State machine in ladder logic in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple 2-state ladder logic state machine

Consider a ladder logic program with two states: S0 and S1. The program starts in S0. When input I0 is ON, it transitions to S1. When input I1 is ON, it transitions back to S0. What is the output Q0 after the following input sequence: I0=ON, I1=OFF, then I0=OFF, I1=ON?

PLC Programming

State S0: If I0 ON then go to S1
State S1: If I1 ON then go to S0
Output Q0 is ON only in state S1

A Q0 is OFF after the sequence

B Q0 remains in its initial state

C Q0 is ON after the sequence

D Q0 toggles rapidly and ends ON

💡 Hint

Trace the states step-by-step with the input changes.

The machine starts in S0 with Q0 OFF. I0 ON moves it to S1 (Q0 ON). Then I0 OFF and I1 ON moves it back to S0 (Q0 OFF). So final Q0 is OFF.

📝 Syntax

intermediate

Identify the correct rung for a state transition

Which ladder logic rung correctly implements a transition from state S0 to S1 when input I0 is ON, using a memory bit M0 to represent S0 and M1 for S1?

A

|---[ M0 ]---[ I0 ]---( M1 )---|
|---[ M1 ]---( )---|  

B

|---[ I0 ]---( M1 )---|
|---[ M0 ]---( )---|  

C

|---[ M0 ]---[ I0 ]---( M1 )---|
|---[ M0 ]---( )---|  

D

|---[ I0 ]---[ M0 ]---( M1 )---|
|---[ M1 ]---( )---|  

💡 Hint

The coil M1 should be energized only when M0 and I0 are both ON.

Option A correctly uses contacts M0 and I0 in series to energize M1, representing the transition from S0 to S1. Other options either have wrong order or missing conditions.

🔧 Debug

advanced

Find the error in this ladder logic state machine snippet

Given this ladder logic snippet for a 3-state machine (states M0, M1, M2), which line causes the machine to get stuck in state M1?

PLC Programming

|---[ M0 ]---[ I0 ]---( M1 )---|
|---[ M1 ]---[ I1 ]---( M2 )---|
|---[ M2 ]---[ I2 ]---( M0 )---|
|---[ M1 ]---( M1 )---|

A The line '|---[ M1 ]---[ I1 ]---( M2 )---|' causes the problem

B The line '|---[ M0 ]---[ I0 ]---( M1 )---|' causes the problem

C The line '|---[ M1 ]---( M1 )---|' causes the problem

D The line '|---[ M2 ]---[ I2 ]---( M0 )---|' causes the problem

💡 Hint

Look for a line that forces M1 to stay ON unconditionally.

The line '|---[ M1 ]---( M1 )---|' latches M1 ON regardless of input I1, preventing transition to M2 and causing the machine to get stuck in M1.

🚀 Application

advanced

Design a ladder logic snippet for a 2-state toggle machine

You want a ladder logic state machine that toggles between states S0 and S1 each time input I0 goes from OFF to ON (rising edge). Which rung correctly implements this toggle using memory bits M0 and M1?

A

|---[ I0 ]---( M1 )---|
|---[ NOT I0 ]---( M0 )---|

B

|---[ I0 ]---[ NOT M1 ]---( M1 )---|
|---[ I0 ]---[ M1 ]---( M0 )---|

C

|---[ I0 ]---[ NOT M0 ]---( M1 )---|
|---[ I0 ]---[ M0 ]---( M0 )---|

D

|---[ I0 ]---[ M0 ]---( M1 )---|
|---[ I0 ]---[ M1 ]---( M0 )---|

💡 Hint

Toggle means when I0 is ON, switch from current state to the other.

Option B uses I0 and the inverse of M1 to set M1, and I0 and M1 to set M0, effectively toggling between M0 and M1 on each I0 rising edge.

🧠 Conceptual

expert

Why is a state machine implemented in ladder logic considered deterministic?

In ladder logic state machines, why is the behavior considered deterministic?

A Because the state machine can jump to any state at any time

B Because ladder logic uses random timers to decide next states

C Because the outputs change randomly to handle faults

D Because for each input and current state, the next state and outputs are uniquely defined

💡 Hint

Think about how state machines respond to inputs and current states.

Deterministic means the next state and outputs depend only on current state and inputs, with no randomness. Ladder logic state machines follow this rule.

Practice

1. What is the main purpose of a state machine in ladder logic?

easy

A. To control a process by remembering and switching between states

B. To perform mathematical calculations

C. To store large amounts of data

D. To display information on an HMI screen

Solution

1. Step 1: Understand state machine concept

   A state machine controls a process by remembering the current step or state and moving to the next based on conditions.

2. Step 2: Compare options to concept

   Only To control a process by remembering and switching between states describes controlling process flow by switching states, which matches the state machine purpose.

3. Final Answer:

   To control a process by remembering and switching between states -> Option A

4. Quick Check:

   State machine controls steps = D [OK]

Hint: State machines remember and switch states to control flow [OK]

Common Mistakes:

• Confusing state machine with data storage
• Thinking it only does math
• Assuming it controls display only

2. Which ladder logic element is typically used to represent a state in a state machine?

easy

A. Timer

B. Bit or coil

C. Counter

D. Analog input

Solution

1. Step 1: Identify state representation in ladder logic

   States are usually represented by bits or coils that can be ON or OFF to show current state.

2. Step 2: Eliminate other options

   Timers and counters are for timing/counting, analog inputs read values, so only bits/coils represent states.

3. Final Answer:

   Bit or coil -> Option B

4. Quick Check:

   States use bits/coils = C [OK]

Hint: States are bits or coils ON/OFF in ladder logic [OK]

Common Mistakes:

• Choosing timers or counters as states
• Confusing analog inputs with states
• Not knowing bits represent ON/OFF

3. Given this ladder logic snippet for a state machine:

--[ State1 ]--+--[ Condition ]--( State2 )
              |
              +--( Reset State1 )

What happens when Condition is TRUE?

medium

A. State2 turns OFF and State1 turns ON

B. State1 and State2 both turn ON

C. State1 stays ON and State2 stays OFF

D. State1 turns OFF and State2 turns ON

Solution

1. Step 1: Analyze ladder logic behavior on Condition TRUE

   When Condition is TRUE, the coil for State2 energizes and the Reset coil for State1 de-energizes State1.

2. Step 2: Understand state transition

   This means State1 turns OFF and State2 turns ON, moving the machine to the next state.

3. Final Answer:

   State1 turns OFF and State2 turns ON -> Option D

4. Quick Check:

   Condition TRUE switches states = A [OK]

Hint: Condition TRUE switches OFF current and ON next state [OK]

Common Mistakes:

• Thinking both states turn ON simultaneously
• Assuming no change happens
• Confusing which state resets

4. You wrote this ladder logic for a state machine:

--[ State1 ]--+--[ Condition ]--( State2 )
              +--( State1 )

Why does the state machine never leave State1?

medium

A. Because State1 coil is never reset, so it stays ON

B. Because Condition is never TRUE

C. Because State2 coil is missing

D. Because the rung has a syntax error

Solution

1. Step 1: Check how State1 coil is controlled

   State1 coil is energized again on the same rung without a reset coil, so it stays ON continuously.

2. Step 2: Understand why state never changes

   Without turning OFF State1, the machine cannot move to State2 even if Condition is TRUE.

3. Final Answer:

   Because State1 coil is never reset, so it stays ON -> Option A

4. Quick Check:

   No reset keeps state ON = B [OK]

Hint: Always reset current state coil to move states [OK]

Common Mistakes:

• Ignoring need to reset current state
• Assuming Condition is always FALSE
• Thinking missing coil causes lock

5. You want to design a 3-step state machine in ladder logic: State1 -> State2 -> State3 -> State1. Which approach correctly implements the transitions?

hard

A. Use timers to switch states automatically without conditions

B. Use one bit for all states and toggle it on each condition

C. Use bits for each state; each transition resets current state bit and sets next state bit based on conditions

D. Use counters to count states and reset after 3

Solution

1. Step 1: Understand state machine design

   A proper state machine uses separate bits for each state and transitions that turn OFF the current state bit and ON the next state bit.

2. Step 2: Evaluate options for correct implementation

   Use bits for each state; each transition resets current state bit and sets next state bit based on conditions matches this approach; the other options do not correctly represent state transitions in ladder logic.

3. Final Answer:

   Use bits for each state; each transition resets current state bit and sets next state bit based on conditions -> Option C

4. Quick Check:

   Separate bits with reset/set transitions = A [OK]

Hint: Reset current and set next state bits for transitions [OK]

Common Mistakes:

• Using one bit for all states
• Relying only on timers or counters
• Not resetting current state bit