The difference between PLC and SCM is:
1. PLC is a relatively mature control system composed of single-chip microcomputers. It is a product that has been debugged mature and stable single-chip microcomputer application system. Stronger versatility.
2. The single-chip microcomputer can constitute a variety of application systems with a wider range of applications. In terms of “single-chip microcomputer” alone, it is only an integrated circuit, and it must be combined with other components and software to be applied.
3. From the point of view of project use, PLC is quick and convenient for single projects or projects with few repetitions. The success rate is high and the reliability is good, but the cost is high.
4. For a large number of supporting projects, the use of SCM system has the advantages of low cost and high efficiency, but it must have considerable R & D strength and industry experience to make the system stable.
In essence, PLC is actually a set of already-successfully-made single-chip microcomputers.
But PLC also has its characteristic: PLC uses ladder diagram to replace computer language extensively, have certain advantage to programming. You can think of ladder diagrams as a calculator language like assembly, but it is a programming language, but the scope of use is different! Also, it is common practice to convert your ladder diagram into C or assembly language (used by the PLC) by PLC software. The CPU decides) and then compiles into machine code using an assembler or C compiler system! The PLC runs just machine code. Ladder diagrams only 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 in some advanced applications such as: a large number of operations (including floating-point operations), embedded systems (now UCOS can also be transplanted to the MCS-51) And so on, some of the power is not enough, but with DSP can already meet the general requirements, but also use ladder programming, we can convert the ladder diagram to C51 and then use KEIL’s C51 to compile. We can also find that different type of PLC will use different CPUs. In fact, it means that PLC is a set of already-made single-chip microcomputer systems.
In this case, of course, you can use the microcontroller to directly develop the control system, but the developers are very demanding (not the average level can be competent), long development cycle, high cost (for some large-scale systems, you need to do experiments, printed circuit boards There is a considerable cost, you can say that you use an emulator to develop with the experiment board, but I want to tell you that you just verified the feasibility of the hardware and software. It does not mean that you can use it in the industrial control system. Because industrial control systems have very high requirements for anti-interference, stability first, not performance first, so your circuit board design must constantly experiment and improve). When you solve the above problem, you find that you have already done a PLC, of course, if you need to be easy to use, you also need a set of software, so you do not need to tell your circuit. You cannot tell anyone else either.
This kind of PLC is actually not mysterious, many PLC is very simple, in addition to its internal CPU speed, other features are not as good as ordinary microcontroller. Usually the PLC uses 16-bit or 32-bit CPU, with 1 or 2 serial channels to communicate with the outside world, there is a timer inside, to solve the problem of adding a watchdog timer to improve reliability.
In addition, the key technology of PLC lies in that it solidifies a program and auxiliary communication program that can explain the ladder language. The efficiency of the interpreter program of the ladder language 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 is usually operated as a stand-alone controller. It does not need to exchange information with the outside world. It only needs to be internally solidified to have a program that can explain the ladder language. In fact, the main task of designing a PLC is to develop a program that explains the ladder language. The microcontroller can now completely replace the PLC. The weaknesses of previous SCMs due to their stability and resistance to electromagnetic interference are not comparable to those of PLCs. Today’s SCMs have achieved high stability and strong anti-jamming capabilities that have been replaced in some areas.