Asynchronous communication and synchronous communication
Serial communication expresses its transmission rate in terms of baud rate, which is the number of bits per second transmitted by the signal.
Serial communication includes both asynchronous communication and synchronous communication.
Asynchronous communication represents the beginning of a character with a start bit and the end of the stop bit. Its transmission format is shown in Figure 1.
Figure 1 Format of asynchronous communication
As can be seen from the figure, the start bit occupies 1 bit, and the data is 5-8 bits. The length depends on the type of data to be transmitted. The low bit is transmitted first, and then the high bit is transmitted. The data is followed by a parity bit. If set to even parity, the number of data and parity bits is even. If set to odd parity, the number of data and check digits is odd. Finally, the stop bit, the stop bit can be set to 1 bit, 1 as needed. Bit or 2 digits. In asynchronous communication, the time occupied by each bit is the reciprocal of the data transmission rate (baud rate). If the transmission baud rate is 2400, the time per bit is 1/2400S, which is 0.416 ms. If an ASCII code is transmitted, the data occupies 7 bits, and the start bit, check bit, and stop bit each occupy 1 bit, then an ASCII code is transmitted occupying 10 bits. With a baud rate of 2400, 240 ASCII codes can be transmitted per second. The transmission baud rate commonly used for asynchronous communication is 150 to 38,400.
Asynchronous communication is transmitted with the start bit indicating the beginning of the character and the stop bit indicating the end of the character. The receiver uses these separators to transform a serial data into parallel data. This communication method is easy to implement, and even if it is not continuously transmitted during transmission, there is no problem of out-of-synchronization. However, since each character is transmitted by adding 2 to 3 bits for synchronization, the transmission efficiency is lowered.
Unlike asynchronous communication, synchronous communication sends a complete data set at a time, and both parties issue a special synchronization symbol before sending. Synchronous communication has a high transmission efficiency, and its communication coordination is complicated, and a synchronous clock is required.
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