Why is a start bit used in asynchronous communication?

Answer

Asynchronous (serial) communication means that all the necessary decoding information is carried in the data stream, with no need for any external timing reference. The start bit is needed to tell the receiver that the next so-many bits are data. Likewise, the stop bit tells the receiver that the byte or packet has ended, and it can check to see if it received the correct number of data bits between the start and stop bits.

Answer

Think of it this way, if you transmitted 0 asychronously without a start bit, how would the client know a byte had been sent? There would be no state change on the line.

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