Asynchronous Communication

The asynchronous communication technique is a physical layer transmission technique which is most widely used for personal computers providing connectivity to printers, modems, fax machines, etc. The most significant aspect of asynchronous communications is that the transmitter and receiver clock are independent and are not synchronised. In fact, there need be no timing relationship between successive characters (or bytes of data). Individual characters may be separated by any arbitrary idle period.

Asynchronous transmission of a series of characters

An asynchronous link communicates data as a series of characters of fixed size and format. Each character is preceded by a start bit and followed by 1-2 stop bits. Parity is often added to provide some limited protection against errors occurring on the link. The use of independent transmit and receive clocks constrains transmission to relatively short characters (<8 bits) and moderate data rates (< 64 kbps, but typically lower). The asynchronous transmitter delimits each character by a start sequence and a stop sequence. The start bit (0), data (usually 8 bits plus parity) and stop bit(s) (1) are transmitted using a shift register clocked at the nominal data rate.

Asynchronous transmission - each character is framed by a start and one or more stop bits

At the receiver, a clock of the same nominal frequency is constructed and used to clock-in the data to the receive shift register. Only data that are bounded by the correct start and stop bits are accepted. This operation is normally performed using a UART - Universal Asynchronous Receiver Transmitter. The receiver is started by detecting the edge of the first start bit as shown below:

The transition from the idle state triggers the UART at the receiver to start reception

The reconstructed receive clock (receive (rx) clock) is normally generated using a local stable high rate clock, frequently operating at 16 or 32 times the intended data rate. Clock generation proceeds by detecting the edge of the start bit and counting sufficient clock cycle from the high frequency clock to identify the mid position of the start bit. From there the centre of the successive bits are located by counting cycles corresponding to the original data speed.

Reconstruction of the clock (red) , by matching of phase to the transmitted data (blue) to the local stable high rate clock (black)

When asynchronous transmission is used to support packet data links (e.g. IP), then special characters have to be used ("framing") to indicate the start and end of each frame transmitted. One character (none as an escape character) is reserved to mark any occurrence of the special characters within the frame. In this way the receiver is able to identify which characters are part of the frame and which are part of the "framing". Packet communication over asynchronous links is used by some users to get access to a network using a modem. Most Wide Area Networks use synchronous links and a more sophisticated link protocol (e.g. HDLC)


If you ever have connected a computer terminal to a storage oscilloscope to look at the waveform you may have been rather puzzled at the result! Click HERE to find out why.

Gorry Fairhurst - Date: 01/10/2001 EG3557