The first is the difference between the two:
Synchronous communication requires the receiving side and the transmission side clock frequency consistent with the clock frequency , the transmitting side transmits a continuous bit stream; asynchronous communication is not required when the receiving end and the transmitting end clock clock synchronization, the transmitting side transmits a byte, may be subjected to arbitrary length time interval and then send the next byte.
Synchronous high communication efficiency; asynchronous communication efficiency is low.
Synchronous communication is more complex, both to allow the clock error is small; simple asynchronous communication, both clocks may allow certain error.
Synchronization can be used for point to multipoint communication; only applies to asynchronous communication point.
Asynchronous communication
Recipient asynchronous communication does not know when the data will arrive, sending and receiving sides can have their own clock . Time interval may be sent by the sender unevenness , recipient information for synchronization is achieved with the help of the start and stop bits of data. Such transmission is usually small packet, such as a character as a set, with start and stop bits for this group. Therefore the efficiency of this transmission is relatively low, after a lot of additional auxiliary bit is added as a load, often used in low-speed transmission.
In Case predetermined RS232 protocol, asynchronous communication a transmitted character by character, each character transmitted bit by bit, and when a character transmission, always "start bit" start (low logical value 0) to "stop bit" end, there is no fixed time interval required between characters. Character data itself by a 5 to 8-bit data bits, followed by a parity bit behind the character is (or may not parity bit), and finally a half or one or two stop bits, stop bits are variable length behind the idle position. Idle stop bit and predetermined bits are high (logical value 1), so that at the beginning of the start bit must ensure that there is a next hop along,
For example, our keyboard presses a button to send a character signal, asynchronous transmission mechanism will assist the synchronization information before and after it with the help us to identify the recipient which key was pressed. Because we percussion rhythm keyboard is not fixed, so asynchronous it is a very appropriate way
Synchronous communication
Synchronous communication parties using the same clock frequency, compared to an asynchronous packet which is much larger, is called a data frame through a unique bit string as start and stop identifier. To the sender transmits data to a fixed rhythm, and the receiver to be always ready to receive data, to identify the preamble of the received data is about to begin. Synchronization in this manner because the packet is large, a long data bit will have additional auxiliary load, therefore more efficient, is more suitable for a high transmission speed requirements, of course, such communication timing requirements are also higher.
Synchronous continuous communication is a serial communication for transmitting data, a communication only send one message , the synchronization character, character data and the check character (CRC) composition.