How PLCs Work – Understanding the Working Cycle

A Programmable Logic Controller (PLC) is an essential part of industrial automation, responsible for processing inputs, executing logic-based control programs, and generating outputs to control machinery and systems. Understanding the PLC working cycle is crucial for engineers, technicians, and system integrators who design, program, and maintain these systems.

Unlike traditional relay-based control systems, PLCs operate using a cyclic scanning process that ensures real-time control and decision-making. This process, known as the PLC scan cycle, occurs continuously and rapidly, often in milliseconds.

This article explains the step-by-step working cycle of a PLC, covering input processing, logic execution, output activation, and communication handling.

1. The PLC Scan Cycle – A Continuous Loop

A PLC operates in a repetitive loop, processing inputs, executing control logic, and updating outputs. This cycle is known as the PLC scan cycle, and it consists of four primary phases:

• Input Scan – Detects the state of connected sensors and input devices
• Program Execution – Processes logic instructions in the PLC program
• Output Update – Activates or deactivates connected actuators
• Communication & Diagnostics – Handles data exchange and error detection

Each scan cycle is completed in milliseconds, ensuring real-time process control.

2. Phase 1: Input Scan – Detecting External Signals

The first step in the PLC scan cycle is the input scan, where the PLC reads the status of all connected input devices. These include:

• Sensors (temperature, pressure, proximity, etc.)
• Push buttons and switches
• Encoders and limit switches
• Barcode scanners and RFID readers

How Input Scanning Works

The PLC continuously checks the status of digital and analog inputs.

• Digital Inputs (ON/OFF): Sensors like photoelectric or limit switches send binary signals (0 or 1).
• Analog Inputs (Variable values): Devices like temperature sensors send continuous signals (e.g., 0–10V or 4–20mA).

Example: ➡️ In an automated bottling plant, a photoelectric sensor detects the presence of a bottle on the conveyor. The PLC reads this sensor input and proceeds with the filling process.

Key Point: The PLC stores the current state of inputs in a memory table, ensuring accurate decision-making in the next step.

3. Phase 2: Program Execution – Processing Control Logic

After gathering input data, the PLC moves to the program execution phase, where it processes logic-based instructions defined by the user. The logic is typically written in Ladder Logic (LD), Structured Text (ST), Function Block Diagram (FBD), or Sequential Function Chart (SFC).

How Program Execution Works

1. The PLC scans the user-defined control logic.
2. It executes logic instructions based on the current input status.
3. It determines which outputs need to be turned ON or OFF.

Example: ➡️ In an automated warehouse, a conveyor system transports boxes. If a barcode scanner detects a specific package, the PLC executes logic to route the package to the correct location using diverter arms.

Logic Execution Order

PLC programs execute sequentially from top to bottom, left to right (in Ladder Logic). The execution time (scan time) depends on:

• Program complexity
• Number of instructions
• CPU processing speed

Key Point: Some PLCs allow priority-based execution, where critical operations are executed before non-essential tasks.

4. Phase 3: Output Update – Controlling Actuators

After processing the program logic, the PLC updates the output states based on the control decisions made in the previous phase.

Common output devices include:

• Motors and pumps
• Solenoids and valves
• Alarms and indicator lights
• Robot arms and conveyors

How Output Processing Works

• The PLC writes new values to the output memory table.
• This activates or deactivates connected devices.

Example: ➡️ In a smart irrigation system, the PLC monitors soil moisture sensors. If moisture levels are low, the PLC activates the water pump to irrigate the field.

Key Point: Outputs are not activated instantly—they are only updated at the end of each scan cycle to prevent logic conflicts.

5. Phase 4: Communication & Diagnostics – Data Handling

Modern PLCs are networked systems, exchanging data with:

• Human-Machine Interfaces (HMIs)
• Supervisory Control and Data Acquisition (SCADA) systems
• Industrial Internet of Things (IIoT) platforms
• Other PLCs and controllers

How Communication & Diagnostics Work

• The PLC transmits data to external systems (e.g., production statistics to SCADA).
• It receives remote control commands (e.g., start/stop commands from an HMI).
• It detects errors or faults and triggers diagnostic alerts (e.g., low voltage, sensor failure).

Example: ➡️ In an automotive production line, the PLC sends real-time assembly data to a SCADA system, allowing managers to monitor production efficiency remotely.

Key Point: Advanced PLCs support Ethernet/IP, Modbus, Profibus, and OPC UA for seamless communication.

6. The Importance of Scan Time in PLCs

The scan time is the total time taken to complete one full scan cycle. This is critical for ensuring real-time process control.

Factors Affecting Scan Time

• Processor speed – Faster CPUs reduce scan time
• Program size – Complex programs increase scan time
• I/O count – More inputs/outputs require longer processing

Key Point: Critical applications (e.g., motion control) require scan times below 1 millisecond for real-time precision.

Example: ➡️ In a high-speed bottling plant, a PLC with a scan time of 2ms ensures that bottles are filled, capped, and labeled without delay.

Conclusion

PLCs operate through a continuous scan cycle, ensuring real-time monitoring and control of industrial processes. The cycle consists of four main phases:

• Input Scan – Collecting data from sensors
• Program Execution – Processing control logic
• Output Update – Activating actuators
• Communication & Diagnostics – Handling data exchange

Understanding how PLCs work efficiently is essential for optimizing automation, troubleshooting faults, and improving system performance. As industries move towards Industry 4.0, PLCs will integrate with AI, IIoT, and cloud-based control systems for enhanced performance and predictive maintenance.