Normally open (NO) contact in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a Normally Open (NO) Contact in Ladder Logic

In a PLC ladder logic program, a Normally Open (NO) contact is connected to an input X and an output coil Y. When input X is energized (true), what will be the state of output Y?

PLC Programming

----[ ]----( )----
     X       Y

A Output Y is always energized regardless of input X.

B Output Y is energized (true) only when input X is false.

C Output Y is energized (true) only when input X is true.

D Output Y is never energized regardless of input X.

💡 Hint

Think about how a Normally Open contact behaves when the input is true or false.

A Normally Open (NO) contact closes the circuit when its input is true, allowing current to flow and energizing the output coil.

🧠 Conceptual

intermediate

Behavior of Normally Open (NO) Contact in a Control Circuit

Which statement best describes the behavior of a Normally Open (NO) contact in a control circuit?

A It allows current to flow only when the controlling input is active (true).

B It never allows current flow regardless of the controlling input.

C It always allows current flow regardless of the controlling input.

D It blocks current flow when the controlling input is active (true).

💡 Hint

Consider what happens to the contact when the input signal changes.

A Normally Open contact closes and allows current flow only when its controlling input is active (true).

🔧 Debug

advanced

Identify the Error in Ladder Logic Using NO Contact

Given the ladder logic snippet below, what is the error that prevents output Y from energizing when input X is true? ----[ ]----( )---- X Y Options:

A The output coil Y is missing from the circuit.

B There is no error; the output Y will energize correctly.

C The input X is not connected to the NO contact.

D The NO contact is incorrectly wired as Normally Closed (NC).

💡 Hint

Check if the NO contact and output coil are correctly represented.

The ladder logic shows a correct NO contact connected to input X and output coil Y. When X is true, Y energizes.

🚀 Application

advanced

Using NO Contact to Control a Motor Start

In a motor control circuit, a Normally Open (NO) push button is used to start the motor. Which of the following describes the correct operation?

A The NO push button has no effect on the motor starter coil.

B Pressing the NO push button opens the contact, de-energizing the motor starter coil.

C The NO push button keeps the motor starter coil energized at all times.

D Pressing the NO push button closes the contact, energizing the motor starter coil.

💡 Hint

Think about what happens when you press a Normally Open push button.

Pressing the NO push button closes the contact, allowing current to energize the motor starter coil and start the motor.

📝 Syntax

expert

Correct Syntax for Representing a Normally Open Contact in Structured Text

Which of the following Structured Text code snippets correctly represents a Normally Open (NO) contact controlling output Y based on input X?

A Y := X;

B Y := NOT X;

C Y := X AND NOT Y;

D Y := TRUE;

💡 Hint

A NO contact passes the input signal directly to the output when true.

In Structured Text, a NO contact is represented by assigning the input directly to the output.

Practice

1. What does a Normally Open (NO) contact do in a PLC circuit?

easy

A. It remains open until activated, then closes to allow current flow.

B. It remains closed until activated, then opens to stop current flow.

C. It always stays closed regardless of activation.

D. It always stays open regardless of activation.

Solution

1. Step 1: Understand NO contact default state

   A Normally Open contact is open by default, meaning no current flows through it initially.

2. Step 2: Behavior when activated

   When the NO contact is activated (e.g., button pressed), it closes, allowing current to flow.

3. Final Answer:

   It remains open until activated, then closes to allow current flow. -> Option A

4. Quick Check:

   NO contact = open by default, closes when activated [OK]

Hint: NO means open until pressed, then closes [OK]

Common Mistakes:

• Confusing NO with Normally Closed (NC) contact
• Thinking NO contact is closed by default
• Assuming NO contact always stays open

2. Which of the following is the correct syntax to represent a Normally Open (NO) contact in a PLC ladder logic program?

easy

A. --[/]-- symbol

B. |/| symbol

C. | | symbol

D. --[ ]-- symbol

Solution

1. Step 1: Identify NO contact symbol

   The Normally Open contact is represented by two parallel vertical lines: | |.

2. Step 2: Differentiate from NC contact

   The Normally Closed contact uses |/| or --[/]-- symbols, which are different.

3. Final Answer:

   | | symbol -> Option C

4. Quick Check:

   NO contact = | | [OK]

Hint: NO contact looks like two parallel lines | | [OK]

Common Mistakes:

• Using NC contact symbol for NO contact
• Confusing ladder logic symbols
• Mixing up contact and coil symbols

3. Given the following ladder logic snippet with a Normally Open (NO) contact named Start_Button controlling a coil Motor, what happens when Start_Button is pressed?

--| |--( )--
  Start_Button  Motor

medium

A. Motor coil de-energizes, turning the motor OFF.

B. Motor coil energizes, turning the motor ON.

C. Nothing happens; the motor state remains unchanged.

D. Motor coil toggles ON and OFF rapidly.

Solution

1. Step 1: Analyze NO contact behavior when pressed

   Pressing the Start_Button closes the NO contact, allowing current to flow.

2. Step 2: Effect on coil Motor

   Current energizes the Motor coil, turning the motor ON.

3. Final Answer:

   Motor coil energizes, turning the motor ON. -> Option B

4. Quick Check:

   NO contact pressed = coil energized [OK]

Hint: Pressing NO contact energizes coil [OK]

Common Mistakes:

• Assuming coil de-energizes when NO contact closes
• Thinking NO contact does not affect coil
• Confusing coil with contact behavior

4. In the ladder logic below, the Normally Open (NO) contact Sensor is supposed to start the Conveyor motor. However, the motor never starts even when the sensor is triggered. What is the likely error?

--|/|--( )--
 Sensor Conveyor

medium

A. The contact is Normally Closed (NC) instead of Normally Open (NO).

B. The coil Conveyor is wired incorrectly.

C. The sensor is not physically connected.

D. The power supply is off.

Solution

1. Step 1: Identify contact type in code

   The contact symbol |/| represents a Normally Closed (NC) contact, not Normally Open (NO).

2. Step 2: Effect of NC contact on motor start

   Since the sensor is triggered (activated), the NC contact opens, breaking the circuit and preventing the motor from starting.

3. Final Answer:

   The contact is Normally Closed (NC) instead of Normally Open (NO). -> Option A

4. Quick Check:

   Wrong contact type = motor won't start [OK]

Hint: Check contact symbol: |/| means NC, not NO [OK]

Common Mistakes:

• Confusing NC and NO contact symbols
• Assuming coil wiring is always the issue
• Ignoring sensor physical connection

5. You want to design a control circuit where a Normally Open (NO) start button must be pressed to start a machine. Which logic setup correctly uses the NO contact to achieve this?

hard

A. Use the NO contact in series with the motor coil; pressing the button breaks the circuit to start the machine.

B. Use the NO contact in parallel with the motor coil; pressing the button breaks the circuit to start the machine.

C. Use the NO contact in parallel with the motor coil; pressing the button energizes the coil to start the machine.

D. Use the NO contact in series with the motor coil; pressing the button energizes the coil to start the machine.

Solution

1. Step 1: Understand NO contact behavior in start circuit

   A Normally Open start button is open by default, meaning no current flows through it initially (machine OFF).

2. Step 2: Effect of pressing NO start button

   When the NO button is pressed, it closes, allowing current to flow and energizing the motor coil to start the machine.

3. Final Answer:

   Use the NO contact in series with the motor coil; pressing the button energizes the coil to start the machine. -> Option D

4. Quick Check:

   NO series press = coil energized to start [OK]

Hint: NO start button in series energizes coil when pressed [OK]

Common Mistakes:

• Placing NO contact in parallel for start function
• Assuming pressing NO button breaks circuit to start
• Confusing NO and NC contacts in control circuits