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Asynchronous means that there is no timing element between the two communicating devices. The synchronization is done by extending a data word with start and stop bits. The most common asynchronous transmission is the RS232 port on a PC.

The word size is normally 7 or 8 bits. By adding start and stop bits to the data a receiving device is able to destillate the clock timing and after that read the data in a correct way. The benefit is that there is no need for clock signals which saves in wires in the cable and electronics on the serial board. The disadvantage is that more bits are necessary to transmit the data. So money is saved at the cost of performance.