PLC scan cycle (input scan, program execution, output update) in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are the three main steps of a PLC scan cycle?

The three main steps are: 1) Input Scan - reading all input devices, 2) Program Execution - running the user program logic, 3) Output Update - updating all output devices based on the program results.

beginner

Why does a PLC scan cycle start with an input scan?

The PLC reads all inputs first to get the current state of sensors and switches. This ensures the program works with the latest real-world information.

beginner

What happens during the program execution step in a PLC scan cycle?

During program execution, the PLC processes the logic written by the user using the input data. It decides what outputs should be turned on or off.

beginner

Explain the output update step in the PLC scan cycle.

In the output update step, the PLC sends signals to output devices like motors or lights based on the program's decisions. This controls the physical devices.

intermediate

What could happen if the PLC did not follow the scan cycle order?

If the order is not followed, the PLC might use old input data or update outputs incorrectly, causing machines to behave unexpectedly or unsafely.

What is the first step in a PLC scan cycle?

A Error Checking

B Program Execution

C Output Update

D Input Scan

The PLC always starts by scanning inputs to get the current state of sensors and switches.

During which step does the PLC run the user program logic?

A Program Execution

B Input Scan

C Output Update

D Diagnostics

Program Execution is when the PLC processes the logic using the input data.

What does the output update step do?

A Reads sensor data

B Runs the user program

C Sends signals to output devices

D Checks for errors

Output Update sends signals to control devices like motors or lights.

Why is the scan cycle order important?

A To save memory

B To ensure correct and safe machine operation

C To speed up the program

D To reduce power consumption

Following the correct order ensures the PLC uses fresh data and controls outputs properly.

Which step uses the latest input data to decide outputs?

A Program Execution

B Output Update

C Input Scan

D Memory Clear

Program Execution uses the input data to decide what outputs to activate.

Describe the PLC scan cycle steps and why each step is important.

Think about how the PLC interacts with sensors and machines step-by-step.

Explain what might happen if the PLC updates outputs before scanning inputs.

Consider the flow of information from sensors to outputs.

Practice

1. What is the correct order of the PLC scan cycle steps?

easy

A. Output update, input scan, program execution

B. Input scan, program execution, output update

C. Program execution, input scan, output update

D. Program execution, output update, input scan

Solution

1. Step 1: Understand the PLC scan cycle process

   The PLC first reads all inputs to get the current state of sensors and switches.

2. Step 2: Follow the sequence of operations

   After reading inputs, the PLC runs the user program logic, then updates outputs accordingly.

3. Final Answer:

   Input scan, program execution, output update -> Option B

4. Quick Check:

   Scan cycle order = Input, Program, Output [OK]

Hint: Remember: Inputs first, then logic, then outputs [OK]

Common Mistakes:

• Mixing up the order of program execution and input scan
• Thinking outputs update before program runs
• Assuming outputs update before inputs are scanned

2. Which of the following is the correct syntax to represent the PLC scan cycle steps in order?

easy

A. InputScan(); ProgramExecution(); OutputUpdate();

B. OutputUpdate(); InputScan(); ProgramExecution();

C. ProgramExecution(); OutputUpdate(); InputScan();

D. OutputUpdate(); ProgramExecution(); InputScan();

Solution

1. Step 1: Identify the correct function call order

   The PLC scan cycle functions must be called in the order: InputScan, ProgramExecution, then OutputUpdate.

2. Step 2: Match the syntax with the correct order

   InputScan(); ProgramExecution(); OutputUpdate(); correctly shows the functions in the right sequence with proper syntax.

3. Final Answer:

   <code>InputScan(); ProgramExecution(); OutputUpdate();</code> -> Option A

4. Quick Check:

   Function calls order = InputScan, ProgramExecution, OutputUpdate [OK]

Hint: Functions follow the scan cycle order: input, program, output [OK]

Common Mistakes:

• Placing OutputUpdate before InputScan
• Swapping ProgramExecution and OutputUpdate calls
• Incorrect function call syntax

3. Given the following PLC scan cycle pseudo-code, what will be the output state after one full scan if input sensor is ON?

InputScan(): sensor = ON
ProgramExecution(): if sensor == ON then output = ON else output = OFF
OutputUpdate(): set output device to output

medium

A. Output device state will not change

B. Output device will be OFF

C. Output device will be ON

D. Output device will toggle randomly

Solution

1. Step 1: Analyze input scan step

   The sensor input is read as ON during InputScan.

2. Step 2: Evaluate program execution logic

   Since sensor == ON, the program sets output = ON.

3. Step 3: Understand output update effect

   OutputUpdate sets the output device state to ON based on program logic.

4. Final Answer:

   Output device will be ON -> Option C

5. Quick Check:

   Sensor ON leads to output ON [OK]

Hint: Input ON means output ON after program runs [OK]

Common Mistakes:

• Confusing input and output states
• Ignoring program logic conditions
• Assuming output updates before program execution

4. A PLC program is not updating outputs correctly. The code snippet is:

OutputUpdate()
InputScan()
ProgramExecution()

What is the main problem here?

medium

A. OutputUpdate is called before inputs are scanned and program runs

B. InputScan should be last in the cycle

C. ProgramExecution should be before OutputUpdate only

D. No problem, this order is correct

Solution

1. Step 1: Review the correct PLC scan cycle order

   The correct order is InputScan, ProgramExecution, then OutputUpdate.

2. Step 2: Identify the error in the given code

   OutputUpdate is called first, before inputs are scanned and program logic runs, causing incorrect output updates.

3. Final Answer:

   OutputUpdate is called before inputs are scanned and program runs -> Option A

4. Quick Check:

   OutputUpdate must be last in scan cycle [OK]

Hint: Output update must come after input scan and program execution [OK]

Common Mistakes:

• Calling OutputUpdate too early
• Swapping InputScan and ProgramExecution order
• Assuming any order works for scan cycle

5. You want to ensure a safety output turns OFF immediately when an emergency stop input is pressed. Which scan cycle step should you prioritize to minimize delay?

hard

A. None, PLC scan cycle cannot be changed

B. Program execution to run safety logic faster

C. Output update to apply changes immediately

D. Input scan to detect emergency stop as early as possible

Solution

1. Step 1: Understand the scan cycle timing

   The input scan step reads all inputs at the start of the cycle, so detecting emergency stop early is critical.

2. Step 2: Prioritize input scan for safety

   By ensuring input scan happens promptly, the PLC can detect emergency stop quickly and then run program logic to turn off outputs.

3. Step 3: Output update applies changes but depends on earlier steps

   Output update applies the result but cannot act before inputs are scanned and logic runs.

4. Final Answer:

   Input scan to detect emergency stop as early as possible -> Option D

5. Quick Check:

   Early input scan = faster emergency response [OK]

Hint: Detect emergency input ASAP in input scan step [OK]

Common Mistakes:

• Thinking output update speed alone improves safety
• Ignoring input scan timing importance
• Assuming program execution speed is the only factor