A Programmable Logic Controller (PLC) is like the brain of an industrial automation system. It continuously monitors input signals, makes decisions based on the programmed logic, and controls outputs such as motors, pumps, or valves.
But have you ever wondered how a PLC actually thinks and acts?
It all happens through something called the PLC Scan Cycle which is nothing but a repeating sequence that runs every few milliseconds.
Let us understand it step by step.
What is the PLC Scan Cycle?
The PLC Scan Cycle is the continuous loop that a PLC follows to read inputs, execute the program and update outputs.
It is similar to how our brain constantly reads information from our senses, decides what to do, and then makes our body respond.
This scan cycle happens thousands of times per second making PLCs incredibly fast and reliable for industrial automation.
A typical PLC scan has four main stages:
Input Scan
Program Execution
Communication and Diagnostics
Output Scan
Step 1: Input Scan
In the first step, the PLC reads the status of all input devices connected to it — such as sensors, switches, and transmitters.
It stores these readings in an internal memory area called the Input Image Table.
Example:
If a limit switch is ON, the PLC stores a “1” for that input address.
If it’s OFF, it stores a “0”.
This means the PLC doesn’t directly read input devices while executing logic. Instead, it uses the stored values from memory.
Step 2: Program Execution
After reading all inputs, the PLC moves to the next step executing the logic program (like ladder logic, function block diagram, or structured text).
The PLC processes the logic line by line from top to bottom, left to right.
It uses the stored input values (from Step 1) to decide what each output should be.
The results are stored in another memory area called the Output Image Table, but the outputs are not yet sent to the field devices.
Example:
If your logic says “Start Motor if Start Button is ON and Stop Button is OFF,”
the PLC checks those conditions and sets the output memory for the motor accordingly.
Step 3: Communication and Housekeeping
Once the program logic is executed, the PLC performs some background tasks before updating outputs.
This includes:
- Communication with HMIs (Human Machine Interfaces) or SCADA systems
- Internal diagnostics (checking module status, battery, etc.)
- PID calculations (if used for control loops)
- Handling timers, counters, and data exchange
These are called housekeeping tasks and make sure the PLC system remains healthy and responsive.
Step 4: Output Scan
Finally, the PLC updates the status of all output devices such as relays, solenoids, lamps, and actuators based on the results stored in the Output Image Table.
Example:
If the logic turned ON the motor output, the PLC sends a signal from the corresponding output terminal to start the motor.
After this step, one complete scan cycle is finished and the PLC immediately starts the next one.
How Fast is the PLC Scan Cycle?
The time it takes to complete one full cycle (input → logic → output) is called the Scan Time.
Typical scan times range from 1 to 50 milliseconds, depending on:
- Program size (number of rungs or instructions)
- Processor speed
- Communication load
- Type of PLC (micro, modular, or advanced type)
Shorter scan times mean the PLC can react faster to input changes, which is important for high-speed processes.
Why Scan Time Matters
Let us say a sensor sends a very short pulse (for example, 5 milliseconds).
If your PLC scan time is 10 milliseconds, the PLC may miss that pulse because it only reads inputs once per scan.
To handle such fast signals, PLCs use high-speed inputs or interrupt routines that react instantly without waiting for the next scan.
Example of a PLC Scan Cycle
Imagine a simple conveyor system:
- Input: Start push button (I0.0)
- Output: Conveyor motor (Q0.0)
The program logic:
If the start button is pressed, turn ON the motor.
Now let’s see what happens during one scan:
Input Scan: PLC reads I0.0 = 1 (button pressed)
Program Execution: Logic evaluates — Start button ON → set motor ON
Output Scan: PLC writes Q0.0 = 1 → Motor runs
Next scan: if the button is released (I0.0 = 0), the PLC detects that change in the next cycle and turns OFF the motor.
This continuous process keeps your system responsive and safe.
Understanding Input and Output Images
It’s important to remember that PLCs don’t work directly with live inputs and outputs while running logic.
Instead, they use copies of data stored in memory known as the Input and Output Image Tables.
This helps the PLC process the program faster and ensures stable operation, even when inputs change rapidly.
How to Reduce Scan Time
If your PLC scan time is too high, try these:
- Simplify your logic and avoid unnecessary rungs or repeated calculations.
- Use subroutines and modular programming.
- Limit communication frequency with external devices.
- Use efficient data types and structured tags.
- Upgrade your CPU if your process is high-speed.
Applications of PLC Scan Cycle
Understanding the scan cycle helps in designing reliable systems such as:
- Packaging machines
- Conveyor control systems
- Water treatment plants
- Boiler automation
- Batch mixing systems
In these systems, quick and predictable PLC response is critical to safety and performance.
What we learn today?
The PLC Scan Cycle is the heart of every automation system.
It works like this:
1) Read inputs
2) Execute program logic
3) Perform internal checks
4) Update outputs
This cycle repeats thousands of times per second making PLCs powerful, accurate and dependable.
By understanding how the scan cycle works, you can write better logic, troubleshoot faster, and design safer automation systems.
Frequently Asked Questions on PLC Scan Cycle
Q1: Why didn’t my PLC react when I pressed a button quickly?
Because the input might have changed between scans. You may need to use a high-speed input or latching logic.
Q2: Why do timers behave differently in large programs?
Longer scan times affect the accuracy of software timers. Always check your actual scan time when tuning delays.
Q3: What happens if a PLC takes too long to complete a scan?
The process becomes slow, and in critical systems, it may cause alarms or unsafe conditions.
I hope you like above blog. There is no cost associated in sharing the article in your social media. Thanks for reading!! Happy Learning!!
3 Comments