State machine in ladder logic in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the initial state in the ladder logic.

PLC Programming

IF current_state = [1] THEN
  next_state := STATE_1;
END_IF

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

ASTATE_0

BSTATE_2

CSTATE_1

DSTATE_3

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a state other than the initial one.

Using a state number that does not exist.

The initial state is usually defined as STATE_0 to start the state machine.

2fill in blank

medium

Complete the code to transition to the next state when the input is true.

PLC Programming

IF input_signal = TRUE THEN
  current_state := [1];
END_IF

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

ASTATE_2

BSTATE_1

CSTATE_0

DSTATE_3

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the wrong next state.

Not updating the current_state variable.

When the input is true, the machine moves to STATE_2 as the next step.

3fill in blank

hard

Fix the error in the state transition condition.

PLC Programming

IF current_state = STATE_1 AND [1] = TRUE THEN
  current_state := STATE_2;
END_IF

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

Areset_signal

Binput_signal

Ctimer_done

Doutput_signal

Attempts:

3 left

💡 Hint

Common Mistakes

Using output_signal or timer_done incorrectly.

Confusing reset_signal with input_signal.

The transition depends on the input_signal being TRUE to move from STATE_1 to STATE_2.

4fill in blank

hard

Fill both blanks to complete the state machine logic for resetting and advancing states.

PLC Programming

IF [1] = TRUE THEN
  current_state := STATE_0;
ELSIF current_state = STATE_0 AND [2] = TRUE THEN
  current_state := STATE_1;
END_IF

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

Areset_signal

Binput_signal

Ctimer_done

Doutput_signal

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up reset and input signals.

Using timer_done or output_signal incorrectly.

The reset_signal resets the state machine to STATE_0, and input_signal moves it from STATE_0 to STATE_1.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps states to their next states based on input.

PLC Programming

state_transitions = {
  [1]: [2],
  STATE_1: STATE_2,
  STATE_2: [3]
}

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

ASTATE_0

BSTATE_1

CSTATE_3

Attempts:

3 left

💡 Hint

Common Mistakes

Incorrectly mapping states out of order.

Repeating the same state as next state.

STATE_0 maps to STATE_1, STATE_1 maps to STATE_2, and STATE_2 maps to STATE_3 in the state machine.

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