The difference between PLC and single chip microcomputer is:

1
PLC is a relatively mature control system composed of single-chip microcomputer, and it is a product that has been debugged mature and stable single-chip application system. Has a strong versatility.




2
The single-chip microcomputer can form a variety of application systems, and the scope of use is wider. As far as the "single chip microcomputer" is concerned, it is only an integrated circuit, and must be combined with other components and software to form a system.




3
From the point of view of the use of the project, PLC is quick and convenient for single projects or projects with few repetitions, with high success rate and good reliability, but the cost is high.




4
For a large number of supporting projects, the use of single-chip systems has the advantages of low cost and high efficiency, but it must have considerable research and development strength and industry experience to make the system stable.




In essence, PLC is actually a set of systems that have been made well (a wide range of microcontrollers).


But PLC also has its characteristics: PLC widely uses ladder diagram instead of computer language, which has certain advantages for programming. You can understand the ladder diagram as a calculator language like assembly. It is a programming language, but the scope of use is different! And it is usually the PLC software to convert your ladder diagram into C or assembly language (used by PLC). The CPU determines), and then compiles into machine code using assembly or C compilation system! The PLC runs only the machine code. Ladder diagrams are just to make it easier for users to use.
As mentioned, then the MCS-51 microcontroller can of course also be used for PLC production, but the 8-bit CPU is used in some advanced applications such as: a large number of operations (including floating-point operations), embedded systems (now UCOS can also be ported to MCS-51) Wait, some power is not enough, but with the DSP can already meet the general requirements, and also use ladder programming, we can convert the ladder diagram to C51 and then use KEIL C51 to compile. We can also find that different types of CPUs will use different CPUs. In fact, PLC is a set of MCU systems that have already been completed.


In this case, of course, you can also use the microcontroller to directly develop the control system, but the requirements for developers are quite high (not the general level can be competent), the development cycle is long, the cost is high (for some large-scale systems you need to do experiments, printed circuit boards You need a considerable fee. You can say that you use the emulator and use the experiment board to develop, but I want to tell you that you just verify the feasibility of the hardware and software, it does not mean that it can be used in industrial control systems. Because the industrial control system has very high requirements for anti-interference and is stable first, not performance first, so your board design must be constantly experimented and improved). When you solve the above problem, you will find that you have already done a PLC. Of course, if you need others to use it easily, you need a set of software, so you don't need to tell your circuit. You can't tell others.


In this way, PLC is not mysterious. Many PLCs are very simple. In addition to the fast CPU, other functions are not as good as ordinary single-chip microcomputers. Usually, the PLC uses a 16-bit or 32-bit CPU with 1 or 2 serial channels to communicate with the outside world. There is a timer inside. If you want to improve the reliability and add a watchdog timer problem, it will be solved.


In addition, the key technology of PLC is that it internally solidifies a program that can interpret the ladder language and the auxiliary communication program. The efficiency of the ladder language interpretation program determines the performance of the PLC. The communication program determines the difficulty of exchanging information between the PLC and the outside world. . For simple applications, it usually operates as a stand-alone controller. There is no need to exchange information with the outside world. It is only necessary to internally embed a program that can interpret the ladder language. In fact, the main job of designing a PLC is to develop a program that interprets the ladder language. Now the microcontroller can completely replace the PLC. The previous single-chip microcomputer has a weak comparison between stability and anti-electromagnetic interference capability, and PLC has no way to compare it. Now the single-chip microcomputer may have achieved high stability and strong anti-interference ability, and has been replaced in some fields.