FOR and WHILE loops in PLC Programming - Mini Project: Build & Apply

Using FOR and WHILE Loops in PLC Programming

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to control a conveyor belt system in a factory. The system needs to count items passing on the belt and stop the belt after a certain number of items have passed.

🎯 Goal: Build a PLC program that uses a FOR loop to simulate counting items and a WHILE loop to keep the conveyor running until the count reaches a limit.

📋 What You'll Learn

Create a variable itemCount to track the number of items counted.

Create a variable maxItems to set the maximum number of items before stopping.

Use a FOR loop to simulate counting items from 1 to maxItems.

Use a WHILE loop to simulate the conveyor running until itemCount reaches maxItems.

Print the current itemCount during each loop iteration.

Print a message when the conveyor stops.

💡 Why This Matters

🌍 Real World

Counting items on a conveyor belt and controlling machinery based on counts is common in manufacturing automation.

💼 Career

Understanding loops and control flow in PLC programming is essential for automation engineers and technicians working with industrial control systems.

1

Create the initial variables

Create a variable called itemCount and set it to 0. Create another variable called maxItems and set it to 5.

PLC Programming

# Create variables itemCount and maxItems
# Your code here

Hint

Use the assignment operator := to set values in PLC code.

2

Set up the FOR loop to count items

Write a FOR loop that uses a variable i to count from 1 to maxItems. Inside the loop, set itemCount to i.

PLC Programming

itemCount := 0;
maxItems := 5;

// Write a FOR loop to count items
// Your code here

Hint

Use FOR i := 1 TO maxItems DO to start the loop and END_FOR; to end it.

3

Use a WHILE loop to run the conveyor

Reset itemCount to 0. Write a WHILE loop that runs while itemCount is less than maxItems. Inside the loop, increase itemCount by 1 and print the current itemCount.

PLC Programming

itemCount := 0;
maxItems := 5;

FOR i := 1 TO maxItems DO
    itemCount := i;
END_FOR;

// Reset itemCount to 0
// Write a WHILE loop to run the conveyor
// Your code here

Hint

Reset with itemCount := 0; Use WHILE condition DO and END_WHILE; to create the loop. Use Print to show messages.

4

Print the conveyor stop message

After the WHILE loop, write a Print statement to display the message 'Conveyor stopped after ' followed by the itemCount and the text ' items.'.

PLC Programming

itemCount := 0;
maxItems := 5;

FOR i := 1 TO maxItems DO
    itemCount := i;
END_FOR;

itemCount := 0;
WHILE itemCount < maxItems DO
    itemCount := itemCount + 1;
    // Simulate printing the count
    Print('Item count: ' + INT_TO_STRING(itemCount));
END_WHILE;

// Print conveyor stop message
// Your code here

Hint

Use Print with string concatenation and INT_TO_STRING to show the final message.

Practice

1. Which statement best describes a FOR loop in PLC programming?

easy

A. It repeats a set of instructions a fixed number of times.

B. It repeats instructions only while a condition is true.

C. It runs instructions once and then stops.

D. It waits for an input before running.

Solution

1. Step 1: Understand the purpose of a FOR loop

   A FOR loop repeats instructions a specific number of times, controlled by a counter.

2. Step 2: Compare with other loop types

   WHILE loops repeat based on a condition, not a fixed count. The other options describe different behaviors.

3. Final Answer:

   It repeats a set of instructions a fixed number of times. -> Option A

4. Quick Check:

   FOR loop = fixed repetitions [OK]

Hint: FOR loops run fixed times; WHILE loops depend on conditions [OK]

Common Mistakes:

• Confusing FOR loops with WHILE loops
• Thinking FOR loops run only once
• Believing FOR loops wait for input

2. Which of the following is the correct syntax for a FOR loop in PLC structured text?

easy

A. for i in range(1,6): ...

B. FOR (i = 1; i <= 5; i++) { ... }

C. FOR i := 1 TO 5 DO ... END_FOR;

D. WHILE i < 5 DO ... END_WHILE;

Solution

1. Step 1: Identify PLC structured text syntax

   PLC uses FOR i := 1 TO 5 DO ... END_FOR; for loops.

2. Step 2: Recognize other languages' syntax

   The syntax FOR (i = 1; i <= 5; i++) { ... } is C/JavaScript. The syntax for i in range(1,6): ... is Python. WHILE i < 5 DO ... END_WHILE; is a WHILE loop.

3. Final Answer:

   FOR i := 1 TO 5 DO ... END_FOR; -> Option C

4. Quick Check:

   PLC FOR loop uses := and TO [OK]

Hint: PLC FOR loops use ':=' and 'TO' keywords [OK]

Common Mistakes:

• Using C or Python syntax in PLC code
• Confusing FOR and WHILE syntax
• Omitting END_FOR statement

3. What will be the output of this PLC structured text code?

VAR i: INT := 1; sum: INT := 0; END_VAR
FOR i := 1 TO 3 DO
  sum := sum + i;
END_FOR;
// Output sum

medium

A. Syntax error

B. 6

C. 0

D. 3

Solution

1. Step 1: Trace the FOR loop iterations

   The loop runs with i = 1, 2, 3. sum starts at 0.

2. Step 2: Calculate sum after each iteration

   sum = 0+1=1, then 1+2=3, then 3+3=6.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Add loop variable each iteration to sum [OK]

Common Mistakes:

• Adding only last value of i
• Starting sum at 1 instead of 0
• Confusing loop range boundaries

4. Identify the error in this PLC code snippet:

VAR i: INT := 0; count: INT := 0; END_VAR
WHILE i < 5 DO
  count := count + 1;
END_WHILE;

medium

A. Missing increment of i inside the WHILE loop

B. Incorrect variable declaration syntax

C. WHILE loop should be FOR loop

D. count variable should be BOOL, not INT

Solution

1. Step 1: Analyze the WHILE loop condition and body

   The loop condition is i < 5, but i never changes inside the loop.

2. Step 2: Identify infinite loop risk

   Without incrementing i, the condition stays true forever, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the WHILE loop -> Option A

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Always update loop variable inside WHILE loops [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Confusing FOR and WHILE loops
• Incorrect variable types

5. You want to use a loop to turn on 4 output coils named Q1 to Q4 in sequence using PLC structured text. Which code snippet correctly uses a FOR loop to do this?

hard

A. FOR i := 1 TO 4 DO
Q[i-1] := TRUE;
END_FOR;

B. FOR i := 1 TO 4 DO
Q + i := TRUE;
END_FOR;

C. FOR i := 1 TO 4 DO
Qi := TRUE;
END_FOR;

D. FOR i := 1 TO 4 DO
Q[i] := TRUE;
END_FOR;

Solution

1. Step 1: Understand coil addressing in arrays

   Using Q[i] accesses coil Q1 to Q4 correctly if Q is an array of outputs.

2. Step 2: Check other options for syntax errors

   FOR i := 1 TO 4 DO Q + i := TRUE; END_FOR; uses invalid syntax Q + i. FOR i := 1 TO 4 DO Qi := TRUE; END_FOR; treats Q1, Q2 as fixed names, not variable. FOR i := 1 TO 4 DO Q[i-1] := TRUE; END_FOR; uses i-1 which may cause off-by-one error if indexing starts at 1.

3. Final Answer:

   FOR i := 1 TO 4 DO Q[i] := TRUE; END_FOR; -> Option D

4. Quick Check:

   Use array indexing for sequential coil control [OK]

Hint: Use array indexing Q[i] for sequential outputs [OK]

Common Mistakes:

• Using invalid syntax for coil names
• Off-by-one errors in indexing
• Treating coil names as strings