A programmable logic controller (PLC) is a digital computer used for automation in industrial processes, composed of five main components: a central processing unit, power supply, input/output modules, communication interface, and programming device. The CPU executes control instructions in a cycle, while I/O modules interface with external devices, and the communication interface enables networking with other systems. The PLC continuously scans inputs, executes programs, and updates outputs to ensure real-time monitoring and control.

1. PLC Architecture
• A Programmable Logic Controller (PLC) is a digital
computer used in industrial and manufacturing
processes to control machinery, production lines, and
other automated systems. The architecture of a PLC
typically consists of five main components:
1.Central Processing Unit
2.Power Supply
3.Input and Output Module
4.Communication Interface
5.Programming Devices

2. PLC Architecture

3. Architecture of PLC

4. Central Processing Unit (CPU)
• The CPU is the brain of the PLC, executing the control
instructions in the programmed logic. It reads inputs,
executes the control program, and writes to outputs.
• It includes a microprocessor, memory (RAM for working
data, ROM or flash for the program), and an interpreter
for ladder logic or other programming languages.
• The CPU operates in cycle
• s: reading inputs, executing the program, updating
outputs, and performing diagnostics and
communication tasks.

5. Power Supply
• The power supply module provides the necessary DC
power to other PLC modules (e.g., CPU, I/O
modules).
• It usually converts standard AC voltage to the DC
voltage level required by the PLC and may offer some
power conditioning and protection.

6. Input/Output (I/O) Modules
• The I/O modules serve as the interface between the
PLC and external devices, allowing it to monitor and
control equipment.
• Input modules receive signals from sensors or other
input devices (e.g., switches, proximity sensors).
• Output modules send control signals to actuators or
other output devices (e.g., motors, relays, solenoids)

7. Communication Interface
• PLCs often include communication interfaces for
networking, which allow them to connect to other
PLCs, SCADA systems, HMIs, or other control
systems.
• Communication can occur via various protocols, such
as Ethernet/IP, Modbus, or Profinet, depending on
the industrial environment and requirements.

8. Programming Device
• The programming device (e.g., PC or dedicated
programming console) is used to create and modify
the control program.
• Programs are written in PLC-specific programming
languages, such as Ladder Logic, Function Block
Diagrams, or Structured Text, which the CPU
executes.

9. Workflow of a PLC System
• Scan Cycle: The PLC performs a continuous scan
cycle:
– Input Scan: Reads the status of input devices.
– Program Execution: Executes the control program
instructions based on input statuses.
– Output Scan: Updates output devices based on
program results.
• Repeat: The cycle repeats continuously, ensuring
real-time control and monitoring of the process.