Asynchronous data transmission

Serial data communication generally employs either synchronous or asynchronous communication scheme. This two scheme used different techniques for synchronizing in the circuits in sending and receiving end.

In asynchronous transmission each character is transmitted separately, that is one character at a time. The character is preceded by a start bit, which tells the receiving end where the character coding begins, and is followed by a stop bit, which tells the receiver where the character coding ends. There will be intervals of ideal time on the channel shown as gaps. Thus there can be gaps between two adjacent characters in the asynchronous communication scheme. In this scheme, the bits within the character frame (including start, parity and stop bits) are sent at the baud rate.

The START BIT and STOP BIT including gaps allow the receiving and sending computers to synchronise the data transmission. Asynchronous communication is used when slow speed peripherals communicate with the computer. The main disadvantage of asynchronous communication is slow speed transmission. Asynchronous communication however, does not require the complex and costly hardware equipments as is required for synchronous transmission.

Advantages and Disadvantages of Asynchronous Transmission

The advantage of asynchronous transmission is that it does not required any local storage at the terminal or the computer and is thus cheaper to implement.

Major disadvantage of asynchronous transmission is that the transmission lines is idle during the time intervals between transmitting characters.

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