IF-THEN-ELSE statement in PLC Programming

Introduction

An IF-THEN-ELSE statement helps a PLC decide what to do based on conditions. It lets the program choose between two paths.

Turn on a motor only if a safety switch is on.

Open a valve if the temperature is too high, otherwise keep it closed.

Start a conveyor belt if a sensor detects an object.

Sound an alarm if a machine stops unexpectedly.

Syntax

PLC Programming

IF condition THEN
    // actions if condition is true
ELSE
    // actions if condition is false
END_IF

The condition is a test that can be true or false.

Actions inside THEN run only if the condition is true; ELSE actions run if false.

Examples

Turn motor on if sensor_input is true, else turn it off.

PLC Programming

IF sensor_input = TRUE THEN
    motor := TRUE;
ELSE
    motor := FALSE;
END_IF

Sound alarm if temperature is above 50.

PLC Programming

IF temperature > 50 THEN
    alarm := TRUE;
ELSE
    alarm := FALSE;
END_IF

Sample Program

This program checks if sensor_input is true. If yes, it turns motor on; otherwise, it turns motor off.

PLC Programming

PROGRAM Main
VAR
    sensor_input : BOOL := TRUE;
    motor : BOOL := FALSE;
END_VAR

IF sensor_input THEN
    motor := TRUE;
ELSE
    motor := FALSE;
END_IF
END_PROGRAM

Important Notes

Always close IF statements with END_IF in PLC languages.

Conditions can use comparison operators like =, >, <.

Use BOOL variables for true/false conditions.

Summary

IF-THEN-ELSE lets your PLC choose between two actions based on a condition.

Use it to control machines safely and smartly.

Remember to end the statement with END_IF.

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