📄 asynchronous communication.htm
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<H1>Asynchronous Communication</H1>
<P>The asynchronous communication technique is a <A
href="http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/phy.html">physical
layer</A> 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.</P>
<P align=center><I><IMG align=bottom height=105
src="Asynchronous Communication.files/async.gif" width=424
NATURALSIZEFLAG="3"></I></P>
<P align=center><I>Asynchronous transmission of a series of characters</I></P>
<P>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.</P>
<P align=center><I><IMG align=bottom height=278
src="Asynchronous Communication.files/async-sr.gif" width=420
NATURALSIZEFLAG="3"></I></P>
<P align=center><I>Asynchronous transmission - each character is framed by a
start and one or more stop bits</I></P>
<P>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:</P>
<P align=center><I><IMG align=bottom height=118
src="Asynchronous Communication.files/async-start.gif" width=406
NATURALSIZEFLAG="3"></I></P>
<P align=center><I>The transition from the idle state triggers the UART at the
receiver to start reception</I></P>
<P>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.</P>
<P align=center><I><IMG align=bottom height=210
src="Asynchronous Communication.files/async-clock.gif" width=449
NATURALSIZEFLAG="3"></I></P>
<P align=center><I>Reconstruction of the clock (red) , by matching of phase to
the transmitted data (blue) to the local stable high rate clock (black)</I></P>
<P>When asynchronous transmission is used to support packet data links (e.g. <A
href="http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/ip.html">IP</A>),
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 <A
href="http://www.erg.abdn.ac.uk/users/gorry/course/intro-pages/wan.html">Wide
Area Networks</A> use synchronous links and a more sophisticated link protocol
(e.g. <A
href="http://www.erg.abdn.ac.uk/users/gorry/course/dl-pages/hdlc.html">HDLC</A>)</P>
<HR align=left>
<B><I><FONT color=#ff0000>Puzzled???</FONT></I></B>
<P>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 <A
href="http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/bit-order.html">HERE</A>
to find out why.
<P></P>
<HR align=left>
<I><A href="http://www.erg.abdn.ac.uk/users/gorry/">Gorry Fairhurst</A> -
email:G.Fairhurst@eng.abdn.ac.uk - Date: 01/10/2001 <A
href="http://www.erg.abdn.ac.uk/users/gorry/course/road-map.html">EG3557</A></I>
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