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Inside a PLC: how scan cycles, I/O modules and logic execution really work
Published on 31 March 26
Programmable Logic Controllers (PLCs) are the backbone of modern industrial automation. From manufacturing lines and packaging machines to water treatment plants and building control systems, PLCs provide the reliable, real-time control that keeps processes running safely and efficiently
But what actually happens inside a PLC once it’s powered on? How does it read sensors, process logic and control outputs in fractions of a second?
In this guide, we’ll explore the internal workings of a PLC - including scan cycles, I/O modules and logic execution - to give you a clear understanding of how these powerful industrial computers operate.
What Is a PLC?
A Programmable Logic Controller (PLC) is a ruggedised industrial computer designed to monitor inputs, execute programmed logic and control outputs in automated systems.
Unlike general-purpose computers, PLCs are built specifically for industrial environments. They are designed to withstand electrical noise, vibration, extreme temperatures and continuous operation.
Typical PLC applications include:
Factory automation
Conveyor and packaging systems
Process control
Robotics integration
Energy and utilities management
Building automation
PLCs continuously monitor inputs from devices such as sensors, switches and transducers, process the data using programmed logic and then activate outputs such as motors, valves or alarms.
The core components inside a PLC
Although PLCs come in many sizes and configurations, most systems share the same fundamental components.
1. CPU (Central Processing Unit)
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The CPU is the brain of the PLC. It executes the control program and coordinates communication between all system components.
Its responsibilities include:
Executing user logic programs
Managing memory
Coordinating input and output updates
Handling communications with other devices
Running diagnostics and system checks
Modern PLC CPUs can process thousands of logic instructions per millisecond.
2. Power supply
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The power supply converts incoming electrical power (often 24V DC or 120/230V AC) into the stable voltages required by the PLC’s internal electronics.
Industrial PLC power supplies are designed with protection features such as:
Over-voltage protection
Short-circuit protection
Noise filtering
This ensures stable operation even in electrically noisy environments.
3. I/O Modules
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Input/Output (I/O) modules are what allow a PLC to interact with the real world.
They act as the interface between field devices and the PLC’s internal logic.
I/O modules typically fall into two main categories.
The PLC converts these signals into numerical values that can be used within the control program.
4. Communication interfaces
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Many PLCs include communication ports that allow them to connect with other devices and systems.
Common industrial communication protocols include:
Modbus
PROFIBUS
EtherNet/IP
PROFINET
These allow PLCs to communicate with:
Human-Machine Interfaces (HMIs)
SCADA systems
Remote I/O
Variable frequency drives
Other PLCs
Understanding the PLC scan cycle
At the heart of PLC operation is something called the scan cycle.
A PLC does not process events continuously in parallel like a human brain might. Instead, it works through a repeating sequence of steps known as a scan.
Each scan typically takes a few milliseconds with the scan cycle usually consisting of four stages.
1. Input scan
First, the PLC reads the current state of all input devices.
Rather than working directly with raw signals, the PLC stores these values in a section of memory called the input image table.
For example:
Input Device - State
Start button - ON
Stop button - OFF
Safety switch - ON
The program then works with these stored values during execution.
2. Program execution
Next, the PLC executes the user program.
This is usually written using industrial programming languages defined by IEC 61131-3, which include:
Ladder Logic (LD)
Function Block Diagram (FBD)
Structured Text (ST)
Sequential Function Chart (SFC)
The program runs from top to bottom, left to right, evaluating conditions and determining whether outputs should turn on or off.
3. Output scan
Once the logic has been evaluated, the PLC updates the outputs.
The calculated results are stored in the output image table and the PLC then sends these signals to the physical output modules.
This might activate:
Motors
Valves
Indicators
Relays
4. Housekeeping tasks
Finally, the PLC performs internal maintenance tasks such as:
Diagnostics
Communication handling
Memory management
Error checking
Once this is complete, the PLC immediately begins the next scan cycle.
This process repeats continuously - often hundreds or thousands of times per second.
Why scan time matters
The total time it takes to complete one scan cycle is known as the scan time.
Scan time depends on factors such as:
Program complexity
Number of I/O points
Communication tasks
PLC processor speed
Typical scan times range from 1 ms to 20 ms.
Fast scan times are critical in applications where timing matters, such as:
High-speed manufacturing
Motion control
Safety systems
Packaging machinery
If the scan time is too slow, the PLC may not respond quickly enough to changing conditions.
Memory areas inside a PLC
PLCs organise data into different memory areas to manage system operations.
Common memory areas include:
Input Image Table
Stores the latest state of all inputs.
Output Image Table
Stores the calculated state of outputs.
Program Memory
Contains the user logic program.
Data Memory
Stores variables, timers, counters and intermediate values.
This structured approach allows PLCs to execute programs consistently and predictably.
Example: A simple PLC control sequence
Imagine a conveyor system with the following logic:
Operator presses the Start button
The PLC starts the conveyor motor
A sensor detects a product
The PLC activates a sorting gate
During each scan cycle the PLC:
Reads the Start button and sensor input
Executes the program logic
Updates the motor and gate outputs
All of this may happen within just a few milliseconds, allowing the system to operate smoothly and reliably.
Why PLCs are still critical to industrial automation
Even with the rise of industrial PCs and advanced controllers, PLCs remain the preferred control platform for many industries.
Key advantages include:
High reliability and uptime
Deterministic operation
Ease of troubleshooting
Modular scalability
Long product lifecycles
Because of their predictable scan-based execution model, PLCs provide the consistent, real-time control required in critical industrial environments.
Final thoughts
Understanding what happens inside a PLC helps engineers, technicians and students design more efficient and reliable automation systems.
By continuously repeating the scan cycle - reading inputs, executing logic and updating outputs - PLCs maintain precise control over industrial processes.
Behind the scenes, this cycle happens thousands of times every second, quietly powering the automated systems that modern industry depends on.
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