Light

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|------[X0]--------------(T0 K500)---------|

|------[T0]--------------(Y0)--------------|

|------[T0]--------------[/R T0]-----------|

• X0: Starts the blinking process.
• T0: Timer with a 0.5-second preset value.
• Y0: Light output blinks every 0.5 seconds.

Conclusion

Self-resetting timers in Mitsubishi PLCs are straightforward to implement and powerful for repetitive tasks. By using the timer (T) and reset (/R) commands, you can automate processes effectively. Experiment with different timer settings and applications to deepen your understanding.