Parallel logic (OR condition) in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if either input A or input B is ON.

PLC Programming

if inputA [1] inputB then
    output := TRUE;
end_if;

Drag options to blanks, or click blank then click option'

AXOR

BOR

CNOT

DAND

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND instead of OR will require both inputs to be true.

Using NOT will invert the condition incorrectly.

Using XOR only returns true if exactly one input is true.

The OR operator checks if either inputA or inputB is TRUE, which is the correct parallel logic condition.

2fill in blank

medium

Complete the code to set the output ON if either sensor1 or sensor2 is active.

PLC Programming

output := sensor1 [1] sensor2;

Drag options to blanks, or click blank then click option'

AOR

BAND

CXOR

DNOT

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND will only set output if both sensors are active.

Using XOR will exclude the case where both sensors are active.

Using OR between sensor1 and sensor2 sets output to TRUE if either sensor is active.

3fill in blank

hard

Fix the error in the condition to correctly implement parallel logic with inputs X and Y.

PLC Programming

if X [1] Y then
    motor := TRUE;
end_if;

Drag options to blanks, or click blank then click option'

ANAND

BAND

CXOR

DOR

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND instead of OR changes the logic to series.

Using NAND or XOR changes the logic behavior.

The OR operator correctly implements parallel logic by activating motor if either X or Y is true.

4fill in blank

hard

Fill both blanks to create a condition that turns on the alarm if either button1 or button2 is pressed.

PLC Programming

if button1 [1] button2 [2] TRUE then
    alarm := TRUE;
end_if;

Drag options to blanks, or click blank then click option'

AOR

BAND

C=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND will require both buttons pressed.

Using <> (not equal) is incorrect for this logic.

Using OR between buttons and comparing the result to TRUE correctly checks if either button is pressed.

5fill in blank

hard

Fill all three blanks to create a parallel logic condition that sets light ON if sensorA or sensorB is active and override is OFF.

PLC Programming

if (sensorA [1] sensorB) [2] override [3] FALSE then
    light := TRUE;
end_if;

Drag options to blanks, or click blank then click option'

AOR

BAND

C=

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND instead of OR between sensors changes logic.

Using <> instead of = for override check is incorrect.

The condition uses OR to check sensors, AND to combine with override status, and compares override to FALSE.

Practice

1. In PLC programming, what does a parallel logic (OR condition) do?
Input1 ---| |---+---(Output)
Input2 ---| |---+

easy

A. Output never activates regardless of inputs

B. Output activates only if both Input1 and Input2 are ON

C. Output activates if either Input1 or Input2 is ON

D. Output activates only if Input1 is OFF and Input2 is ON

Solution

1. Step 1: Understand parallel logic in PLC

   Parallel contacts represent an OR condition, meaning output activates if any input is ON.

2. Step 2: Analyze given inputs

   Since Input1 and Input2 are in parallel, output activates if either Input1 or Input2 is ON.

3. Final Answer:

   Output activates if either Input1 or Input2 is ON -> Option C

4. Quick Check:

   Parallel logic (OR) = Output ON if any input ON [OK]

Hint: Parallel contacts mean OR condition: any input ON activates output [OK]

Common Mistakes:

• Thinking parallel means AND condition
• Assuming output needs both inputs ON
• Confusing series and parallel contacts

2. Which of the following is the correct ladder logic syntax for an OR condition between two inputs Start and Reset to control Motor?

easy

A. Motor := Start AND Reset;

B. Motor := Start XOR Reset;

C. Motor := NOT(Start OR Reset);

D. Motor := Start OR Reset;

Solution

1. Step 1: Identify OR operator syntax

   In PLC structured text, OR is written as 'OR' between conditions.

2. Step 2: Match correct expression

   Motor := Start OR Reset; uses 'Start OR Reset' which correctly represents OR condition for Motor.

3. Final Answer:

   Motor := Start OR Reset; -> Option D

4. Quick Check:

   OR operator syntax = Motor := Start OR Reset [OK]

Hint: Use 'OR' keyword to combine inputs for parallel logic [OK]

Common Mistakes:

• Using AND instead of OR
• Using XOR which is exclusive OR
• Negating the OR condition incorrectly

3. Given the following ladder logic snippet:

---[ ]---+---( )---
  Start  |  Motor
---[ ]---+
  Stop   

Where [ ] is a normally open contact and ( ) is the coil, what is the output state of Motor if Start is OFF and Stop is ON?

medium

A. Motor is ON

B. Motor is OFF

C. Motor toggles ON and OFF rapidly

D. Motor state is undefined

Solution

1. Step 1: Understand parallel contacts behavior

   Motor coil energizes if either Start or Stop contact is closed (ON).

2. Step 2: Check input states

   Start is OFF (open), Stop is ON (closed), so current flows through Stop contact to Motor coil.

3. Final Answer:

   Motor is ON -> Option A

4. Quick Check:

   Parallel OR logic = Motor ON if any input ON [OK]

Hint: If any parallel input is ON, output coil energizes [OK]

Common Mistakes:

• Assuming both inputs must be ON
• Confusing normally open with normally closed contacts
• Ignoring parallel wiring effect

4. Identify the error in this ladder logic snippet intended to implement an OR condition:

---[ ]---[ ]---( )---
  Input1  Input2  Output

medium

A. Contacts are in series, not parallel, so it's an AND condition

B. Output coil is missing

C. Input2 contact is normally closed instead of normally open

D. No error, logic is correct

Solution

1. Step 1: Analyze contact arrangement

   Contacts are placed one after another (series), which means both must be ON for output.

2. Step 2: Compare with OR condition requirement

   OR requires parallel contacts, so this is an AND condition, not OR.

3. Final Answer:

   Contacts are in series, not parallel, so it's an AND condition -> Option A

4. Quick Check:

   Series contacts = AND, parallel contacts = OR [OK]

Hint: OR needs parallel contacts, series means AND [OK]

Common Mistakes:

• Confusing series and parallel wiring
• Ignoring contact types
• Assuming output coil is missing

5. You want to design a PLC rung that turns ON an alarm if any of three sensors (SensorA, SensorB, SensorC) detect a fault. Which ladder logic arrangement correctly implements this parallel OR condition?

hard

A. Use NOT contacts for all sensors in series feeding the alarm coil

B. Place all three sensor contacts in parallel feeding the alarm coil

C. Place all three sensor contacts in series feeding the alarm coil

D. Use XOR contacts between sensors feeding the alarm coil

Solution

1. Step 1: Understand alarm activation logic

   Alarm should activate if any sensor detects fault, so OR condition is needed.

2. Step 2: Choose correct contact arrangement

   Parallel contacts represent OR logic, so placing sensors in parallel activates alarm if any sensor is ON.

3. Final Answer:

   Place all three sensor contacts in parallel feeding the alarm coil -> Option B

4. Quick Check:

   Parallel contacts = OR = Alarm ON if any sensor ON [OK]

Hint: Use parallel contacts for multiple inputs triggering one output [OK]

Common Mistakes:

• Using series contacts which require all sensors ON
• Using NOT contacts incorrectly
• Using XOR which only activates if exactly one sensor is ON