Introduction to Data Communications
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20. Asynchronous Communications

Asynchronous communications or transmission sends individual characters one at a time framed by a start bit and 1 or 2 stop bits.

20a. Start/Stop bits

The purpose of the Start bit is to notify the receiving station of a new character arriving. Typically data is shown moving left to right. This is how it would appear on a Storage Oscilloscope or Network Analyser. The MSB ( Most Significant Bit) is sent first and the LSB (Least Significant Bit) is sent last.

The purpose of the Stop bits is to indicate the end of data. There could be 1 or 2 stop bits with 1 being the typical number of stop bits used today. In Asynchronous transmission, the characters are sent individually with a quiet period in between (quiet meaning 0 bit level). Asynchronous communications requires the transmitting station and the receiving station to have individual internal free-running clocks operating at the same frequency. Free-running means that the clocks are not locked together.

Both clocks operating at same frequency:

The receive station starts checking for data after the Start bit is received (Start bit is a wake up call!).

The receive station samples the transmitted data in the middle of each data bit. The samples are evenly spaced and match the transmitted data because both transmit and receive clocks are operating at the same frequency.

Introduction to Data Communications
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