About synchronous communication and asynchronous communication

    Synchronous communication is a communication method that transmits data serially, and only one frame of information is transmitted at a time. The information frame here is different from the character frame in asynchronous communication and usually contains several data characters.

    When synchronous communication is used, many characters are grouped into a message group, so that characters can be transmitted one by one. However, a synchronization character is added at the beginning of each group of information (usually called a frame), and a blank character is filled when there is no information to be transmitted, because synchronous transmission does not allow gaps. In the synchronous transmission process, one character can correspond to 5 to 8 bits. Of course, for the same transmission process, all characters correspond to the same number of bits, for example, n bits. In this way, when transmitting, every n bits are divided into a time slice, the sender sends one character in one time slice, and the receiver receives one character in one time slice.

    In synchronous transmission, a message frame contains many characters. Each message frame starts with a synchronization character. Generally, the synchronization character and the space character are coded with the same code. In the whole system, a unified clock controls the sending end to send and use the same code for the space character. The receiving end should be able to recognize the synchronization character. When a string of digits matching the synchronization character is detected, it is considered to start a message frame, and the subsequent digits are treated as actual transmission information.

PS: "0-bit insertion and deletion technology" commonly used in synchronous communication 
　　　In synchronous communication, a frame of information starts with one (or several) special characters, for example, F field = 01111110B.
However, these special characters may appear in other positions of the information frame. In order to prevent the receiver from mistaking these special characters for the beginning of the frame, the sender uses "0-bit insertion technology", and accordingly, the receiver uses "0-bit deletion technology". The
　　sender's 0-bit insertion: In addition to the start character, when 5 consecutive 1s appear, the sender automatically inserts a 0. In the entire information frame, only the start character contains 6 consecutive 1s. The
　　receiver's "0-bit deletion technology": The receiver receives 6 consecutive 1s as the start of the frame and automatically deletes the 0 after 5 consecutive 1s.  
 

Principles of asynchronous communication

    Asynchronous communication is a very common communication method. When sending characters in asynchronous communication, the time interval between the characters sent can be arbitrary. Of course, the receiving end must always be ready to receive. The sending end can start sending characters at any time, so it is necessary to add marks at the beginning and end of each character, that is, add start bits and stop bits, so that the receiving end can correctly receive each character. The advantage of asynchronous communication is that the communication equipment is simple and cheap, but the transmission efficiency is low (because the overhead of start bits and stop bits accounts for a large proportion).

The difference between synchronous communication and asynchronous communication:

   1. Synchronous communication requires that the clock frequency of the receiving end and the clock frequency of the sending end are consistent, and the sending end sends a continuous bit stream; asynchronous communication does not require the clock of the receiving end and the clock of the sending end to be synchronized. After the sending end sends a byte, it can send the next byte after an arbitrary time interval.
    2. Synchronous communication is efficient; asynchronous communication is less efficient.
    3. Synchronous communication is more complex, and the allowable error of the clocks of both parties is smaller; asynchronous communication is simple, and the clocks of both parties can allow a certain error.
    4. Synchronous communication can be used for point-to-multipoint; asynchronous communication is only applicable to point-to-point.