Move instruction (MOV) in PLC Programming

Introduction

The Move instruction (MOV) copies a value from one place to another. It helps transfer data inside a PLC program without changing the original value.

Copy a sensor reading to a memory location for later use.

Transfer a setpoint value from an input to a control register.

Save a calculated result into a different register for output.

Initialize a variable with a fixed value at the start of a program.

Move data between registers during a sequence of operations.

Syntax

PLC Programming

MOV source, destination

The source can be a constant, input, or register.

The destination is usually a register or memory location.

Examples

Moves the constant value 100 into register D0.

PLC Programming

MOV 100, D0

Copies the value from input X0 into register D1.

PLC Programming

MOV X0, D1

Copies the value from register D2 into register D3.

PLC Programming

MOV D2, D3

Sample Program

This program moves the value 50 into D10, then copies D10 to D11, and finally copies D11 to D12.

PLC Programming

MOV 50, D10
MOV D10, D11
MOV D11, D12

Important Notes

MOV does not change the source value; it only copies it.

Make sure the destination register is large enough to hold the source value.

MOV is useful for organizing data and preparing values for other instructions.

Summary

MOV copies a value from source to destination without changing the source.

Use MOV to transfer data between inputs, constants, and registers.

MOV helps keep data organized and ready for processing.

Practice

1. What does the MOV instruction do in PLC programming?

easy

A. Compares two values and sets a flag

B. Copies a value from a source to a destination without changing the source

C. Deletes a value from memory

D. Adds two values and stores the result

Solution

1. Step 1: Understand the MOV instruction purpose

   The MOV instruction copies data from one place to another without altering the original data.

2. Step 2: Compare with other options

   Other options describe different instructions like addition, deletion, or comparison, which are not MOV's function.

3. Final Answer:

   Copies a value from a source to a destination without changing the source -> Option B

4. Quick Check:

   MOV copies value unchanged [OK]

Hint: MOV copies data without changing source [OK]

Common Mistakes:

• Thinking MOV adds or deletes data
• Confusing MOV with comparison instructions
• Assuming MOV changes the source value

2. Which of the following is the correct syntax for the MOV instruction in PLC programming?

easy

A. MOV source, destination

B. MOV(destination, source)

C. MOV source -> destination

D. MOV = source to destination

Solution

1. Step 1: Recall standard MOV syntax

   The MOV instruction syntax is typically written as MOV followed by source and destination separated by a comma.

2. Step 2: Evaluate other options

   Options A, B, and D use incorrect syntax formats not recognized in PLC MOV instructions.

3. Final Answer:

   MOV source, destination -> Option A

4. Quick Check:

   Correct MOV syntax uses comma [OK]

Hint: MOV uses source, destination format [OK]

Common Mistakes:

• Reversing source and destination order
• Using parentheses or arrows incorrectly
• Adding extra symbols like '='

3. Given the following PLC code snippet:

MOV 100, D0
MOV D0, D1
MOV D1, D2

What is the final value stored in register D2?

medium

A. D0

B. 0

C. 100

D. D1

Solution

1. Step 1: Trace the first MOV instruction

   MOV 100, D0 sets D0 to 100.

2. Step 2: Trace the second and third MOV instructions

   MOV D0, D1 copies 100 to D1; MOV D1, D2 copies 100 to D2.

3. Final Answer:

   100 -> Option C

4. Quick Check:

   Value 100 moves through D0 to D2 [OK]

Hint: Follow value moves step-by-step [OK]

Common Mistakes:

• Assuming D2 remains 0
• Confusing register names with values
• Ignoring the order of MOV instructions

4. Identify the error in this MOV instruction:

MOV D1 D2

What is the problem?

medium

A. MOV requires parentheses

B. Source and destination are reversed

C. MOV cannot move between registers

D. Missing comma between source and destination

Solution

1. Step 1: Check syntax of MOV instruction

   MOV requires a comma between source and destination to separate them.

2. Step 2: Verify other options

   Source and destination order is correct; MOV can move between registers; parentheses are not required.

3. Final Answer:

   Missing comma between source and destination -> Option D

4. Quick Check:

   Comma separates source and destination [OK]

Hint: Always separate source and destination with a comma [OK]

Common Mistakes:

• Omitting the comma
• Swapping source and destination
• Adding unnecessary parentheses

5. You want to copy the value from input register X10 to output register Y20 only if X10 is greater than 50. Which PLC code snippet correctly uses the MOV instruction with this condition?

hard

A. LD X10
GT 50
MOV X10, Y20

B. LD X10 > 50
MOV X10, Y20

C. CMP X10, 50
MOV X10, Y20

D. LD X10
MOV X10, Y20
IF X10 > 50

Solution

1. Step 1: Understand conditional MOV usage

   To move only if X10 > 50, first load X10, then check if greater than 50, then move.

2. Step 2: Analyze each option

   LD X10 GT 50 MOV X10, Y20 correctly loads X10, tests GT 50, then moves X10 to Y20. Others lack proper condition or order.

3. Final Answer:

   LD X10 GT 50 MOV X10, Y20 -> Option A

4. Quick Check:

   Condition before MOV ensures correct transfer [OK]

Hint: Check condition before MOV to control data transfer [OK]

Common Mistakes:

• Placing MOV before condition check
• Using incorrect comparison instructions
• Missing condition entirely