serial interface (rs-232) 1.how rs232 works.mht

用FPGA实现RS232通信,此代码是用VHDL语言编写

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      <H4>How the RS-232 serial interface works</H4>Most computers have =
one or=20
      two serial RS-232 interface as standard equipment. Each interface =
has the=20
      following characteristics:=20
      <UL>
        <LI>Uses a 9 pins connector "DB-9" (older PCs use 25 pins =
"DB-25").=20
        <LI>Allows bidirectional full-duplex communication (the PC can =
send and=20
        receive data at the same time).=20
        <LI>Can communicate at a maximum speed of roughly 10KBytes/s.=20
      </LI></UL>You can use serial communication to connect two PCs =
together,=20
      for example.=20
      <H4>DB-9 connector</H4>You probably already saw this connector on =
the back=20
      of your PC.<BR><BR><IMG=20
      =
src=3D"http://www.fpga4fun.com/images/SerialConnector.jpg"><BR><BR>It =
has 9=20
      pins, but the 3 important ones are:=20
      <UL>
        <LI>pin 2: RxD (receive data).=20
        <LI>pin 3: TxD (transmit data).=20
        <LI>pin 5: GND (ground). </LI></UL>Using just 3 wires, you can =
send and=20
      receive data.=20
      <H4>Serial communication</H4>Data is sent one bit at a time; so =
one wire=20
      is enough for each direction. Since computers usually need at =
least=20
      several bits of data, the data is "serialized" before being sent. =
Data is=20
      commonly sent by chunks of 8 bits. The data bit 0 is sent first, =
bit 7=20
      last.=20
      <H4>Asynchronous communication</H4>This interface uses an =
"asynchronous"=20
      protocol. That means that no clock signal is transmitted along the =
data.=20
      The receiver has to have a way to "time" itself to the incoming =
data=20
      bits.<BR><BR>In the case of RS-232, that's done this way:=20
      <OL>
        <LI>Both side of the cable agree in advance on the communication =

        parameters (speed, format...). That's done manually before =
communication=20
        starts.=20
        <LI>The transmitter sends a "1" when and as long as the line is =
idle.=20
        <LI>The transmitter sends a "start" (a "0") before each byte=20
        transmitted, so that the receiver can figure out that data is =
coming.=20
        <LI>After the "start", data comes in the agreed speed and =
format, so the=20
        receiver can interpret it.=20
        <LI>The transmitter sends a "stop" (a "1") after each data byte. =

      </LI></OL>Let's see how looks the byte 0x55 when =
transmitted:<BR><BR><IMG=20
      =
src=3D"http://www.fpga4fun.com/images/SerialCommunication55.gif"><BR><BR>=
Byte=20
      0x55 is 01010101 in binary.<BR>But since it is transmitted LSB =
(bit-0)=20
      first, the line toggles like that: 1-0-1-0-1-0-1-0.<BR><BR>Here's =
another=20
      example:<BR><BR><IMG=20
      =
src=3D"http://www.fpga4fun.com/images/SerialCommunication.gif"><BR><BR>He=
re=20
      the data is 0xC4, can you see it?<BR>The bits transitions are =
harder to=20
      see. That illustrates how important it is for the receiver to know =
at=20
      which speed the data is sent.=20
      <H4>How fast can we send data?</H4>The speed is specified in baud, =
i.e.=20
      how many bits-per-seconds can be sent. For example, 1000 bauds =
would mean=20
      1000 bits-per-seconds, or that each bit lasts one=20
      millisecond.<BR><BR>Common implementations of the RS-232 interface =
(like=20
      the one used in PCs) don't allow just any speed to be used. If you =
want to=20
      use 123456 bauds, you're out of luck. You have to settle to some=20
      "standard" speed. Common values are:=20
      <UL>
        <LI>1200 bauds.=20
        <LI>9600 bauds.=20
        <LI>38400 bauds.=20
        <LI>115200 bauds (usually the fastest you can go). </LI></UL>At =
115200=20
      bauds, each bit lasts (1/115200) =3D 8.7&micro;s. If you transmit =
8-bits data,=20
      that lasts 8 x 8.7&micro;s =3D 69&micro;s. But each byte requires =
an extra start and=20
      stop bit, so you actually need 10 x 8.7&micro;s =3D 87&micro;s. =
That translates to a=20
      maximum speed of 11.5KBytes per second.<BR><BR>At 115200 bauds, =
some PCs=20
      with buggy chips require a "long" stop bit (1.5 or 2 bits long...) =
which=20
      make the maximum speed drop to around 10.5KBytes per second.=20
      <H4>Physical layer</H4>The signals on the wires use a =
positive/negative=20
      voltage scheme.=20
      <UL>
        <LI>"1" is sent using -10V (or between -5V and -15V).=20
        <LI>"0" is sent using +10V (or between 5V and 15V). </LI></UL>So =
an idle=20
      line carries something like -10V.=20
      <H4>Links</H4>
      <UL>
        <LI>A nice <A=20
        =
href=3D"http://developer.apple.com/documentation/mac/NetworkingOT/graphic=
s/OTSE-L-01.gif">drawing</A>,=20
        from this <A=20
        =
href=3D"http://developer.apple.com/documentation/mac/NetworkingOT/Network=
ingWOT-79.html">Apple=20
        Developer Connection</A> page.=20
        <LI>A short <A=20
        =
href=3D"http://penguin.dcs.bbk.ac.uk/academic/technology/physical-layer/a=
synchronous/">Asynchronous=20
        Communication</A> page=20
        <LI>An <A=20
        =
href=3D"http://www.taltech.com/TALtech_web/resources/intro-sc.html">Intro=
duction=20
        to RS232 Serial Communications</A>=20
        <LI><A =
href=3D"http://tldp.org/HOWTO/Serial-HOWTO-20.html">Voltage=20
        Waveshapes</A>, part of this huge <A=20
        href=3D"http://tldp.org/HOWTO/Serial-HOWTO.html">Serial =
HOWTO</A> <!--<li><a =
href=3D"http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/async.html=
">here</a></li>--></LI></UL><BR><BR><A=20
      href=3D"http://www.fpga4fun.com/SerialInterface2.html"><FONT =
color=3Dblue=20
      size=3D2>&gt;&gt;&gt; NEXT: Baud generator=20
      &gt;&gt;&gt;</FONT></A><BR><BR><BR>
      <HR>
      This page was last updated on May 10=20
2004.<BR><BR></TD></TR></TBODY></TABLE></BODY></HTML>

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