What is USB to serial cable?
USB to serial port is to realize the conversion between the computer USB interface and general serial port. Some computers or notebooks without serial port need to install USB to serial port driver, so that the traditional serial port can be converted into a plug-and-play USB device.
What is serial communication?
Serial communication means that only one line of data is transmitted, and only one bit can be transmitted at a time. To transmit one byte, it needs to be transmitted 8 times. Serial communication is to transmit data strings on a line, so it is called a serial port.
In the case of low speed requirements, using one line to send data brings great convenience and practical value.
In order to send and receive correct data normally, the asynchronous serial communication requires the format as shown below.
In the communication parameters of the serial port, there are several parameters such as baud rate, start bit, data bit, parity bit, and stop bit.
Serial communication is mainly divided into 232, 485, 422 three communication methods,
What is the difference between 232, 485, and 422?
1. 232
232 communication is mainly composed of RX, TX, GND three lines.
RX is connected to TX, TX is connected to RX, and GND is connected to GND. Here, sending and receiving are processed by different lines, that is, data can be sent and received at the same time, which is the so-called full-duplex communication.
To expand here, there is another function of serial communication called full-featured serial communication, also called standard serial port. Because of the data transfer between two devices, some devices process faster and some data slower. In order to ensure the normal transmission of data, on the basis of RX and TX, several control pins were added, and finally became 9 pins, which is the common DB9.
However, nowadays, many controllers, human-machine interfaces, PLCs, etc. generally do not use standard serial ports in serial communication, but directly use three lines of RX, TX, and GND to communicate.
2. 485
The emergence of 485 is to solve the problem of limited communication distance of 232.
485 communication only needs + and - two lines, or also called A and B lines. The differential level signals of the two lines A and B are transmitted as data signals. Since the two wires are used for both sending and receiving, that is to say, they can only be used for sending or receiving each time. So, 485 is half-duplex communication.
This is how 485 sacrifices the efficiency of 232 full-duplex to achieve the cost of its long transmission distance.
3. 422
The appearance of 422 is to realize the full-duplex communication mode of 232 and improve the transmission distance like 485. The 422 is also often labeled 485-4, while the 485 is labeled 485-2. Because 485-2 is 2 wires and 485-4 is 4 wires.
422 is to divide the RX of 232 into two lines, RX+, RX-, and divide TX into TX+, TX-. In this way, you can send and receive at the same time, and you can also have a longer transmission distance like 485. However, such a very advantageous communication method is not used much. The most commonly used are 232 and 485.
