The difference between PLC control and single chip microcomputer control

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

    You can think of the ladder diagram of PLC 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 a few machine codes. The ladder diagram just makes it easier for users to use.

    Similarly, MCS-51 microcontrollers can also be used for 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. Our company uses an industrial system that is made with MCS-51 microcontrollers, but with the addition of DSP, it can already meet our requirements (our equipment is slow and mainly logic control, but the number of points is not small, 128 points of I/O!!), and it also uses ladder diagram programming. We convert our ladder diagram into C51 and then use KEIL's C51 to compile it. Have you noticed that different models of PLCs use different CPUs?!

    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 the printed circuit board will cost 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 use it easily, you also need a set of software, so that you don’t need to tell others about your circuit (you can’t tell others either).

    Many people think that PLC is very mysterious. In fact, PLC is very simple. Its internal CPU is fast, but its other functions are not as good as ordinary single-chip microcomputer. Usually PLC uses 16-bit or 32-bit CPU, with 1 or 2 serial channels to communicate with the outside world, and one internal timer is enough. If you want to improve reliability, adding a watchdog timer is enough.

    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. 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 the need to exchange information with the outside world. It only needs to have a program that can interpret the ladder diagram language solidified inside.

    In fact, a major part of designing a PLC is developing a program that interprets the ladder diagram language.