Move instruction (MOV) in PLC Programming - Step-by-Step Execution

Concept Flow - Move instruction (MOV)

Start

↓

Read Source Value

↓

Copy Value to Destination

↓

Update Destination

↓

End

The MOV instruction reads a value from a source and copies it directly to a destination, updating the destination with the source's value.

Execution Sample

PLC Programming

MOV 10, D100
MOV D100, D101
MOV D101, D102

This code moves the value 10 into D100, then copies D100 to D101, and finally copies D101 to D102.

Execution Table

Step	Instruction	Source Value	Destination Before	Action	Destination After
1	MOV 10, D100	10	Undefined	Copy 10 to D100	10
2	MOV D100, D101	10	Undefined	Copy 10 to D101	10
3	MOV D101, D102	10	Undefined	Copy 10 to D102	10
4	End	-	-	All moves completed	-

💡 All MOV instructions executed, destination registers updated with source values.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
D100	Undefined	10	10	10
D101	Undefined	Undefined	10	10
D102	Undefined	Undefined	Undefined	10

Key Moments - 2 Insights

Why does the destination value change immediately after the MOV instruction?

Because the MOV instruction copies the source value directly to the destination in that step, as shown in execution_table rows 1 to 3.

What happens if the source is a constant versus a register?

The MOV instruction treats both the same by copying the value; for example, step 1 uses a constant 10, steps 2 and 3 use register values, all copied to destinations.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of D101 after step 2?

A Undefined

B 10

C 0

D D100

💡 Hint

Check the 'Destination After' column for step 2 in the execution_table.

At which step does D102 get its value updated?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Instruction' column and find when D102 is the destination.

If the first MOV instruction was MOV 20, D100 instead of MOV 10, D100, what would be the value of D102 after step 3?

A 10

B Undefined

C 20

D D100

💡 Hint

Follow the value flow from source to destination in the variable_tracker.

Concept Snapshot

MOV instruction copies a value from source to destination.
Source can be a constant or a register.
Destination is updated immediately.
Syntax: MOV source, destination
Used to transfer data in PLC programming.

Full Transcript

The MOV instruction in PLC programming copies a value from a source to a destination. It can move constants or register values. Each MOV updates the destination immediately. For example, MOV 10, D100 sets D100 to 10. Then MOV D100, D101 copies 10 to D101. Finally, MOV D101, D102 copies 10 to D102. This process is step-by-step, updating each destination as the instruction runs.

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