Can microcontrollers replace PLCs? This question is akin to asking if flour can replace noodles, and the answer is no. Many may question this answer at first, as microcontrollers have such powerful and diverse functionalities. Why can’t they replace PLCs?
Today, we will explore what microcontrollers and PLCs are, and what the differences between them are.
1. Microcontroller
A Single Chip Microcomputer (SCM), also known as a Microcontroller Unit (MCU), is an integrated circuit chip that uses very large scale integration (VLSI) technology to combine a Central Processing Unit (CPU), Random Access Memory (RAM), Read-Only Memory (ROM), various I/O ports, an interrupt system, timers/counters, and possibly other functions (such as display driver circuits, pulse width modulation circuits, analog multiplexers, A/D converters, etc.) into a single silicon chip, forming a compact and complete microcomputer system widely used in various fields. Examples include mobile phones, PC peripherals, remote controls, automotive electronics, and industrial applications such as stepper motor and robotic arm control, where the presence of MCUs can be observed.
The history of microcontrollers is relatively short, but their development has been rapid. Their emergence and evolution have generally synchronized with that of microprocessors. Since Intel first launched the 4-bit microprocessor in 1971, the development can be roughly divided into five stages.
The initial stage of microcontroller development (1971 to 1976): In November 1971, Intel designed the 4-bit microprocessor Intel 4004, which integrated 2000 transistors per chip, along with RAM, ROM, and shift registers, forming the first MCS-4 microprocessor. This was followed by the introduction of the 8-bit microprocessor Intel 8008 and other 8-bit microprocessors from various companies.
The low-performance microcontroller stage (1976 to 1980): Represented by the MCS-48 series launched by Intel in 1976, which integrated an 8-bit CPU, 8-bit parallel I/O interface, 8-bit timer/counter, RAM, and ROM onto a single semiconductor chip. Although its addressing range was limited (not exceeding 4 KB), and it lacked serial I/O, with small RAM and ROM capacity and a simple interrupt system, its functionality met the needs of general industrial control and intelligent instruments.
The high-performance microcontroller stage (1980 to 1990): During this period, high-performance 8-bit microcontrollers were generally equipped with serial ports, multi-level interrupt handling systems, and multiple 16-bit timers/counters. The on-chip RAM and ROM capacity increased, with addressing ranges reaching up to 64 KB, and some even included A/D conversion interfaces.
The 16-bit microcontroller stage (1983 to 1989): In 1983, Intel launched the high-performance 16-bit microcontroller MCS-96 series, which utilized the latest manufacturing processes, achieving an integration level of 120,000 transistors per chip.
The comprehensive high-level development stage (1990 to present): To date, microcontrollers have shown a trend of transitioning from traditional 8-bit processor platforms to 32-bit advanced RISC processor platforms, yet 8-bit microcontrollers remain irreplaceable. They are low-cost, affordable, and easy to develop, meeting the needs of most applications. Only in high-tech fields such as aerospace, automotive, and robotics, where high-speed processing of large amounts of data is required, are 16/32-bit microcontrollers selected. In general industrial fields, 8-bit general-purpose microcontrollers are still the most widely used. Microcontrollers are advancing in integration, functionality, speed, reliability, and application fields.
The characteristics of microcontrollers include relatively complex programming and maintenance, commonly using C language or assembly language for programming, low cost, and relatively limited I/O interfaces.
2. PLC
PLC, short for Programmable Logic Controller, is a digital operation electronic system specifically designed for industrial environments. It uses a programmable memory to store instructions for executing logical operations, sequential control, timing, counting, and arithmetic operations, controlling various types of machinery or production processes through digital or analog input/output.
3. Why can’t microcontrollers replace PLCs?
1. Stability and Reliability
Some say this is a pseudo-question; microcontrollers are components, while PLCs are systems composed of components and extensive software, making them incomparable in this regard. This is true; most PLC control chips are actually microcontrollers, meaning PLCs can be seen as a secondary development of microcontrollers. In terms of industrial protection levels, the stability and reliability of microcontrollers cannot compare to PLCs, which are IP67 rated products (IP is a marking letter; the first digit indicates contact protection and foreign object protection levels, while the second digit indicates water protection levels). Moreover, PLCs have developed a redundant system to cope with harsh industrial environments. If comparing stability and reliability is meaningless, we can analyze from other aspects.
2. I/O Functionality
Microcontrollers have very limited I/O points, while PLCs have corresponding I/O points for different field signals that can directly connect to industrial devices (such as buttons, switches, current transmitters, motor starters, or control valves) and connect to the CPU motherboard via a bus. Almost any production line in industry has hundreds or even thousands of I/O points, which microcontrollers cannot match.
3. Expansion Functionality
A complete industrial production line requires not only control but also communication, upper-level control, configuration, motion control, and display, all of which rely on a complete industrial system and communication protocols, such as Siemens’ PROFIBUS-DP communication and Mitsubishi Heavy Industries’ CC-LINK. Communication between microcontrollers and PCs or between microcontrollers is mostly done via serial ports. The serial port of a microcontroller is full-duplex asynchronous communication. Can microcontrollers implement communication protocols like MODBUS, PROFIBUS, CANopen, and Ethernet? Perhaps they can, but this leads to the next analysis point: development cycle.
4. Development Cycle
There are over 200 brands of PLCs, each with different programming software, which are constantly being improved to better serve electrical engineers. Various program blocks are becoming increasingly user-friendly, such as PID modules and motion control modules, significantly reducing the development pressure on engineers and shortening the development cycle. How can microcontrollers achieve this? Without ready-made modules, development is necessary. Engineers who have worked on non-standard automation equipment often encounter a common issue—insufficient construction time. PLCs, being highly integrated and modular products, face challenges in meeting the development cycle required for equipment, let alone microcontrollers, which are like a blank slate.
5. Communication Distance
Most production lines now require cross-regional integration and monitoring, using communication methods such as Ethernet with repeaters or directly using civilian broadband fiber optics. The final product may end up using Microsoft’s IE browser. Clearly, PLCs have RJ-45 interfaces, and even if the main unit does not have RJ-45, Ethernet modules can be added. Can a microcontroller’s PCB board be equipped with this interface and develop Ethernet communication? How long will development take?
6. Programming Language
This point is both an advantage and a disadvantage for microcontrollers. As mentioned, there are over 200 brands of PLCs, and even more programming software. Although most PLC programming languages are quite similar, every time an electrical engineer encounters a different brand of PLC, they must understand the hardware parameters, software components, and programming software from scratch to use it effectively. In contrast, microcontrollers typically use C language or assembly language for programming, which is universal across all microcontrollers. In other words, learning C or assembly language allows one to develop desired functionalities on any microcontroller (provided they have a basic understanding of electrical and electronic principles). However, it should be noted that electrical engineers are not electronic engineers; their work does not solely involve how microcontrollers drive relays to control machine tools, and some electrical engineers may not even know C or assembly language. In recent years, the promotion of the IEC-61131-3 standard has led to more PLCs supporting multiple programming languages, such as structured text (ST) similar to C language and continuous function chart (CFC) language similar to circuit diagrams. This convenient feature is something traditional microcontroller development environments cannot achieve.
4. Conclusion
From the above discussion, we can see that PLCs can actually be viewed as a secondary application development of microcontrollers, yet they possess their own distinct characteristics. To date, China’s microcontroller applications and embedded system development have gone through over twenty years, impacting various fields such as national economic construction, military, and household appliances. Industries such as mobile phones, automotive navigation devices, PDAs, smart toys, smart home appliances, and medical devices have all utilized microcontrollers. Currently, there are over 100,000 engineers engaged in microcontroller development and application in high-end industries.
Source:
https://www.sohu.com/a/274772593_100279726
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