Classification of PLCs

Classification of PLCs

PLC (Programmable Logic Controller) can be classified in various ways, primarily based on its hardware structure, I/O point count, and functional range. This is similar to classifying cars by structure (sedan, SUV), size (compact, mid-size), or performance (family, performance).

The following are the most common and important classification methods for PLCs:

1. Classification by I/O Point Count and Scale (Most Common Classification Method)

This is the most intuitive classification method, dividing PLCs based on the number of input/output points they can control, which directly determines their control capacity and application range.

Type	I/O Point Count Range	Characteristics and Applications	Common Brand Models
Small (Micro) PLC	Below 256 points	– Structure: Integrated (all-in-one), CPU, power supply, and I/O points are integrated into one body. – Characteristics: Small size, low cost, specialized functions. – Applications: Replacing traditional relays for logic control of single machines and small-scale systems, such as packaging machines, conveyors, elevators, and automatic doors.	Siemens S7-200 SMART / 1200, Mitsubishi FX Series, Omron CP1 / CP2E Series
Medium PLC	256 to 2048 points	– Structure: Modular, consisting of CPU modules, power modules, various I/O modules, and function modules connected via a backplane. – Characteristics: Flexible expansion, powerful functions, strong analog processing, network communication, and complex computation capabilities. – Applications: Complex machinery, production line process control, large building automation.	Siemens S7-1500 / S7-300, Rockwell ControlLogix, Schneider Modicon M340
Large PLC	Above 2048 points	– Structure: Modular rack structure, large scale, can expand multiple racks. – Characteristics: Extremely fast computation speed, large control scale, very powerful communication network capabilities (can form large distributed control systems), high reliability requirements (often uses redundant design). – Applications: Large process industries such as steel, petrochemical, electric power, and automotive manufacturing, serving as central control systems.	Siemens S7-400 / S7-1500H, Rockwell ControlLogix, Schneider Modicon Quantum

2. Classification by Hardware Structure

This classification focuses on the physical composition of the PLC.

1. Integrated PLC (All-in-One PLC)**Characteristics: Integrates CPU, memory, I/O interfaces, power supply, etc., into one housing.Advantages: Compact structure, small size, low cost, easy installation.Disadvantages: Fixed I/O points, poor expansion capability.Corresponds to: Typically small PLCs**.
2. Modular PLC (Combination PLC)**Characteristics: Composed of independent modules, such as CPU modules, I/O modules, power modules, communication modules, etc., which are plugged into a bus-equipped baseboard or rack.Advantages: Flexible configuration, modules can be selected as needed like “building blocks”, easy to maintain and expand.Disadvantages: Complex structure, higher cost.Corresponds to: Mainly medium and large PLCs**.
3. Stackable PLCThis is a compromise between the first two types. Each functional module (CPU, I/O, power) is physically independent but can be tightly connected together via cables without a baseboard. It is more flexible than integrated types and more compact than modular types. For example, Siemens S7-200 series.

3. Classification by Functional Strength

This classification focuses more on the performance of the PLC and its ability to handle complex tasks.

1. Low-end PLCOnly has basic logic operations, timing, and counting functions. Mainly used for sequential control and discrete control. Typically small PLCs.
2. Mid-range PLCIn addition to low-end functions, it has powerful analog processing (such as PID control), data transmission, arithmetic operations, interrupts, and network communication capabilities. Programming languages are also more diverse. Typically medium PLCs.
3. High-end PLCHas strong mathematical and functional operation capabilities. Supports complex motion control, closed-loop control, and redundant systems. Possesses powerful network communication capabilities, can form distributed control systems, and integrate with upper-level management systems like MES and ERP. Typically large PLCs.

Summary Table

Classification Dimension	Type	Core Features	Typical Applications
I/O Point Count/Scale	Small PLC	< 256 points, integrated, economical	Single machine, simple logic control
Medium PLC	256~2048 points, modular, flexible	Complex equipment, production lines
Large PLC	> 2048 points, modular rack, powerful and reliable	Large process industries, distributed control
Hardware Structure	Integrated	All components integrated	Small PLC
Modular	Independent functional components	Medium and large PLCs
Functional Level	Low-end	Basic logic control	Replacing relays
Mid-range	Analog, network communication	Process control
High-end	Complex calculations, motion control, system integration	Smart manufacturing, large projects

When selecting a PLC, it is usually first determined based on I/O point count and project scale to establish a general direction (small/medium/large), and then specific brands and models are chosen.

I/O is the abbreviation for Input/Output, which refers to input/output. It is the absolute core concept for any computing or control system to interact with the external world.

It can be imagined as the system’s “sensory system” and “motor system”:

• Input is the system’s senses, responsible for perceiving the state of the external world.
• Output is the system’s limbs, responsible for executing actions to influence the external world.

The I/O point count range refers to the maximum number of “signal wires” a PLC can connect.

Core Metaphor: Imagine the PLC as a “Power Strip”

You can take a look at your home power strip:

• It has 8 sockets, which means this power strip can supply power to 8 devices at the same time.
• This “8” is its maximum output point count.