Output coil in PLC Programming - Step-by-Step Execution

Concept Flow - Output coil

Input Condition

↓

Evaluate Condition

Yes↓

Set Output Coil ON

↓

END

No↓

Set Output Coil OFF

↓

END

The output coil turns ON if the input condition is true; otherwise, it turns OFF.

Execution Sample

PLC Programming

LD X0
OUT Y0

If input X0 is ON, output coil Y0 turns ON; if X0 is OFF, Y0 turns OFF.

Execution Table

Step	Input X0	Action	Output Y0
1	ON	Set Y0 ON	ON
2	OFF	Set Y0 OFF	OFF

💡 Execution ends after setting output coil based on input condition.

Variable Tracker

Variable	Start	After Step 1	After Step 2
X0	OFF	ON	OFF
Y0	OFF	ON	OFF

Key Moments - 2 Insights

Why does the output coil Y0 turn OFF when input X0 is OFF?

Because the output coil directly follows the input condition. When X0 is OFF (see execution_table step 2), the coil is set OFF to match the input state.

Can the output coil be ON if the input is OFF?

No, the output coil only turns ON when the input condition is true (ON). This is shown in execution_table step 1 where X0 is ON and Y0 is set ON.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of output Y0 at step 1?

A OFF

B ON

C Undefined

D Flashing

💡 Hint

Check the 'Output Y0' column in execution_table row for step 1.

At which step does the input X0 become OFF?

A Step 2

B Never

C Step 1

D Both steps

💡 Hint

Look at the 'Input X0' column in execution_table for each step.

If input X0 stays ON, what will happen to output Y0?

A Y0 will toggle ON and OFF

B Y0 will turn OFF

C Y0 will stay ON

D Y0 will stay OFF

💡 Hint

Refer to variable_tracker to see how Y0 changes when X0 is ON.

Concept Snapshot

Output coil turns ON when input condition is true.
If input is OFF, output coil turns OFF.
Syntax example: LD X0; OUT Y0
Used to control devices like motors or lights.
Directly reflects input state in output coil.

Full Transcript

An output coil in PLC programming is a command that controls an output device. It turns ON when the input condition is true and turns OFF when the input is false. For example, if input X0 is ON, the output coil Y0 will be set ON, activating the connected device. If X0 is OFF, Y0 will be OFF, turning the device off. This simple logic ensures the output coil always matches the input condition. The execution table shows two steps: when X0 is ON, Y0 is ON; when X0 is OFF, Y0 is OFF. The variable tracker confirms these state changes. Beginners often wonder why the coil turns OFF when input is OFF; it is because the coil directly follows the input condition. This concept is fundamental for controlling outputs in automation.

Practice

1. What is the main purpose of an output coil in PLC programming?

easy

A. To control devices like lights or motors by turning them ON or OFF

B. To read input signals from sensors

C. To store data temporarily

D. To perform mathematical calculations

Solution

1. Step 1: Understand the role of output coils

   Output coils are used to control physical devices by turning them ON or OFF based on program logic.

2. Step 2: Differentiate from other PLC elements

   Input signals are read by input instructions, data storage is done by registers, and calculations are done by math instructions, not output coils.

3. Final Answer:

   To control devices like lights or motors by turning them ON or OFF -> Option A

4. Quick Check:

   Output coil controls devices = B [OK]

Hint: Output coils always control devices ON/OFF [OK]

Common Mistakes:

• Confusing output coils with input instructions
• Thinking output coils store data
• Assuming output coils perform calculations

2. Which of the following is the correct syntax to define an output coil named Motor in ladder logic?

easy

A. ( ) Motor

B. COIL Motor

C. OUT Motor

D. SET Motor

Solution

1. Step 1: Recognize ladder logic output coil symbol

   In ladder logic, output coils are represented by parentheses around the coil name, like ( ) Motor.

2. Step 2: Compare other options

   OUT and SET are instructions but not standard coil syntax; COIL is not a typical keyword.

3. Final Answer:

   ( ) Motor -> Option A

4. Quick Check:

   Output coil uses parentheses = A [OK]

Hint: Output coils use parentheses in ladder logic [OK]

Common Mistakes:

• Using keywords like OUT or SET instead of parentheses
• Confusing coil syntax with input instructions
• Omitting parentheses around coil name

3. Given the ladder logic snippet:

( ) Light
--[ ] Switch

What happens when the Switch input is ON?

medium

A. Nothing changes

B. The Light output coil turns ON

C. The Switch input turns OFF

D. The Light output coil turns OFF

Solution

1. Step 1: Understand the ladder logic flow

   The normally open contact [ ] Switch controls the output coil ( ) Light. When Switch is ON (closed), current flows to Light coil.

2. Step 2: Determine output coil state

   Since Switch is ON, the Light coil energizes and turns ON the Light output.

3. Final Answer:

   The Light output coil turns ON -> Option B

4. Quick Check:

   Switch ON energizes Light coil = D [OK]

Hint: Input ON energizes output coil ON [OK]

Common Mistakes:

• Thinking output coil turns OFF when input is ON
• Confusing input and output roles
• Assuming no change occurs

4. Identify the error in this ladder logic snippet:

( ) Motor
--[ ] Start
--[/] Stop

where [/] is a normally closed contact. The Motor never stays ON even when Start is pressed and Stop is released.

medium

A. The output coil syntax is incorrect

B. The Stop contact should be normally open, not normally closed

C. The contacts are not connected in parallel

D. The Start contact is wired incorrectly

Solution

1. Step 1: Analyze contact wiring logic

   The Start and Stop contacts are in series, so both must be ON for Motor coil to energize.

2. Step 2: Understand typical start/stop control

   Usually, Start and Stop contacts are wired in parallel or with a latch to maintain Motor ON after Start is pressed.

3. Step 3: Identify wiring issue

   Because contacts are in series without latching, Motor never stays ON; wiring should be parallel or use latch logic.

4. Final Answer:

   The contacts are not connected in parallel -> Option C

5. Quick Check:

   Start/Stop need parallel or latch wiring = C [OK]

Hint: Start/Stop usually wired parallel or latched [OK]

Common Mistakes:

• Assuming output coil syntax is wrong
• Changing Stop contact type incorrectly
• Not recognizing wiring arrangement

5. You want to control a valve using an output coil named Valve. The valve should turn ON only if both Pressure_OK and Temperature_OK inputs are ON. Which ladder logic snippet correctly implements this?

hard

A. ( ) Valve
--[/] Pressure_OK
--[ ] Temperature_OK

B. ( ) Valve
--[ ] Pressure_OK
--[/] Temperature_OK

C. ( ) Valve
--[ ] Pressure_OK
--| | Temperature_OK

D. ( ) Valve
--[ ] Pressure_OK
--[ ] Temperature_OK

Solution

1. Step 1: Understand AND condition in ladder logic

   To turn Valve ON only if both inputs are ON, the contacts must be in series, both normally open.

2. Step 2: Check each option

   ( ) Valve
   --[ ] Pressure_OK
   --[ ] Temperature_OK
   

   has both [ ] Pressure_OK and [ ] Temperature_OK in series, correct for AND logic.

   ( ) Valve
   --[ ] Pressure_OK
   --[/] Temperature_OK
   

   uses normally closed for Temperature_OK, wrong.

   ( ) Valve
   --[ ] Pressure_OK
   --| | Temperature_OK
   

   uses invalid symbol | |.

   ( ) Valve
   --[/] Pressure_OK
   --[ ] Temperature_OK
   

   uses normally closed for Pressure_OK, wrong.

3. Final Answer:

   with both normally open contacts in series -> Option D

4. Quick Check:

   Both inputs ON in series = Valve ON [OK]

Hint: AND logic uses series normally open contacts [OK]

Common Mistakes:

• Using normally closed contacts incorrectly
• Confusing AND with OR logic
• Using invalid contact symbols