MELSEC Tutorial 04 – Create a Ladder Program

Industrial automation relies heavily on programming to implement control logic for machines and processes. Ladder Programming, the graphical representation of logic circuits, is widely used for configuring Mitsubishi Electric’s MELSEC PLCs. In this tutorial, we’ll cover the step-by-step process how to create a MELSEC ladder program. You’ll learn how to design and implement logic structures, incorporate advanced functions, and test your program effectively.

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What is Ladder Programming?

Ladder Logic mimics electrical relay circuits using graphical elements like contacts, coils, and rungs. Each rung represents a logical condition or operation, enabling simple and effective programming of industrial systems. It is especially popular for programming PLCs due to its intuitive design.

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Step 1: Setting Up MELSOFT GX Works3

Before starting your ladder program, ensure you have the necessary software and hardware setup.

Prerequisites:

• MELSEC PLC hardware: A configured and connected PLC.
• MELSOFT GX Works3: Mitsubishi’s programming software.
• Connection Cable: USB, Ethernet, or RS232, depending on your PLC model.

Installing the Software:

1. Download and install GX Works3 from the Mitsubishi Electric website.
2. Launch the software and connect your MELSEC PLC.
3. Create a new project and select your PLC model (e.g., iQ-F or iQ-R series).

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Step 2: Understanding Ladder Diagram Components

Basic Elements:

1. Contacts: Represent input conditions (e.g., a sensor or switch).
2. Coils: Represent output actions (e.g., a motor or alarm).
3. Timers and Counters: Used for time-based and count-based logic.

Logical Structures:

• AND Logic: Two or more contacts in series.
• OR Logic: Two or more contacts in parallel.
• Latch Circuits: Maintain the state of an output until explicitly reset.

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Step 3: Create the MELSEC ladder program

Start with the Input and Output Definitions:

1. Open the Ladder Editor in GX Works3.
2. Define the input devices (e.g., start button, stop button) and assign them to input addresses.
3. Define the output devices (e.g., motor, light) and assign them to output addresses.

Write the Ladder Logic:

Example: Motor Control Logic

Create a circuit to turn on a motor when a start button is pressed and turn it off with a stop button.

1. Add a normally open contact for the start button.
2. Add a normally closed contact for the stop button.
3. Connect these to a coil representing the motor.

plaintextCopy code  Start Button --] [-- Stop Button --]/[-- Motor Coil --( )--

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Step 4: Adding Advanced Functions

Using Timers:

1. Select a TON (On-delay Timer) from the function block library.
2. Configure the timer with a specific delay (e.g., 5 seconds).
3. Connect the timer to the input and output for time-dependent operations.

Timer Example:

• A conveyor belt starts after a 5-second delay when a sensor is activated.

plaintextCopy code  Sensor --] [-- Timer (TON) -- Conveyor --( )--

Using Counters:

1. Add a Counter Block (CTU) to count events like part detection.
2. Configure the counter with a preset value (e.g., count 10 parts before activating an output).

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Step 5: Testing and Debugging the Ladder Program

Testing ensures your program works as intended.

1. Simulate the Program:
   • Use the GX Works3 simulator to test logic without connecting to hardware.
   • Monitor the virtual inputs and outputs for proper functionality.
2. Load the Program:
   • Download the ladder logic to the PLC.
   • Connect physical inputs (e.g., buttons, sensors) and verify that outputs (e.g., motors, alarms) respond correctly.
3. Debug Errors:
   • Use the debugging tools in GX Works3 to identify and fix issues.
   • Check the wiring and input/output addresses if problems persist.

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Conclusion