IF-THEN-ELSE statement in PLC Programming - Mini Project: Build & Apply

Basic IF-THEN-ELSE Statement in PLC Programming

📖 Scenario: You are programming a simple PLC controller for a water tank system. The tank has a sensor that measures the water level as an integer value. You want to control a pump that fills the tank only when the water level is below a certain threshold.

🎯 Goal: Build a PLC program using an IF-THEN-ELSE statement to turn the pump ON if the water level is below 50, and turn it OFF otherwise.

📋 What You'll Learn

Create an integer variable water_level with the value 45

Create a boolean variable pump_on to control the pump

Use an IF-THEN-ELSE statement to set pump_on to TRUE if water_level is less than 50

Set pump_on to FALSE if water_level is 50 or more

Output the value of pump_on

💡 Why This Matters

🌍 Real World

Controlling pumps and actuators based on sensor inputs is common in industrial automation.

💼 Career

Understanding IF-THEN-ELSE logic is essential for PLC programmers to implement control decisions.

1

Create the water level variable

Create an integer variable called water_level and set it to 45.

PLC Programming

# Your code here

Hint

Use the assignment operator := to set the value.

2

Create the pump control variable

Create a boolean variable called pump_on to control the pump.

PLC Programming

water_level : INT := 45;
# Your code here

Hint

Declare pump_on as a boolean variable.

3

Write the IF-THEN-ELSE logic

Write an IF-THEN-ELSE statement that sets pump_on to TRUE if water_level is less than 50, otherwise set pump_on to FALSE.

PLC Programming

water_level : INT := 45;
pump_on : BOOL;
# Your code here

Hint

Use IF, THEN, ELSE, and END_IF; keywords properly.

4

Output the pump status

Write a statement to output the value of pump_on.

PLC Programming

water_level : INT := 45;
pump_on : BOOL;
IF water_level < 50 THEN
    pump_on := TRUE;
ELSE
    pump_on := FALSE;
END_IF;
# Your code here

Hint

Output methods vary by PLC. For this exercise, ensure pump_on is assigned correctly.

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