Assignment and arithmetic in PLC Programming - Step-by-Step Execution

Concept Flow - Assignment and arithmetic

Start

↓

Assign initial value

↓

Perform arithmetic operation

↓

Store result in variable

↓

Use or output result

↓

End

This flow shows how a value is assigned to a variable, then arithmetic is done, and the result is stored back for use.

Execution Sample

PLC Programming

VAR
  a : INT := 5;
  b : INT := 3;
  c : INT;
END_VAR

c := a + b;

Assigns 5 to a, 3 to b, then adds a and b and stores the result in c.

Execution Table

Step	Action	Variable a	Variable b	Variable c	Notes
1	Initialize a with 5	5	undefined	undefined	a is set to 5
2	Initialize b with 3	5	3	undefined	b is set to 3
3	Compute c = a + b	5	3	8	c stores sum of a and b
4	End of execution	5	3	8	All variables assigned

💡 Execution stops after assigning sum to c.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
a	undefined	5	5	5	5
b	undefined	undefined	3	3	3
c	undefined	undefined	undefined	8	8

Key Moments - 2 Insights

Why is variable c undefined before the arithmetic operation?

Because c is declared but not assigned until step 3 where c := a + b happens, as shown in execution_table row 3.

Can we use variable c before it is assigned?

No, using c before assignment would cause an error or undefined behavior, as c only gets a value at step 3.

Visual Quiz - 3 Questions

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

A 5

B 3

C 8

D undefined

💡 Hint

Check the 'Variable b' column in execution_table row 2.

At which step does variable c get its value assigned?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Variable c' columns in execution_table.

If we change a to 7, what will be the value of c after step 3?

A 10

B 8

C 5

D undefined

💡 Hint

c = a + b, so update a in variable_tracker and sum.

Concept Snapshot

Assignment and arithmetic in PLC:
- Declare variables with VAR
- Assign values using :=
- Perform arithmetic like c := a + b
- Variables hold values for later use
- Order matters: assign before use

Full Transcript

This example shows how in PLC programming, variables are declared and assigned values. First, variable a is set to 5, then b to 3. Next, c is assigned the sum of a and b, which is 8. Variables must be assigned before use to avoid errors. The execution table traces each step and variable state. Changing a's value changes c accordingly.

Practice

1. In PLC programming, which symbol is used to assign a value to a variable?
speed := 100;

easy

A. The ':=' symbol

B. The '=' symbol

C. The '==' symbol

D. The '->' symbol

Solution

1. Step 1: Understand assignment syntax in PLC

   PLC uses ':=' to assign values to variables, not '=' or '=='.

2. Step 2: Identify correct assignment operator

   The ':=' operator updates the variable with the new value.

3. Final Answer:

   The ':=' symbol -> Option A

4. Quick Check:

   Assignment operator = ':=' [OK]

Hint: Remember ':=' means assign value in PLC [OK]

Common Mistakes:

• Using '=' instead of ':='
• Confusing comparison '==' with assignment
• Using arrow '->' mistakenly
• Forgetting semicolon after assignment

2. Which of the following is the correct syntax to add 5 to a variable count in PLC?

easy

A. count := count + 5;

B. count = count + 5;

C. count += 5;

D. count :=+ 5;

Solution

1. Step 1: Check assignment operator usage

   PLC uses ':=' for assignment, so '=' and '+=' are incorrect here.

2. Step 2: Verify arithmetic addition syntax

   'count := count + 5;' correctly adds 5 to count and assigns it back.

3. Final Answer:

   count := count + 5; -> Option A

4. Quick Check:

   Use ':=' for assignment and '+' for addition [OK]

Hint: Use ':=' for assignment, '+' for addition [OK]

Common Mistakes:

• Using '=' instead of ':='
• Trying '+=' which is not valid in PLC
• Writing ':=+' which is invalid
• Missing semicolon

3. What is the value of result after executing the following PLC code?

temp := 10;
result := temp * 3 + 2;

medium

A. 12

B. 36

C. 30

D. 32

Solution

1. Step 1: Calculate multiplication first

   temp * 3 = 10 * 3 = 30

2. Step 2: Add 2 to the multiplication result

   30 + 2 = 32

3. Final Answer:

   32 -> Option D

4. Quick Check:

   10 * 3 + 2 = 32 [OK]

Hint: Multiply before adding, follow order of operations [OK]

Common Mistakes:

• Adding before multiplying
• Using wrong variable value
• Ignoring operator precedence
• Confusing temp and result variables

4. Identify the error in this PLC code snippet:

speed := 50
speed := speed + 10;

medium

A. Cannot add numbers in PLC

B. Using '=' instead of ':='

C. Missing semicolon after first assignment

D. Variable 'speed' not declared

Solution

1. Step 1: Check statement endings

   PLC statements must end with a semicolon ';'. The first line misses it.

2. Step 2: Confirm other syntax is correct

   Second line uses correct ':=' and addition syntax.

3. Final Answer:

   Missing semicolon after first assignment -> Option C

4. Quick Check:

   Each statement ends with ';' [OK]

Hint: Always end PLC statements with semicolon ';' [OK]

Common Mistakes:

• Forgetting semicolon
• Confusing '=' with ':='
• Assuming addition not allowed
• Ignoring variable declaration rules

5. You want to calculate the average speed from three sensor readings s1, s2, and s3 in PLC. Which code correctly assigns the average to avg_speed?

s1 := 40;
s2 := 50;
s3 := 60;
avg_speed := (s1 + s2 + s3) / 3;

hard

A. avg_speed = (s1 + s2 + s3) / 3;

B. avg_speed := (s1 + s2 + s3) / 3;

C. avg_speed := s1 + s2 + s3 / 3;

D. avg_speed := (s1 + s2 + s3) * 3;

Solution

1. Step 1: Check correct assignment operator

   Use ':=' to assign the calculated value to avg_speed.

2. Step 2: Verify arithmetic expression correctness

   Sum all three readings and divide by 3 inside parentheses for average.

3. Final Answer:

   avg_speed := (s1 + s2 + s3) / 3; -> Option B

4. Quick Check:

   Use ':=' and parentheses for correct average [OK]

Hint: Use parentheses to sum before dividing [OK]

Common Mistakes:

• Using '=' instead of ':='
• Missing parentheses causing wrong division
• Multiplying instead of dividing
• Forgetting semicolon