PLC is an electronic device specially designed for application in industrial environments. In use, external devices are an integral part of the PLC system. They are programming device, monitoring device, storage device, and input/output device. . Then we will come together to understand the four major external devices and its role of PLC 1, programming equipment: simple as a simple programmer, more than just accept mnemonics will be programming, individual graphics programming can also be used (such as the Japanese Toshiba Corporation EX Type programmable controller). A more complicated graphic programmer can be programmed in ladder language. There are also dedicated computers that can be programmed in other high-level languages. In addition to being used for programming, the programmer can also make some settings for the system to determine the PLC control mode or operating mode. The programmer can also monitor the working status of the PLC and PLC controlled system to debug the PLC user program.

2, monitoring equipment: small data monitor, can monitor data; large may also have a graphic monitor, monitor data through the screen. In addition to the PLC user program can not be changed, the programmer can do it, is to use PLC a very good interface. Good performance PLC, this kind of external equipment has been more and more abundant.

3, storage device: It is used to permanently store user data, so that the user program is not lost. These devices, such as memory cards, storage tapes, floppy disks, or read-only memory. In order to implement these memories, there are corresponding memory cards, tape drives, floppy or ROM writers, and corresponding interface components. Various PLCs generally have supporting facilities in this area.

4, input and output devices: It is used to receive signals or output signals, to facilitate the human-machine dialogue with the PLC. Input barcode reader, input analog potentiometer and so on. Although there are printers, programmers, and monitors that can output information to the PLC and output information from the PLC, the input and output devices are more convenient for human-machine dialogue and can be implemented under field conditions and are easy to use. As technology advances, this device will be more abundant.