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Output coil in PLC Programming

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Introduction
An output coil turns a device on or off in a PLC program, like switching a light or motor.
To turn on a motor when a start button is pressed.
To switch on a light when a sensor detects something.
To activate an alarm when a condition is met.
To control a valve opening or closing.
To signal a machine to stop or start.
Syntax
PLC Programming
OutputCoil (OutputName)
OutputCoil is the command to control an output device.
OutputName is the label for the device you want to control.
Examples
Turns on the motor when conditions before this coil are true.
PLC Programming
OutputCoil (MotorStart)
Switches on a light output coil.
PLC Programming
OutputCoil (LightOn)
Activates an alarm output coil.
PLC Programming
OutputCoil (Alarm)
Sample Program
This program simulates pressing a start button. When pressed, the output coil LightOn turns TRUE, meaning the light is on.
PLC Programming
(* Simple PLC program to turn on a light when a start button is pressed *)
VAR
  StartButton : BOOL := FALSE; (* Input *)
  LightOn : BOOL := FALSE;     (* Output coil *)
END_VAR

(* Logic: If StartButton is TRUE, turn on LightOn coil *)
IF StartButton THEN
  LightOn := TRUE;
ELSE
  LightOn := FALSE;
END_IF;

(* Simulate pressing the start button *)
StartButton := TRUE;

(* Run logic again to update output coil *)
IF StartButton THEN
  LightOn := TRUE;
ELSE
  LightOn := FALSE;
END_IF;

(* Print output coil state *)
WriteLn('LightOn output coil state: ' + TO_STRING(LightOn));
OutputSuccess
Important Notes
Output coils represent physical devices controlled by the PLC.
The coil turns ON (TRUE) or OFF (FALSE) based on the logic before it.
Always label output coils clearly to match the device they control.
Summary
Output coils control devices like lights, motors, or alarms.
They turn ON or OFF based on program logic.
Use clear names to keep your program easy to understand.

Practice

(1/5)
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
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