Asynchronous communications is a method of transmitting serial data over a communication link between two hosts in a serial fashion. Two hosts communicate by sending burst of data across the link to each other and they use their shared configured frequency in order to synchronize, and decode the bit streams.
One of the most well known techniques for serial communication in an asynchronous fashion is RS232, which is how computers with serial ports, used to communicate with peripherals with serial interfaces such as modems. Asynchronous communications do not use a clock to generate a clock signal so there must be another method for both host systems to be able to synchronize the bit stream. The method they use is a fixed transmit/receive (Rx/Tx )frequency.
After all that is all that is happening, one computer is sending a bursting stream of data bits in binary to the other. Therefore, the receiving end system must be able to distinguish what are the relevant bits in the stream. They do this by identifying the start and stop bits within the stream, and then they know that everything between is data. There are other signal bits such a parity bit, which is used to detect and recover transmission errors. A parity bit can be set at having parity or commonly, no parity.
A common asynchronous communication configuration would be 96,000, 1 start bit, 1 stop bit, no parity.
Asynchronous communication ports were found on PC’s and older peripherals such as modems or printers. An RS232 serial cable connects the two 9 pin or 25 pin connectors up to a distance to 1,000 feet. However the fluctuating voltage swing used in the RS232 specification made power supply unit design costly and RS232 in Pc’s and peripherals has been replaced by USB ports which have smaller connectors, much higher speeds, and better power requirements.
Asynchronous communication interfaces are still commonplace in telecoms and serial data communication interfaces and cards, and they can still found be found in routers for low speed telecom links.