NOT logic with NC contact in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of NOT logic with NC contact in ladder logic

Given a Normally Closed (NC) contact labeled X0 in a PLC ladder logic rung, what is the output Y0 when X0 is OFF (open)?

PLC Programming

Rung: --[NC X0]--(Y0)

A Y0 is ON

B Y0 remains in previous state

C Y0 toggles ON and OFF rapidly

D Y0 is OFF

💡 Hint

Remember that NC contact passes current when input is OFF.

A Normally Closed (NC) contact allows current flow when the input is OFF (open). So if X0 is OFF, the NC contact is closed, energizing output Y0.

🧠 Conceptual

intermediate

Understanding NOT logic with NC contact behavior

Which statement correctly describes the behavior of a Normally Closed (NC) contact in PLC logic?

A It always passes current regardless of input

B It passes current only when the input is ON

C It never passes current

D It passes current only when the input is OFF

💡 Hint

Think about the meaning of 'Normally Closed'.

An NC contact is closed (passes current) when the input is OFF (not energized). It opens when the input is ON.

🔧 Debug

advanced

Identify the error in NOT logic with NC contact rung

Consider this ladder logic rung: --[NC X0]--[NC X1]--(Y0). What is the output Y0 when X0 is ON and X1 is OFF?

PLC Programming

Rung: --[NC X0]--[NC X1]--(Y0)

A Y0 is OFF

B Y0 toggles ON and OFF

C Y0 is ON

D Y0 is in error state

💡 Hint

Evaluate each NC contact state based on input.

NC X0 is open when X0 is ON, so no current flows. Even though NC X1 is closed (X1 OFF), the series connection means Y0 is OFF.

📝 Syntax

advanced

Correct syntax for NOT logic using NC contact in structured text

Which of the following structured text snippets correctly implements NOT logic using NC contact X0 to set output Y0?

A Y0 := X0 NOT;

B Y0 := !X0;

C Y0 := NOT X0;

D Y0 := NOT(X0);

💡 Hint

Structured Text uses the NOT operator as a prefix before the variable.

In IEC 61131-3 Structured Text, NOT is a unary logical operator used as Y0 := NOT X0;.

🚀 Application

expert

Design a rung using NC contacts to energize output only when both inputs are OFF

You need to energize output Y0 only when both inputs X0 and X1 are OFF. Which ladder logic rung correctly implements this using NC contacts?

A --[NC X0]--[NO X1]--(Y0)

B --[NC X0]--[NC X1]--(Y0)

C --[NO X0]--[NO X1]--(Y0)

D --[NO X0]--[NC X1]--(Y0)

💡 Hint

NC contacts pass current when input is OFF.

Using NC contacts for both X0 and X1 in series means Y0 is energized only when both inputs are OFF.

Practice

1. What does a Normally Closed (NC) contact represent in PLC logic?

easy

A. The circuit is ON when the switch is NOT pressed

B. The circuit is ON only when the switch is pressed

C. The circuit is always OFF

D. The circuit toggles ON and OFF automatically

Solution

1. Step 1: Understand NC contact behavior

   An NC contact means the circuit is closed (ON) when the switch is not pressed.

2. Step 2: Compare options with NC contact logic

   Only The circuit is ON when the switch is NOT pressed correctly states the circuit is ON when the switch is NOT pressed.

3. Final Answer:

   The circuit is ON when the switch is NOT pressed -> Option A

4. Quick Check:

   NC contact = ON when NOT pressed [OK]

Hint: NC means circuit closed when switch is NOT pressed [OK]

Common Mistakes:

• Confusing NC with Normally Open (NO) contact
• Thinking circuit is ON when switch is pressed
• Assuming NC contact toggles automatically

2. Which of the following is the correct PLC ladder logic symbol for a Normally Closed (NC) contact?

easy

A. --|&|--

B. --| |--

C. --|/|--

D. --|X|--

Solution

1. Step 1: Recall PLC contact symbols

   Normally Open (NO) contact is --| |--, Normally Closed (NC) contact is --|/|--.

2. Step 2: Match symbol to NC contact

   --|/|-- shows --|/|--, which is the correct NC contact symbol.

3. Final Answer:

   --|/|-- -> Option C

4. Quick Check:

   NC contact symbol = --|/|-- [OK]

Hint: NC contact symbol has a slash / inside brackets [OK]

Common Mistakes:

• Choosing NO contact symbol instead of NC
• Confusing symbols with AND/OR logic
• Using invalid or made-up symbols

3. Given this PLC ladder logic snippet with an NC contact labeled Stop controlling output Motor:

--|/|--(Stop)----( )--(Motor)

What happens to Motor when the Stop button is pressed?

medium

A. Motor turns OFF

B. Motor toggles ON and OFF rapidly

C. Motor turns ON

D. Motor state does not change

Solution

1. Step 1: Understand NC contact effect when pressed

   Pressing the NC contact opens the circuit, breaking the path.

2. Step 2: Determine output state

   Since the circuit is open, the Motor output coil is not energized and turns OFF.

3. Final Answer:

   Motor turns OFF -> Option A

4. Quick Check:

   Pressing NC contact = circuit open = output OFF [OK]

Hint: Pressing NC opens circuit, output turns OFF [OK]

Common Mistakes:

• Assuming pressing NC turns output ON
• Thinking output toggles automatically
• Ignoring NC contact behavior

4. A PLC program uses an NC contact named EmergencyStop to stop a conveyor motor. The motor never stops when the emergency button is pressed. What is the most likely error?

medium

A. The PLC is powered off

B. The NC contact is wired as Normally Open (NO)

C. The emergency button is not pressed long enough

D. The output coil is missing

Solution

1. Step 1: Analyze symptom

   Motor never stops when emergency button pressed means circuit does not open.

2. Step 2: Identify wiring error

   If NC contact is wired as NO, pressing button closes circuit instead of opening it, so motor stays ON.

3. Final Answer:

   The NC contact is wired as Normally Open (NO) -> Option B

4. Quick Check:

   Wrong contact type wiring = motor won't stop [OK]

Hint: Check contact type wiring if stop fails [OK]

Common Mistakes:

• Blaming output coil without checking input wiring
• Assuming button press duration causes issue
• Ignoring PLC power status

5. You want to design a safety stop circuit using an NC contact SafetySwitch and an output coil MachineRun. Which ladder logic rung correctly implements the NOT logic with the NC contact to stop the machine when the switch is pressed?

hard

A. --| |--(SafetySwitch)----( )--(MachineRun)

B. --| |--(MachineRun)----( )--(SafetySwitch)

C. --|/|--(MachineRun)----( )--(SafetySwitch)

D. --|/|--(SafetySwitch)----( )--(MachineRun)

Solution

1. Step 1: Identify correct NC contact usage

   Safety stop uses NC contact so circuit is closed (MachineRun ON) when switch NOT pressed.

2. Step 2: Match ladder logic rung

   --|/|--(SafetySwitch)----( )--(MachineRun) uses NC contact --|/|-- controlling MachineRun output correctly.

3. Final Answer:

   --|/|--(SafetySwitch)----( )--(MachineRun) -> Option D

4. Quick Check:

   NC contact before output coil = safety stop logic [OK]

Hint: Use NC contact before output coil for safety stop [OK]

Common Mistakes:

• Using NO contact instead of NC for safety stop
• Reversing contact and coil positions
• Confusing contact labels with output coil