Parallel logic (OR condition) in PLC Programming - Mini Project: Build & Apply

Parallel logic (OR condition)

📖 Scenario: You are programming a simple control system for a factory machine. The machine should start if either of two buttons is pressed. This is a common safety and convenience feature in industrial automation.

🎯 Goal: Build a PLC program that uses parallel logic (OR condition) to start the machine when either start_button_1 or start_button_2 is pressed.

📋 What You'll Learn

Create two boolean variables named start_button_1 and start_button_2 representing the two start buttons.

Create a boolean variable named machine_running to represent the machine's running state.

Use an OR condition to set machine_running to true if either start_button_1 or start_button_2 is true.

Print the value of machine_running to show if the machine is running.

💡 Why This Matters

🌍 Real World

Parallel logic (OR condition) is used in industrial automation to allow multiple inputs to control a single output, such as starting a machine from different locations.

💼 Career

Understanding how to implement OR conditions in PLC programming is essential for automation engineers and technicians to design safe and flexible control systems.

1

Create input variables for start buttons

Create two boolean variables called start_button_1 and start_button_2. Set start_button_1 to False and start_button_2 to True to simulate that the second button is pressed.

PLC Programming

# Create two boolean variables for the start buttons
# Your code here

Hint

Use = to assign False or True to the variables.

2

Create output variable for machine state

Create a boolean variable called machine_running and set it to False initially.

PLC Programming

start_button_1 = False
start_button_2 = True
# Create a boolean variable called machine_running and set it to False
# Your code here

Hint

Remember to use = to assign False to machine_running.

3

Use OR condition to update machine state

Use an OR condition with start_button_1 and start_button_2 to set machine_running to True if either button is pressed.

PLC Programming

start_button_1 = False
start_button_2 = True
machine_running = False
# Use OR condition to update machine_running
# Your code here

Hint

Use the or keyword to combine the two button variables.

4

Display the machine running state

Print the value of machine_running to show if the machine is running.

PLC Programming

start_button_1 = False
start_button_2 = True
machine_running = False
machine_running = start_button_1 or start_button_2
# Print machine_running
# Your code here

Hint

Use print(machine_running) to display the result.

Practice

1. In PLC programming, what does a parallel logic (OR condition) do?
Input1 ---| |---+---(Output)
Input2 ---| |---+

easy

A. Output never activates regardless of inputs

B. Output activates only if both Input1 and Input2 are ON

C. Output activates if either Input1 or Input2 is ON

D. Output activates only if Input1 is OFF and Input2 is ON

Solution

1. Step 1: Understand parallel logic in PLC

   Parallel contacts represent an OR condition, meaning output activates if any input is ON.

2. Step 2: Analyze given inputs

   Since Input1 and Input2 are in parallel, output activates if either Input1 or Input2 is ON.

3. Final Answer:

   Output activates if either Input1 or Input2 is ON -> Option C

4. Quick Check:

   Parallel logic (OR) = Output ON if any input ON [OK]

Hint: Parallel contacts mean OR condition: any input ON activates output [OK]

Common Mistakes:

• Thinking parallel means AND condition
• Assuming output needs both inputs ON
• Confusing series and parallel contacts

2. Which of the following is the correct ladder logic syntax for an OR condition between two inputs Start and Reset to control Motor?

easy

A. Motor := Start AND Reset;

B. Motor := Start XOR Reset;

C. Motor := NOT(Start OR Reset);

D. Motor := Start OR Reset;

Solution

1. Step 1: Identify OR operator syntax

   In PLC structured text, OR is written as 'OR' between conditions.

2. Step 2: Match correct expression

   Motor := Start OR Reset; uses 'Start OR Reset' which correctly represents OR condition for Motor.

3. Final Answer:

   Motor := Start OR Reset; -> Option D

4. Quick Check:

   OR operator syntax = Motor := Start OR Reset [OK]

Hint: Use 'OR' keyword to combine inputs for parallel logic [OK]

Common Mistakes:

• Using AND instead of OR
• Using XOR which is exclusive OR
• Negating the OR condition incorrectly

3. Given the following ladder logic snippet:

---[ ]---+---( )---
  Start  |  Motor
---[ ]---+
  Stop   

Where [ ] is a normally open contact and ( ) is the coil, what is the output state of Motor if Start is OFF and Stop is ON?

medium

A. Motor is ON

B. Motor is OFF

C. Motor toggles ON and OFF rapidly

D. Motor state is undefined

Solution

1. Step 1: Understand parallel contacts behavior

   Motor coil energizes if either Start or Stop contact is closed (ON).

2. Step 2: Check input states

   Start is OFF (open), Stop is ON (closed), so current flows through Stop contact to Motor coil.

3. Final Answer:

   Motor is ON -> Option A

4. Quick Check:

   Parallel OR logic = Motor ON if any input ON [OK]

Hint: If any parallel input is ON, output coil energizes [OK]

Common Mistakes:

• Assuming both inputs must be ON
• Confusing normally open with normally closed contacts
• Ignoring parallel wiring effect

4. Identify the error in this ladder logic snippet intended to implement an OR condition:

---[ ]---[ ]---( )---
  Input1  Input2  Output

medium

A. Contacts are in series, not parallel, so it's an AND condition

B. Output coil is missing

C. Input2 contact is normally closed instead of normally open

D. No error, logic is correct

Solution

1. Step 1: Analyze contact arrangement

   Contacts are placed one after another (series), which means both must be ON for output.

2. Step 2: Compare with OR condition requirement

   OR requires parallel contacts, so this is an AND condition, not OR.

3. Final Answer:

   Contacts are in series, not parallel, so it's an AND condition -> Option A

4. Quick Check:

   Series contacts = AND, parallel contacts = OR [OK]

Hint: OR needs parallel contacts, series means AND [OK]

Common Mistakes:

• Confusing series and parallel wiring
• Ignoring contact types
• Assuming output coil is missing

5. You want to design a PLC rung that turns ON an alarm if any of three sensors (SensorA, SensorB, SensorC) detect a fault. Which ladder logic arrangement correctly implements this parallel OR condition?

hard

A. Use NOT contacts for all sensors in series feeding the alarm coil

B. Place all three sensor contacts in parallel feeding the alarm coil

C. Place all three sensor contacts in series feeding the alarm coil

D. Use XOR contacts between sensors feeding the alarm coil

Solution

1. Step 1: Understand alarm activation logic

   Alarm should activate if any sensor detects fault, so OR condition is needed.

2. Step 2: Choose correct contact arrangement

   Parallel contacts represent OR logic, so placing sensors in parallel activates alarm if any sensor is ON.

3. Final Answer:

   Place all three sensor contacts in parallel feeding the alarm coil -> Option B

4. Quick Check:

   Parallel contacts = OR = Alarm ON if any sensor ON [OK]

Hint: Use parallel contacts for multiple inputs triggering one output [OK]

Common Mistakes:

• Using series contacts which require all sensors ON
• Using NOT contacts incorrectly
• Using XOR which only activates if exactly one sensor is ON