NOT logic with NC contact in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an NC (Normally Closed) contact represent in PLC logic?

An NC contact is closed (allows current) when the input is OFF, and opens (breaks current) when the input is ON. It acts like a NOT gate in logic.

beginner

How does NOT logic work with an NC contact in a PLC ladder diagram?

The NC contact inverts the input signal: if the input is ON, the NC contact is OFF; if the input is OFF, the NC contact is ON.

intermediate

Why use an NC contact to implement NOT logic instead of a NO contact?

Because an NC contact naturally provides the inverse signal without needing extra logic, simplifying the ladder diagram.

beginner

In a ladder logic rung, what happens if the NC contact input is energized?

The NC contact opens, breaking the circuit, so the output coil will not energize.

beginner

Explain the difference between NO and NC contacts in terms of their default state.

NO (Normally Open) contacts are open by default and close when energized. NC (Normally Closed) contacts are closed by default and open when energized.

What is the default state of an NC contact in PLC logic?

A Closed

B Open

C Energized

D De-energized

An NC contact is closed by default, allowing current flow until energized.

If an NC contact input is ON, what is the state of the contact in the ladder diagram?

A Closed

B Open

C No change

D Depends on the output

When the input is ON, the NC contact opens, breaking the circuit.

Which contact type naturally implements NOT logic in PLC programming?

A NO contact

B Timer

C Output coil

D NC contact

NC contacts invert the input signal, acting as NOT logic.

Why might you choose an NC contact over a NO contact in a ladder diagram?

A To simplify NOT logic without extra components

B To increase power consumption

C To make the circuit slower

D To add more timers

NC contacts provide inverse logic naturally, simplifying the design.

What happens to the output coil if the NC contact is open?

A Output coil toggles

B Output coil energizes

C Output coil de-energizes

D No effect

An open NC contact breaks the circuit, so the output coil does not energize.

Describe how NOT logic is implemented using an NC contact in PLC ladder programming.

Think about how the NC contact changes state opposite to the input.

Explain the difference between NO and NC contacts and how each affects the circuit when energized.

Focus on default states and what happens when input is ON.

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