Can microcontroller replace PLC?

This question is like whether flour can replace noodles. The answer is no. When hearing this answer for the first time, many people may have questions. MCU is obviously so powerful and resource-rich, why can't it replace PLC?

Microcontroller

As we all know, a single chip microcomputer (Single Chip Microcomputer), also known as a microcontroller unit (MCU), is an integrated circuit chip that uses very large-scale integrated circuit technology to combine a central processing unit (Central Processor) with data processing capabilities. Unit; CPU), random access memory (Random Access Memory; RAM), read-only memory (Read-Only Memory; ROM), various I/O ports and interrupt systems, timers/counters and other functions (may also include display driver circuits , pulse width modulation circuit, analog multiplexer, A/D converter and other circuits) integrated on a silicon chip to form a small and complete microcomputer system, which is widely used in various fields. MCUs can be seen in applications such as mobile phones, PC peripherals, remote controls, automotive electronics, industrial stepper motors, and robot arm control.

The history of microcontrollers is not long, but it has developed very rapidly. Its production and development are generally synchronized with the production and development of microprocessors. Since Intel Corporation of the United States first launched a 4-bit microprocessor in 1971, its development so far can be roughly divided into five stages:

The initial stage of the development of single-chip microcomputer (1971 to 1976): In November 1971, Intel first designed the Intel 4004, a 4-bit microprocessor with an integration level of 2000 transistors/chip, and equipped with RAM, ROM and shift register. It formed the first MCS-4 microprocessor, and then launched the 8-bit microprocessor Intel 8008, as well as 8-bit microprocessors launched by other companies.

Low-performance microcontroller stage (1976 to 1980): Represented by the MCS-48 series launched by Intel in 1976, it uses an 8-bit CPU, 8-bit parallel I/O interface, 8-bit timer/counter, RAM and ROM, etc. A monolithic structure integrated on a semiconductor chip. Although its addressing range is limited (no more than 4KB), there is no serial I/O, RAM and ROM capacity is small, and the interrupt system is relatively simple, its functions can satisfy general industrial control. and the needs of intelligent instruments, meters, etc.

High-performance microcontroller stage (1980 to 1990): The high-performance 8-bit microcontrollers launched in this stage generally have serial ports, multi-level interrupt processing systems, and multiple 16-bit timers/counters. The capacity of the on-chip RAM and ROM is increased, and the addressing range can reach 64KB. Some chips also have an A/D conversion interface.

PLC

The full name of PLC is Programmable Logic Controller, which is a digital computing operation electronic system specially designed for application in industrial environments. It uses a programmable memory to store instructions for performing operations such as logical operations, sequence control, timing, counting and arithmetic operations, and controls various types of mechanical equipment or production through digital or analog input and output. process.

16-bit microcontroller stage (1983 to 1989): In 1983, Intel launched the high-performance 16-bit microcontroller MCS-96 series. Due to its adoption of the latest manufacturing technology, the chip integration level is as high as 120,000 transistors/chip.

All-round high-level development stage (1990 to present): So far, microcontrollers have also had a trend of transforming from traditional 8-bit processor platforms to 32-bit advanced RISC processor platforms, but 8-bit computers are still difficult to replace. 8-bit microcontrollers have low cost, low price, and are easy to develop. Their performance can meet most needs. Only in high-tech fields such as aerospace, automobiles, and robots, when large amounts of data need to be processed at high speed, 16/32-bit microcontrollers need to be selected. In the general industrial field, 8-bit general-purpose microcontrollers are still the most widely used microcontrollers at present. Microcontrollers are developing to a higher level in all aspects of integration, functionality, speed, reliability, and application fields.

The characteristics of single-chip microcomputer are that programming and maintenance are relatively complicated. C language or assembly language is commonly used for programming. The cost is low and the I/O interface is relatively limited.

Why can't microcontroller replace PLC?

1. Stability and reliability

Some people say this is a false question. Microcontrollers are components, and PLCs are systems composed of components and huge software. There is no comparison between the two in this regard. This is true. The control chips of most PLCs are actually single-chip microcomputers. In other words, PLC can be regarded as a secondary development of single-chip microcomputers. In terms of industrial protection level alone, the stability and reliability of single-chip microcomputers are simply not comparable to that of PLCs. IP67 products (IP is a marked letter, the first marked number indicates the level of contact protection and foreign object protection, the second marked number indicates the waterproof protection level), and a set of products such as PLC that can cope with harsh industrial environments has also been developed. Redundant systems. If the comparison between stability and reliability is meaningless, then we will analyze it from other aspects.

2. I/O function

The I/O points of a microcontroller are really limited, but what about a PLC? For different field signals, there are corresponding I/O points that can be directly connected to industrial field devices (such as buttons, switches, sensing current transmitters, motor starters or control valves, etc.), and connected to the CPU motherboard through the bus. connect. Almost any production line in the industry has hundreds or even thousands of I/O points, which are completely unmatched by microcontrollers.

3. Extended functions

In addition to control, a complete industrial production line also has communication, host, configuration, motion control and display, etc. These things all need to rely on a complete industrial system and communication protocols, such as Siemens' PROFIBUS-DP communication and Mitsubishi Heavy Industries' CC-LINK etc. The communication between microcontroller and PC, microcontroller and microcontroller mostly uses serial port. The serial port of the microcontroller is a full-duplex asynchronous communication serial port, so can the microcontroller implement communication protocols such as MODBUS, PROFIBUS, CAN open, and Ethernet one by one? Maybe a microcontroller can do it, but this involves the next analysis point, the development cycle.

4. Development cycle

There are more than 200 brands of PLC. Almost every brand has different programming software, and they are constantly improving their programming software to make it easier and easier to serve electrical engineers, and various program blocks are also becoming more and more popular. The more convenient and user-friendly it is to call it at will, such as PID module, motion control module, etc., which greatly reduces the development pressure of engineers and shortens the development cycle.

How to implement the microcontroller? If there are no ready-made modules to use, you can only develop them. Then engineers who have worked on non-standard automation equipment will encounter a problem - insufficient construction period. PLC, a highly integrated and modular product, has a very limited development cycle to meet the requirements of the equipment, let alone a single-chip microcomputer that is like a blank piece of paper.

5. Communication distance

Nowadays, most assembly lines need to be integrated and monitored across regions. The communication methods used are mostly Ethernet plus repeaters, or direct civilian broadband optical fiber. What is used in the end is probably Microsoft's IE browser. Obviously, PLC has an RJ-45 interface. Even if the main body does not have RJ-45, it can still be equipped with an Ethernet module. Can the PCB board equipped with the microcontroller be added with this interface and develop Ethernet communication? How long does development take?

6. Programming language

This is an advantage and a disadvantage for microcontrollers. As mentioned above, there are more than 200 brands of PLC, and there are even more programming software. Although the programming languages of most PLCs are similar, every time they come into contact with a PLC of a different brand, electrical engineers must learn from the PLC's hardware parameters, soft components, Programming software and other aspects need to be learned from scratch before you can use it easily. The programming language of the microcontroller is C language or assembly language, which is common to any microcontroller. In other words, if you learn C language or assembly language, you can use any microcontroller to develop the desired functions (provided that you have relevant electrical and electronics foundation).

But then, electrical engineers are not electronic engineers. Their job is not just to consider how the microcontroller drives the relay to control the machine tool. Some electrical engineers even do not know MCU development languages such as C language and assembly language. In recent years, with the promotion of the IEC-61131-3 standard, more and more PLCs support multiple programming languages, such as ST language, which is similar to C language, and CFC language, which is similar to circuit diagrams. This convenient function is really impossible to achieve in the traditional microcontroller development environment.

Summarize

After the above explanation, we can see that PLC can actually be regarded as the secondary application development of single-chip microcomputer, but it has its own distinctive characteristics. So far, China's microcontroller applications and embedded system development have gone through more than 20 years of development in various fields such as national economic construction, military and household appliances, especially mobile phones, automatic car navigation equipment, PDAs, smart toys, smart phones, etc. Home appliances, medical equipment and other industries all use microcontrollers. At the high end of the industry, there are currently more than 100,000 engineers engaged in the development and application of microcontrollers.

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