Normally open (NO) contact in PLC Programming - Step-by-Step Execution

Concept Flow - Normally open (NO) contact

Start

↓

Check NO Contact State

↓

Output OFF

↓

End

↓

Output ON

↓

End

The NO contact is checked; if it is closed (activated), the output turns ON, otherwise it stays OFF.

Execution Sample

PLC Programming

IF NO_contact = TRUE THEN
  Output := ON
ELSE
  Output := OFF
END_IF

This code checks if the normally open contact is closed (TRUE). If yes, it turns the output ON; otherwise, OFF.

Execution Table

Step	NO_contact	Condition (NO_contact=TRUE?)	Action	Output
1	FALSE	FALSE	Set Output OFF	OFF
2	TRUE	TRUE	Set Output ON	ON
3	FALSE	FALSE	Set Output OFF	OFF
4	TRUE	TRUE	Set Output ON	ON

💡 Execution stops after setting output based on NO contact state each cycle.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
NO_contact	FALSE	FALSE	TRUE	FALSE	TRUE
Output	OFF	OFF	ON	OFF	ON

Key Moments - 2 Insights

Why does the output stay OFF when the NO contact is FALSE?

Because the NO contact is normally open, it only closes (TRUE) when activated. When FALSE, the condition fails and output is set OFF as shown in execution_table rows 1 and 3.

What happens when the NO contact changes from FALSE to TRUE?

The output changes from OFF to ON immediately, as seen in execution_table rows 2 and 4, because the condition NO_contact=TRUE becomes true.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at Step 3 when NO_contact is FALSE?

A OFF

B ON

C Undefined

D Remains previous state

💡 Hint

Check the Output column in execution_table row for Step 3.

At which step does the output turn ON?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the rows where Output is ON in execution_table.

If NO_contact stayed TRUE all the time, what would the output be?

A Alternates ON and OFF

B Always OFF

C Always ON

D Depends on previous state

💡 Hint

Refer to variable_tracker for Output when NO_contact is TRUE.

Concept Snapshot

Normally open (NO) contact is open by default.
It closes (becomes TRUE) when activated.
Output turns ON only when NO contact is TRUE.
If NO contact is FALSE, output stays OFF.
Used to control devices that activate on contact closure.

Full Transcript

A normally open (NO) contact is a switch that is open when not activated. When the contact closes, it allows current or signal to pass. In PLC programming, we check if the NO contact is TRUE (closed). If yes, we set the output ON; if not, output stays OFF. The execution table shows steps where NO_contact changes state and output responds accordingly. Beginners often wonder why output is OFF when NO_contact is FALSE; this is because the contact is open and does not allow signal. When NO_contact changes to TRUE, output immediately turns ON. If NO_contact stayed TRUE all the time, output would remain ON continuously.

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