MELSEC Tutorial 07 – Ladder Logic Programming: Data Transfer

In ladder logic programming, data transfer instructions are crucial for moving data between devices, memory locations, or registers in a MELSEC PLC system. These instructions are foundational for creating dynamic and efficient control systems that require seamless communication and data management. This tutorial will guide you through the concepts, implementation, and best practices for using data transfer instructions in GX Works3.

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Understanding Data Transfer in Ladder Logic

Data transfer instructions allow you to:

1. Move data between two locations, such as registers (D, W) or counters.
2. Exchange data with external devices connected via networks.
3. Prepare variables for further processing in mathematical or logical operations.

Key instructions in MELSEC ladder logic include:

• MOV (Move): Transfers a single value from one location to another.
• BMOV (Block Move): Transfers a block of consecutive data.
• ZRST (Zero Reset): Resets a block of registers to zero.

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Step 1: Setting Up Data Registers

Before transferring data, identify the source and destination registers:

1. Open GX Works3 and access the Device/Label Editor.
2. Define the source register (e.g., D0) and destination register (e.g., D10).
3. Specify initial values for testing, if required.

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Step 2: Implementing the MOV Instruction

Example: Simple Data Transfer

Scenario: Transfer the value 123 from data register D0 to register D10.

1. Insert the MOV instruction in the ladder logic editor.
2. Specify D0 as the source and D10 as the destination.

Ladder Logic Code:

 Condition --] [-- MOV D0 D10 --( )--

When the condition is true, the value in D0 will transfer to D10.

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Step 3: Using BMOV for Block Data Transfer

Scenario: Transfer five consecutive values starting at D20 to registers starting at D50.

1. Use the BMOV instruction.
2. Specify D20 as the starting source address and D50 as the starting destination address.
3. Set the number of words to transfer as K5.

Ladder Logic Code:

Condition --] [-- BMOV D20 D50 K5 --( )--

This will move values from D20-D24 to D50-D54.

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Step 4: Resetting Data with ZRST

Scenario: Reset a block of registers starting from D30 for 10 words.

1. Insert the ZRST instruction.
2. Specify D30 as the starting address and K10 as the number of registers to reset.

Ladder Logic Code:

 Condition --] [-- ZRST D30 K10 --( )--

This resets D30-D39 to zero when the condition is true.

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Step 5: Monitoring Data Transfers

Using the Device Monitor:

1. Open the Device/Monitor tool in GX Works3.
2. Add the registers involved in data transfer (D0, D10, etc.) to the watch list.
3. Observe the values in real time to verify that data moves correctly.

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Advanced Tip: Conditional Data Transfers

For more complex scenarios, use comparison instructions to determine whether data should transfer.

Scenario: Transfer data from D40 to D50 only if the value in D40 exceeds 100.

1. Insert a comparison instruction (>) to check the condition.
2. Trigger the MOV instruction based on the comparison result.

Ladder Logic Code:

 D40 > K100 --] [-- MOV D40 D50 --( )--

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Conclusion