MELSEC Tutorial 10 – Ladder Logic Programming: Input Instructions

MELSEC Ladder logic programming depends on input instructions to define how a PLC interprets signals from external devices. These instructions form the foundation for controlling processes in industrial automation. In this tutorial, we will explore in MELSEC how to input instructions effectively in GX Works3, focusing on key instruction types such as contacts and coils.

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Understanding Input Instructions in Ladder Logic

Input instructions enable a PLC to monitor and respond to signals from field devices like sensors, switches, and buttons. The most common input instructions include:

• Normally Open (NO) Contact: Activates when the input signal is ON.
• Normally Closed (NC) Contact: Activates when the input signal is OFF.
• Coil Instruction: Controls outputs based on input logic.

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Step 1: Setting Up Input Devices

Connecting Field Devices

Ensure your input devices (e.g., push buttons, limit switches) are properly wired to the PLC’s input terminals. Use the system’s input wiring diagram to connect each device to its designated terminal.

Assigning Device Addresses

Assign appropriate device addresses (e.g., X0, X1) for the input terminals. These addresses correspond to the input signals in ladder logic.

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Step 2: Inputting Basic Instructions

Example 1: Using a Normally Open Contact

Scenario: Activate a motor (Y0) when a start button (X0) is pressed.

1. Open the ladder logic editor in GX Works3.
2. Insert a Normally Open (NO) Contact instruction for X0.
3. Add a coil instruction for Y0, connecting it to X0.

Ladder Logic Code:

--[ X0 ]--------------------( Y0 )--

When X0 (start button) is ON, the motor (Y0) turns ON.

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Example 2: Using a Normally Closed Contact

Scenario: Stop the motor (Y0) when a stop button (X1) is pressed.

1. Add a Normally Closed (NC) Contact instruction for X1.
2. Place the NC contact in series with the NO contact from Example 1.

Ladder Logic Code:

--[ X0 ]---|/| X1 ---------( Y0 )--

When X1 (stop button) is pressed, Y0 turns OFF.

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Example 3: Using Multiple Inputs

Scenario: Activate a conveyor belt (Y1) when both sensors (X2 and X3) detect items.

1. Insert two NO contact instructions for X2 and X3.
2. Connect the contacts in series to a coil instruction for Y1.

Ladder Logic Code:

--[ X2 ]---[ X3 ]---------( Y1 )--

The conveyor belt activates only when both X2 and X3 are ON.

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Step 3: Advanced Input Techniques

Example: Parallel Inputs for OR Logic

Scenario: Start a pump (Y2) when either of two buttons (X4 or X5) is pressed.

1. Add two NO contact instructions for X4 and X5.
2. Connect the contacts in parallel to a coil instruction for Y2.

Ladder Logic Code:

--[ X4 ]--|   |--[ X5 ]----( Y2 )--

The pump starts when either button is pressed.

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Latching an Output

Scenario: Keep a light (Y3) ON even after the button (X6) is released.

1. Insert an NO contact instruction for X6.
2. Add a coil instruction for Y3.
3. Create a feedback loop by inserting a parallel NO contact linked to Y3.

Ladder Logic Code:

--[ X6 ]--------------------( Y3 )--
   |                        |
   --[ Y3 ]------------------

The light remains ON until manually turned OFF.

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Step 4: Testing and Debugging

Use the Simulation Mode in GX Works3 to test your ladder logic program. Monitor input statuses and verify the output behavior in real-time to ensure all instructions work as intended.

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Conclusion