IF-THEN-ELSE statement in PLC Programming - Step-by-Step Execution

Concept Flow - IF-THEN-ELSE statement

Start

↓

Evaluate Condition

↓

Execute THEN block

↓

Continue

↓

Execute ELSE block

↓

Continue

The PLC checks a condition. If true, it runs the THEN part; if false, it runs the ELSE part, then continues.

Execution Sample

PLC Programming

IF sensor_input = TRUE THEN
  motor_output := ON;
ELSE
  motor_output := OFF;
END_IF;

This code turns the motor ON if the sensor input is TRUE; otherwise, it turns the motor OFF.

Execution Table

Step	Condition (sensor_input = TRUE)	Branch Taken	Action	motor_output
1	TRUE	THEN	Set motor_output to ON	ON
2	FALSE	ELSE	Set motor_output to OFF	OFF

💡 Execution ends after setting motor_output based on condition.

Variable Tracker

Variable	Start	After Step 1	After Step 2
sensor_input	TRUE	TRUE	FALSE
motor_output	OFF	ON	OFF

Key Moments - 2 Insights

Why does the motor_output change only once per execution?

Because the IF-THEN-ELSE runs once per cycle, it sets motor_output based on the current sensor_input value as shown in execution_table rows 1 and 2.

What happens if the condition is FALSE?

The ELSE branch runs, setting motor_output to OFF, as shown in execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution table, what is motor_output after step 1 when sensor_input is TRUE?

A ON

B OFF

C Undefined

D Remains unchanged

💡 Hint

Check the motor_output column in execution_table row 1.

At which step does the ELSE branch execute?

A Step 1

B Step 2

C Never

D Both steps

💡 Hint

Look at the Branch Taken column in execution_table.

If sensor_input stays TRUE for multiple cycles, what happens to motor_output?

A It toggles ON and OFF

B It stays OFF

C It stays ON

D It becomes undefined

💡 Hint

Refer to variable_tracker sensor_input and motor_output values after step 1.

Concept Snapshot

IF-THEN-ELSE checks a condition.
If TRUE, runs THEN block.
If FALSE, runs ELSE block.
Only one branch runs per cycle.
Used to control outputs based on inputs.

Full Transcript

In PLC programming, the IF-THEN-ELSE statement checks a condition once each cycle. If the condition is true, it executes the THEN block; otherwise, it executes the ELSE block. For example, if sensor_input is TRUE, motor_output is set ON; if FALSE, motor_output is set OFF. This decision happens every cycle, updating outputs accordingly.

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