IF-THEN-ELSE statement in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple IF-THEN-ELSE in PLC

What will be the value of the output variable Q after running this PLC code snippet?

IF I = TRUE THEN
  Q := TRUE;
ELSE
  Q := FALSE;
END_IF;

PLC Programming

I := TRUE;
IF I = TRUE THEN
  Q := TRUE;
ELSE
  Q := FALSE;
END_IF;

A Q = TRUE

B Q = FALSE

C Q is unchanged

D Syntax error

💡 Hint

Think about what happens when the input I is TRUE in the IF condition.

Since I is TRUE, the IF condition is met, so Q is set to TRUE.

💻 Command Output

intermediate

IF-THEN-ELSE with numeric comparison

Given the following PLC code, what is the value of Output after execution?

VAR
  Temp : INT := 25;
  Output : STRING;
END_VAR

IF Temp > 30 THEN
  Output := 'Hot';
ELSE
  Output := 'Cold';
END_IF;

PLC Programming

Temp := 25;
IF Temp > 30 THEN
  Output := 'Hot';
ELSE
  Output := 'Cold';
END_IF;

A Output = 'Hot'

B Output = 'Cold'

C Output is empty

D Runtime error

💡 Hint

Check if 25 is greater than 30.

Since 25 is not greater than 30, the ELSE branch runs, setting Output to 'Cold'.

📝 Syntax

advanced

Identify the syntax error in IF-THEN-ELSE

Which option contains a syntax error in the IF-THEN-ELSE statement in PLC Structured Text?

A

IF X THEN
  Y := TRUE;
ELSE
  Y := FALSE;
END_IF;

B

IF X = TRUE THEN
  Y := TRUE;
ELSE
  Y := FALSE;
END_IF;

C

IF X = TRUE THEN
  Y := TRUE;
ELSE
  Y := FALSE;
END_IF

D

IF X = TRUE THEN
  Y := TRUE
ELSE
  Y := FALSE;
END_IF;

💡 Hint

Look for missing semicolons or missing END_IF.

Option D is missing a semicolon after 'Y := TRUE', which causes a syntax error.

🔧 Debug

advanced

Why does this IF-THEN-ELSE not set the output?

Given this PLC code snippet, why does Output remain FALSE after execution?

VAR
  Input : BOOL := FALSE;
  Output : BOOL := FALSE;
END_VAR

IF Input THEN
  Output := TRUE;
END_IF;

PLC Programming

Input := FALSE;
Output := FALSE;
IF Input THEN
  Output := TRUE;
END_IF;

A Because the IF statement is missing an ELSE branch.

B Because Output is initialized to FALSE, it cannot be changed.

C Because Input is FALSE, the IF condition is not met, so Output stays FALSE.

D Because the variable Output is not declared.

💡 Hint

Think about what happens when the IF condition is FALSE.

The IF condition is FALSE, so the code inside the IF block does not run, leaving Output unchanged.

🚀 Application

expert

Determine the final value after nested IF-THEN-ELSE

What is the final value of Result after running this nested IF-THEN-ELSE PLC code?

VAR
  A : INT := 5;
  B : INT := 10;
  Result : STRING;
END_VAR

IF A > B THEN
  Result := 'A is greater';
ELSE
  IF A = B THEN
    Result := 'A equals B';
  ELSE
    Result := 'B is greater';
  END_IF;
END_IF;

PLC Programming

A := 5;
B := 10;
IF A > B THEN
  Result := 'A is greater';
ELSE
  IF A = B THEN
    Result := 'A equals B';
  ELSE
    Result := 'B is greater';
  END_IF;
END_IF;

A Result = 'B is greater'

B Result = 'A equals B'

C Result = 'A is greater'

D Result is empty

💡 Hint

Compare A and B values carefully through the nested conditions.

Since A (5) is not greater than B (10), and A is not equal to B, the ELSE branch of the nested IF sets Result to 'B is greater'.

Practice

1. What does an IF-THEN-ELSE statement do in PLC programming?

easy

A. It repeats a set of instructions multiple times.

B. It sends data to an external device.

C. It stores data in a table format.

D. It chooses between two actions based on a condition.

Solution

1. Step 1: Understand the purpose of IF-THEN-ELSE

   The IF-THEN-ELSE statement checks a condition and decides which action to take based on that condition.

2. Step 2: Compare with other options

   Repeating instructions is done by loops, storing data is done by variables or tables, and sending data is communication, not decision making.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option D

4. Quick Check:

   IF-THEN-ELSE = Choose action based on condition [OK]

Hint: IF-THEN-ELSE picks one of two paths [OK]

Common Mistakes:

• Confusing IF-THEN-ELSE with loops
• Thinking it stores data
• Assuming it sends data externally

2. Which of the following is the correct syntax to start an IF statement in PLC structured text?

easy

A. IF condition THEN

B. IF condition DO

C. WHEN condition THEN

D. IF condition THEN DO

Solution

1. Step 1: Recall PLC IF statement syntax

   The correct way to start an IF statement is with 'IF condition THEN'.

2. Step 2: Check other options for errors

   'DO' is not used after IF in PLC structured text, 'WHEN' is for CASE statements, and 'THEN DO' is invalid syntax.

3. Final Answer:

   IF condition THEN -> Option A

4. Quick Check:

   IF starts with 'IF condition THEN' [OK]

Hint: IF always pairs with THEN, no DO needed [OK]

Common Mistakes:

• Adding DO after THEN
• Using WHEN instead of IF
• Mixing CASE syntax with IF

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

IF SensorInput = TRUE THEN
  Motor := ON;
ELSE
  Motor := OFF;
END_IF;

Assuming SensorInput is FALSE initially.

medium

A. Motor will keep previous state

B. Motor will be ON

C. Motor will be OFF

D. Syntax error occurs

Solution

1. Step 1: Evaluate the condition with SensorInput = FALSE

   The condition 'SensorInput = TRUE' is false, so the ELSE branch runs.

2. Step 2: Determine the action in ELSE branch

   In ELSE, Motor is set to OFF.

3. Final Answer:

   Motor will be OFF -> Option C

4. Quick Check:

   Condition false -> ELSE -> Motor OFF [OK]

Hint: False condition triggers ELSE action [OK]

Common Mistakes:

• Assuming Motor stays ON by default
• Confusing TRUE and FALSE conditions
• Thinking ELSE is optional and skipped

4. Identify the error in this PLC IF-THEN-ELSE code:

IF Temperature > 100 THEN
  Alarm := TRUE
ELSE
  Alarm := FALSE;
END_IF;

medium

A. Missing END_IF statement

B. Missing semicolon after Alarm := TRUE

C. Incorrect comparison operator

D. ELSE should be ELSEIF

Solution

1. Step 1: Check statement endings

   In PLC structured text, each assignment must end with a semicolon. 'Alarm := TRUE' is missing a semicolon.

2. Step 2: Verify other parts

   Comparison operator '>' is correct, END_IF is present, and ELSE is correct for two branches.

3. Final Answer:

   Missing semicolon after Alarm := TRUE -> Option B

4. Quick Check:

   Statements need semicolons [OK]

Hint: Every assignment ends with a semicolon [OK]

Common Mistakes:

• Omitting semicolons after assignments
• Confusing ELSE with ELSEIF
• Forgetting END_IF

5. You want a PLC program to turn ON a fan only if the temperature is above 30°C and the humidity is below 70%. Which IF-THEN-ELSE statement correctly implements this?

hard

A. IF Temperature > 30 AND Humidity < 70 THEN
Fan := ON;
ELSE
Fan := OFF;
END_IF;

B. IF Temperature > 30 OR Humidity < 70 THEN
Fan := ON;
ELSE
Fan := OFF;
END_IF;

C. IF Temperature > 30 THEN
Fan := ON;
ELSE IF Humidity < 70 THEN
Fan := ON;
ELSE
Fan := OFF;
END_IF;

D. IF Temperature > 30 THEN
Fan := ON;
END_IF;
IF Humidity < 70 THEN
Fan := ON;
END_IF;

Solution

1. Step 1: Understand the condition requirements

   The fan should turn ON only if both conditions are true: temperature above 30 and humidity below 70.

2. Step 2: Check each option's logic

   IF Temperature > 30 AND Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; uses AND, requiring both conditions. IF Temperature > 30 OR Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; uses OR, which turns fan ON if either condition is true. IF Temperature > 30 THEN Fan := ON; ELSE IF Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; turns fan ON if either condition is true via ELSE IF. IF Temperature > 30 THEN Fan := ON; END_IF; IF Humidity < 70 THEN Fan := ON; END_IF; turns fan ON separately for each condition, possibly turning ON incorrectly.

3. Final Answer:

   IF Temperature > 30 AND Humidity < 70 THEN Fan := ON; ELSE Fan := OFF; END_IF; -> Option A

4. Quick Check:

   Both conditions must be true -> use AND [OK]

Hint: Use AND to require both conditions true [OK]

Common Mistakes:

• Using OR instead of AND
• Using separate IFs causing wrong ON state
• Misusing ELSE IF for combined conditions