IF-THEN-ELSE statement in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an IF-THEN-ELSE statement in PLC programming?

It allows the program to make decisions by executing different actions based on whether a condition is true or false.

beginner

In an IF-THEN-ELSE statement, what happens if the condition is false?

The program executes the code inside the ELSE block, if it exists, or skips the IF block if there is no ELSE.

beginner

Write a simple IF-THEN-ELSE statement in PLC structured text to turn on a motor if a start button is pressed, otherwise turn it off.

IF StartButton THEN Motor := TRUE; ELSE Motor := FALSE; END_IF;

intermediate

Can IF-THEN-ELSE statements be nested in PLC programming?

Yes, you can place an IF-THEN-ELSE statement inside another to check multiple conditions step-by-step.

intermediate

Why is it important to use IF-THEN-ELSE statements carefully in PLC programs?

Because incorrect conditions or missing ELSE parts can cause unexpected machine behavior or safety issues.

What does the ELSE part of an IF-THEN-ELSE statement do?

A Runs when the IF condition is false

B Runs when the IF condition is true

C Runs before the IF condition is checked

D Runs only if there is an error

The ELSE block runs only when the IF condition is false.

Which keyword ends an IF-THEN-ELSE statement in PLC structured text?

A END_ELSE

B ENDIF

C END_IF

D STOP

END_IF marks the end of the IF-THEN-ELSE block in PLC structured text.

What happens if you omit the ELSE part in an IF-THEN-ELSE statement?

A The program skips the IF block when the condition is false

B The program crashes

C The program runs the IF block twice

D The program always runs the ELSE block

Without ELSE, the program just skips the IF block if the condition is false.

Can you use multiple IF-THEN-ELSE statements inside each other?

A Only in special programs

B No, it causes errors

C Only if the PLC supports it

D Yes, this is called nesting

Nesting means placing IF-THEN-ELSE statements inside others to check multiple conditions.

Which of these is a correct IF-THEN-ELSE syntax in PLC structured text?

A IF Condition DO Action1 ELSE Action2 END;

B IF Condition THEN Action1; ELSE Action2; END_IF;

C IF Condition { Action1 } ELSE { Action2 }

D IF Condition: Action1 ELSE: Action2 END

Option B uses the correct PLC structured text syntax with IF, THEN, ELSE, and END_IF.

Explain how an IF-THEN-ELSE statement controls the flow of a PLC program.

Think about how the program chooses what to do based on a yes/no question.

Describe a real-life example where you might use an IF-THEN-ELSE statement in a machine control program.

Consider a start button or sensor input controlling a motor or light.

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