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Understanding the core components of a Programmable Logic Controller (PLC) system is essential for anyone working in automation and controls. Whether dealing with a compact, all-in-one unit or a larger modular setup, each system is built on fundamental parts that allow it to manage complex tasks in real-world industrial settings.
PLC System Sizes: Micro vs. Modular
PLCs come in a variety of sizes and configurations, from micro PLCs designed for smaller machines to modular systems that scale up to control the most complex processes. Micro PLCs are ideal for simple applications with fewer inputs and outputs, while modular systems support extensive expansions, allowing you to add more I/O modules, power supplies, and communication options as needed.
Regardless of the size, all PLCs have a few core components that allow them to function reliably.
Essential Components of a PLC System
Let’s walk through each of these key components, from the mounting rack to the vital I/O sections, explaining how each contributes to a PLC’s functionality.
1. Rack or Base Unit (Modular Systems Only)
In modular PLC systems, the rack (also known as a backplane or chassis) serves as the base that holds the other components, such as the power supply, CPU, and I/O modules. Modular systems need this dedicated structure to support the easy interchange of components, making it easier to upgrade and troubleshoot. Micro PLCs, by contrast, often integrate all components into a single unit, eliminating the need for a separate rack.
2. Power Supply
The power supply can be a standalone external unit, a built-in feature for smaller PLCs, or a plug-in module in a rack system. Depending on the power available at your site, you can find PLC power supplies that accept 24V DC, 120V AC, or 240V AC.
Typically, the PLC’s power supply is used solely to power its internal components, and a separate source is preferred for field devices like sensors and indicators to avoid interference with PLC operations.
3. CPU (Central Processing Unit)
The CPU is the “brain” of the PLC, containing the processor and memory. It executes the user program and manages the system’s inputs and outputs. Here’s a closer look at how the CPU works:
- RAM (Random Access Memory): This is where user programs, input/output status, and real-time data are stored temporarily. Since RAM is volatile, it requires a battery backup to retain information during power loss.
- ROM (Read-Only Memory): The PLC’s operating system and firmware are stored here. Because ROM is non-volatile, it preserves essential functions even when the system is powered down. Some systems allow firmware updates via EEPROM, but flashing firmware can be risky without experience, as it may corrupt data and “brick” the CPU.
On many CPUs, you’ll find LED indicators or a display panel showing system status (Stop, Run, or Error) and communication ports (such as Ethernet or USB) for programming and network connections.
4. Input/Output (I/O) Sections
The I/O sections are the connection points between the PLC and external field devices. These devices, connected as inputs (sensors, switches) or outputs (indicators, solenoids), allow the PLC to monitor and control physical components in the environment.
- Input Interface: Converts incoming signals from field devices into digital data that the CPU can process.
- Output Interface: Translates processed data from the CPU to manage the operation of output devices.
In smaller PLCs, input and output terminals are typically found on the main unit. In larger, modular systems, you’ll have dedicated I/O modules for connecting field devices, which provides flexibility for complex systems.
Putting It All Together
Every PLC, whether micro or modular, is built around these essential components to perform as a powerful control system. Understanding each part allows technicians and engineers to configure, troubleshoot, and scale systems effectively.
If you find this discussion of PLC basics interesting, then you might want to take a look at this course on Udemy called the Pre PLC Programming Course. These blogs are based on material from that course. It is not geared toward any particular brand of PLC our programming software. The fundamentals are relevant to all types of PLCs.
Up Next – PLC Basics: Discrete Inputs and Outputs