Series logic (AND condition) in PLC Programming - Step-by-Step Execution

Concept Flow - Series logic (AND condition)

Start

↓

Check Input A

↓

Is A ON?

No→Output OFF

Yes↓

Check Input B

↓

Is B ON?

No→Output OFF

Yes↓

Set Output ON

↓

End

The series logic checks each input in order; output turns ON only if all inputs are ON.

Execution Sample

PLC Programming

IF InputA AND InputB THEN
  Output := TRUE;
ELSE
  Output := FALSE;
END_IF;

This PLC code sets Output ON only if both InputA and InputB are ON.

Execution Table

Step	InputA	InputB	Condition (InputA AND InputB)	Output Action
1	OFF	OFF	FALSE	Output = FALSE
2	ON	OFF	FALSE	Output = FALSE
3	OFF	ON	FALSE	Output = FALSE
4	ON	ON	TRUE	Output = TRUE

💡 Output is TRUE only when both InputA and InputB are ON; otherwise FALSE.

Variable Tracker

Variable	Start	Step 1	Step 2	Step 3	Step 4
InputA	OFF	OFF	ON	OFF	ON
InputB	OFF	OFF	OFF	ON	ON
Output	FALSE	FALSE	FALSE	FALSE	TRUE

Key Moments - 2 Insights

Why is the output OFF when only one input is ON?

Because the AND condition requires both InputA and InputB to be ON. The execution_table rows 2 and 3 show the output remains FALSE when only one input is ON.

What happens if both inputs are OFF?

The output stays OFF since the AND condition is FALSE. See execution_table row 1 where both inputs are OFF and output is FALSE.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when InputA is ON and InputB is OFF?

A Output is TRUE

B Output is FALSE

C Output toggles ON and OFF

D Output is undefined

💡 Hint

Check execution_table row 2 where InputA=ON and InputB=OFF, output is FALSE.

At which step do both inputs become ON, causing the output to turn ON?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

See execution_table row 4 where InputA=ON and InputB=ON, output=TRUE.

If InputB changes from OFF to ON at step 3, how does the output change?

A Output stays FALSE until InputA is ON

B Output stays TRUE regardless

C Output turns TRUE immediately

D Output toggles randomly

💡 Hint

Refer to variable_tracker and execution_table rows 3 and 4; output only TRUE when both inputs ON.

Concept Snapshot

Series logic (AND condition):
- Output is ON only if all inputs are ON.
- Syntax: IF InputA AND InputB THEN Output := TRUE ELSE Output := FALSE END_IF;
- If any input is OFF, output is OFF.
- Used to ensure multiple conditions are met before action.

Full Transcript

Series logic with AND condition means the output turns ON only when all inputs are ON. The PLC code checks InputA and InputB. If both are ON, output becomes TRUE; otherwise, it stays FALSE. The execution table shows four steps with different input combinations and the resulting output. Beginners often wonder why output stays OFF if only one input is ON; this is because AND requires all inputs ON. The visual quiz tests understanding of input-output relations at each step.

Practice

1. What does series logic with an AND condition mean in PLC programming?

easy

A. The output is true when no input conditions are true

B. Only one input condition needs to be true for the output to be true

C. The output toggles randomly regardless of inputs

D. All input conditions must be true for the output to be true

Solution

1. Step 1: Understand AND condition meaning

   AND means all inputs must be true together to get true output.

2. Step 2: Apply to series logic

   In series logic, inputs are connected so output is true only if every input is true.

3. Final Answer:

   All input conditions must be true for the output to be true -> Option D

4. Quick Check:

   AND means all true = output true [OK]

Hint: AND means every input must be true for output true [OK]

Common Mistakes:

• Thinking only one input needs to be true
• Confusing AND with OR logic
• Assuming output toggles without input conditions

2. Which of the following is the correct syntax for a series AND condition in a PLC ladder logic rung?

easy

A. --( )--[ ]--[ ]-- output before inputs

B. --[ ]--+--[ ]--( )-- where contacts are in parallel

C. --[ ]--[ ]--( )-- where both contacts are in series

D. --[ ]--[ ]--[ ]-- without output coil

Solution

1. Step 1: Identify series AND syntax

   Series AND means contacts connected one after another in a line before output coil.

2. Step 2: Match with options

   --[ ]--[ ]--( )-- where both contacts are in series shows two contacts in series before output coil, which is correct.

3. Final Answer:

   --[ ]--[ ]--( )-- where both contacts are in series -> Option C

4. Quick Check:

   Series contacts before coil = AND logic [OK]

Hint: Series contacts before coil mean AND logic [OK]

Common Mistakes:

• Choosing parallel contacts which mean OR logic
• Placing output coil before inputs
• Missing output coil in the rung

3. Given the following PLC ladder logic snippet with two inputs I1 and I2 in series controlling output Q1:
--[ I1 ]--[ I2 ]--( Q1 )--
What will be the state of Q1 if I1 = TRUE and I2 = FALSE?

medium

A. Q1 will be FALSE

B. Q1 will be TRUE

C. Q1 will toggle ON and OFF

D. Q1 will remain in previous state

Solution

1. Step 1: Analyze input states in series

   Since I1 is TRUE but I2 is FALSE, the series AND condition fails because both must be TRUE.

2. Step 2: Determine output Q1 state

   Output Q1 will be FALSE because the AND condition requires all inputs TRUE.

3. Final Answer:

   Q1 will be FALSE -> Option A

4. Quick Check:

   AND needs all TRUE, here I2 is FALSE, so output FALSE [OK]

Hint: If any input FALSE in series, output is FALSE [OK]

Common Mistakes:

• Assuming output is TRUE if one input is TRUE
• Thinking output toggles without input change
• Ignoring the FALSE input in series

4. Identify the error in this PLC ladder logic snippet intended to implement series AND logic:
--[ I1 ]--+--[ I2 ]--( Q1 )--
|
( )
What is wrong with this design?

medium

A. Inputs I1 and I2 are in parallel, not series

B. Output coil Q1 is missing

C. Input I1 is not connected to power rail

D. There is no error; logic is correct

Solution

1. Step 1: Examine input connection style

   The '+' symbol and branching show I1 and I2 are connected in parallel, not series.

2. Step 2: Understand effect on logic

   Parallel inputs mean OR logic, not AND series logic as intended.

3. Final Answer:

   Inputs I1 and I2 are in parallel, not series -> Option A

4. Quick Check:

   Parallel = OR, not series AND [OK]

Hint: Parallel branches mean OR, series means one after another [OK]

Common Mistakes:

• Confusing parallel with series wiring
• Ignoring ladder diagram symbols
• Assuming output coil is missing

5. You want a motor to start only when three safety switches S1, S2, and S3 are all ON. Which ladder logic snippet correctly implements this series AND condition for output M1?

hard

A. --[ S1 ]--+--[ S2 ]--+--[ S3 ]--( M1 )--

B. --[ S1 ]--[ S2 ]--[ S3 ]--( M1 )--

C. --[ S1 ]--[ S2 ]--+--[ S3 ]--( M1 )--

D. --[ S1 ]--( M1 )--[ S2 ]--[ S3 ]--

Solution

1. Step 1: Identify series AND condition

   All switches must be connected in series (one after another) before output coil.

2. Step 2: Check each option

   --[ S1 ]--[ S2 ]--[ S3 ]--( M1 )-- shows S1, S2, S3 in series before M1 coil, correct for AND logic.

3. Final Answer:

   --[ S1 ]--[ S2 ]--[ S3 ]--( M1 )-- -> Option B

4. Quick Check:

   Series contacts = AND condition for motor start [OK]

Hint: Connect all switches in series before output coil for AND [OK]

Common Mistakes:

• Using parallel branches which create OR logic
• Placing output coil before inputs
• Mixing series and parallel incorrectly