What is the essential difference between PLC and microcontroller?

I want to understand the difference between PLC and MCU. I searched a lot on the Internet, but I am still confused. Finally, I combined some sentences that hit the point, analyzed and summarized them carefully. I think the difference between PLC and MCU should be:

1.PLC is a relatively mature control system composed of single-chip microcomputers. It is a product of a single-chip microcomputer application system that has been debugged and matured and is highly versatile.

2. Single-chip microcomputers can form a variety of application systems and have a wider range of uses. As far as a "single-chip microcomputer" is concerned, it is just an integrated circuit and must be combined with other components and software to form a system before it can be used.

3. From the perspective of engineering use, for single projects or projects with very few repetitions, the use of PLC is quick and convenient, has a high success rate, and good reliability, but the cost is relatively high.

4. For large-scale supporting projects, the use of single-chip microcomputer system has the advantages of low cost and high efficiency, but this requires considerable R&D strength and industry experience to make the system stable.

In essence, PLC is actually a ready-made single-chip microcomputer (single-chip microcomputer has a wide range of) system.

But PLC also has its own characteristics: PLC widely uses ladder diagrams instead of computer languages, which has certain advantages for programming. You can understand ladder diagrams as a programming language like assembly and other computer languages, but the scope of use is different! And the usual practice is that the PLC software converts your ladder diagram into C or assembly language (determined by the CPU used by the PLC), and then compiles it into machine code using the assembly or C compilation system! PLC runs only machine code. Ladder diagrams just make it easier for users to use.

As mentioned, the MCS-51 microcontroller can certainly be used in PLC production, but the 8-bit CPU is not capable of some advanced applications such as: large-scale calculations (including floating-point calculations), embedded systems (UCOS can now be ported to MCS-51), etc. However, with the addition of DSP, it can meet general requirements. In addition, we can also use ladder diagram programming, and we can convert the ladder diagram into C51 and then use KEIL's C51 to compile. We can also find that different models of PLCs will use different CPUs, which actually shows that PLC is a set of ready-made microcontroller systems.

In this case, of course, you can also use a single-chip microcomputer to directly develop a control system, but the requirements for developers are quite high (not something that ordinary people can do), the development cycle is long, and the cost is high (for some larger systems, you need to do experiments, and printed circuit boards require a considerable amount of money. You can say that you use simulators and experimental boards to develop, but I want to tell you that by doing so, you only verify the feasibility of hardware and software, and it does not mean that it can be used in industrial control systems, because industrial control systems have very high requirements for anti-interference, stability first, not performance first, so your circuit board design must be constantly experimented and improved). When you solve the above problems, you will find that you have made a PLC. Of course, if you need others to be able to use it easily, you also need a set of software, so you don’t need to tell others about your circuit. You can’t tell others.

In this way, PLC is not mysterious. Many PLCs are very simple. Except for the high speed, the other functions of the CPU inside are not as good as those of ordinary single-chip microcomputers. Usually, PLC uses a 16-bit or 32-bit CPU with 1 or 2 serial channels to communicate with the outside world. It only needs one timer inside. If you want to improve reliability, you can add a watchdog timer to solve the problem.

In addition, the key technology of PLC lies in the fact that it has a program that can interpret the ladder diagram language and an auxiliary communication program solidified inside. The efficiency of the ladder diagram language interpretation program determines the performance of the PLC, and the communication program determines the difficulty of the PLC exchanging information with the outside world. For simple applications, it usually operates as an independent controller, without exchanging information with the outside world, and only needs to have a program that can interpret the ladder diagram language solidified inside. In fact, the main work of designing PLC is to develop a program that interprets the ladder diagram language. Today's single-chip microcomputer can completely replace PLC. The previous single-chip microcomputer was not comparable to PLC due to its relatively weak stability and anti-electromagnetic interference ability. Today's single-chip microcomputer has achieved high stability and strong anti-interference ability, and has been replaced in some fields.