Parallel logic (OR condition) in PLC Programming - Step-by-Step Execution

Concept Flow - Parallel logic (OR condition)

Input A

↓

OR Condition

↓

Output

↓

END

Two inputs are checked in parallel; if either is true, the output is true.

Execution Sample

PLC Programming

A = TRUE
B = FALSE
Output = A OR B
// Output is TRUE because A is TRUE

This code checks two inputs A and B; output is true if either A or B is true.

Execution Table

Step	Input A	Input B	OR Condition (A OR B)	Output
1	TRUE	FALSE	TRUE OR FALSE	TRUE
2	FALSE	FALSE	FALSE OR FALSE	FALSE
3	FALSE	TRUE	FALSE OR TRUE	TRUE
4	TRUE	TRUE	TRUE OR TRUE	TRUE
5	-	-	-	END of logic evaluation

💡 All input combinations evaluated; output reflects OR logic result.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
A	TRUE	FALSE	FALSE	TRUE	TRUE	-
B	FALSE	FALSE	TRUE	TRUE	TRUE	-
Output	-	TRUE	FALSE	TRUE	TRUE	-

Key Moments - 2 Insights

Why is the output TRUE when only one input is TRUE?

Because the OR condition returns TRUE if at least one input is TRUE, as shown in execution_table rows 1, 3, and 4.

What happens if both inputs are FALSE?

The output is FALSE since OR requires at least one TRUE input; see execution_table row 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 2?

A FALSE

B Depends on input A

C TRUE

D Depends on input B

💡 Hint

Check execution_table row 2 under Output column.

At which step are both inputs TRUE?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look at Input A and Input B columns in execution_table.

If input A changes from TRUE to FALSE at step 1, what happens to output?

A Output stays TRUE

B Output depends on input B

C Output becomes FALSE

D Output becomes TRUE only if input B is TRUE

💡 Hint

Refer to execution_table rows 1 and 3 for output changes.

Concept Snapshot

Parallel logic (OR condition):
- Checks multiple inputs at the same time
- Output is TRUE if any input is TRUE
- Output is FALSE only if all inputs are FALSE
- Common in PLC for safety and control
- Syntax: Output = InputA OR InputB

Full Transcript

Parallel logic with OR condition means checking two or more inputs at the same time. If any input is TRUE, the output becomes TRUE. If all inputs are FALSE, the output is FALSE. For example, if input A is TRUE and input B is FALSE, the output is TRUE because OR needs only one TRUE. This logic is common in PLC programming to control machines safely. The execution table shows all input combinations and their outputs step by step.

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