First of all, let's understand the meaning of the word "serial port". In fact, it is a kind of interface, which is widely used in serial communication, referred to as serial communication. So what is serial communication? That is to say, our data transmission can be completed by only one wire. No matter how large or complex the information is, it will be carried out in bit order, and it can realize long-distance two-way communication, ranging from several meters to several kilometers, which is greatly reduced. In order to reduce the cost, the disadvantage is that the transmission speed is not ideal. And what we call the "parallel port", the data transmission volume is large, 8 bits of data can be transmitted at the same time, but its speed is slower than the serial port, because of the mutual interference (crosstalk) between the 8-bit channels, the transmission speed is limited , And the transmission is prone to errors, while the serial line does not have such a problem, so it is slowly being taken seriously.
The RS-232 we usually use is called a standard serial port, and it adopts an unbalanced transmission method, which is the so-called single-ended communication.
The receiving pin and the sending pin of the same serial port are directly connected by wires, two serial ports are connected, or one serial port is connected to multiple serial ports. That is to say, the serial port transmission data can be realized as long as there are 2 pins on the receiving data end and 3 pins on the sending data end. Connect the data receiving end of one device with the data sending end of another device. That is, receiving and sending cross each other, and the signal ground is connected correspondingly.
The serial port line is divided into direct connection and crossover, the purpose is slightly different, direct connection is generally used for the connection between the PC and the device, and the crossover is used for the docking between PCs.
In general, there are three types of RS232 DB-9 interface cables: male-to-male, female-to-female and male-to-female. These three types of connecting lines are divided into crossover lines and straight lines.
