470 lines
9.7 KiB
HTML
470 lines
9.7 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Why use a serial console?</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Remote Serial Console HOWTO"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Introduction"
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HREF="intro.html"><LINK
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REL="PREVIOUS"
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TITLE="What is a console?"
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HREF="intro-what.html"><LINK
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REL="NEXT"
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TITLE="Alternative meanings of console"
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HREF="intro-word.html"></HEAD
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><BODY
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CLASS="SECTION"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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VLINK="#840084"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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BORDER="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Remote Serial Console HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="intro-what.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 1. Introduction</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="intro-word.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="INTRO-WHY"
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></A
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>1.2. Why use a serial console?</H1
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><P
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>For the average user a serial console has no advantage over a
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console offered by a directly attached keyboard and screen. Serial
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consoles are much slower, taking up to a second to fill a 80 column
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by 24 line screen. Serial consoles generally only support
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non-proportional <SPAN
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CLASS="ACRONYM"
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>ASCII</SPAN
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> text, with limited
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support for languages other than English. A new terminal can be
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more expensive than an old <SPAN
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CLASS="ACRONYM"
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>PC</SPAN
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>.</P
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><P
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>There are some scenarios where serial consoles are
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useful. These are:</P
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><P
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></P
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><DIV
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CLASS="VARIABLELIST"
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><DL
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><DT
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>Systems administration of remote computers</DT
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><DD
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><P
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><SPAN
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CLASS="SYSTEMITEM"
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>Linux</SPAN
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> is a good
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operating system for deployment at unstaffed sites. <SPAN
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CLASS="SYSTEMITEM"
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>Linux</SPAN
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> is also good for hosting
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critical network infrastructure such as <SPAN
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CLASS="ACRONYM"
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>DNS</SPAN
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>
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and <SPAN
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CLASS="ACRONYM"
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>DHCP</SPAN
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> services. These services are
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generally installed at every site of an organisation including
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sites which may be too small or too remote to have information
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technology staff.</P
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><P
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>System administration of these remote computers is usually
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done using <SPAN
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CLASS="APPLICATION"
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><SPAN
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CLASS="ACRONYM"
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>SSH</SPAN
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></SPAN
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>, but
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there are times when access to the console is the only way to
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diagnose and correct software failures. Major upgrades to the
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installed distribution may also require console access.</P
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><P
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>In these cases the serial console is attached to a modem.
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Access to the console is gained from a remote computer by
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dialing into the modem. This allows the console to be reached
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from any telephone socket.</P
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></DD
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><DT
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>High density racks of computers</DT
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><DD
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><P
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>Clusters of personal computers can outperform mainframe
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computers and form competitive supercomputers for some
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applications. See the <A
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HREF="http://www.tldp.org/HOWTO/Cluster-HOWTO.html"
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TARGET="_top"
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><I
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CLASS="CITETITLE"
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>Cluster-HOWTO</I
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></A
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>
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for more information on clustering.</P
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><P
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>These clusters are typically assembled into 19 inch
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telecommunications equipment racks and the system unit of each
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computer is typically one rack unit (or 1.75 inches) tall. It
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is not desirable to put a keyboard and monitor on each computer,
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as a small cathode ray tube monitor would consume the space used
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by sixteen rack units.</P
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><P
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>A first glance it seems that a monitor and keyboard switch
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is the best solution. However the <SPAN
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CLASS="ACRONYM"
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>VGA</SPAN
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> signal
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to the monitor is small, so even with the switch the monitor
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cannot be placed very far away from the rack of
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computers.</P
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><P
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>It is desirable to allow the consoles to be monitored in
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the operators' room of the computer center, rather than in the
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very expensive space of the machine room. Although monitor
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switches with remote control and fiber optical extensions are
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available, this solution can be expensive.</P
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><P
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>A standard <SPAN
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CLASS="ACRONYM"
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>RS-232</SPAN
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> cable can be 15
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meters in length. Longer distances are easily possible. The
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cabling is cheap. Terminal servers can be used to allow one
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terminal to access up to 90 serial consoles.</P
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></DD
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><DT
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>Recording console messages</DT
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><DD
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><P
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>This is useful in two very different cases.</P
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><P
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>Kernel programmers are often faced with a kernel error
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message that is displayed a split second before the computer
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reboots. A serial console can be used to record that
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message. Another <SPAN
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CLASS="SYSTEMITEM"
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>Linux</SPAN
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>
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machine can be used as the serial terminal.</P
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><P
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>Some secure installations require all security events to
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be unalterably logged. One way to meet this requirement is to
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print all console messages. Connecting the serial console to a
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serial printer can achieve this.<A
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NAME="AEN140"
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HREF="#FTN.AEN140"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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></P
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></DD
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><DT
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>Embedded software development</DT
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><DD
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><P
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><SPAN
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CLASS="SYSTEMITEM"
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>Linux</SPAN
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> is
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increasingly being used as an operating system for embedded
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applications. These computers do not have keyboards or
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screens.</P
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><P
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>A serial port is a cheap way to allow software developers
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to directly access the embedded computer. This is invaluable
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for debugging. Most chip sets designed for embedded computers
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have a serial port precisely for this purpose.</P
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><P
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>The shipping product need not present the
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<SPAN
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CLASS="ACRONYM"
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>RS-232</SPAN
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> port on an external connector.
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Alternatively the <SPAN
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CLASS="ACRONYM"
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>RS-232</SPAN
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> port is often used for
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downloading software updates.</P
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></DD
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><DT
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>Craft terminal for telecommunications equipment</DT
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><DD
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><P
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><SPAN
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CLASS="SYSTEMITEM"
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>Linux</SPAN
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> is
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increasingly being used as the operating system inside
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telecommunications equipment. The <A
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HREF="http://www.osdlab.org/projects/cgl/"
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TARGET="_top"
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>Carrier Grade
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Linux</A
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> consortia hopes to accelerate and coordinate this
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trend.</P
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><P
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>Most telecommunications equipment is remotely managed from
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a distant computer. However, site technicans (called
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<I
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CLASS="WORDASWORD"
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>craft personnel</I
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> in telco-speak) need to
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access the equipment to test installation changes, check the
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status of reported faults, and so on. The terminal used by the
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craft personnel is called the <I
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CLASS="WORDASWORD"
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>craft
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terminal</I
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>. The craft terminal plugs into the
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<I
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CLASS="WORDASWORD"
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>craft interface</I
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> on the equipment. The
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serial console makes an ideal craft interface.</P
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></DD
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></DL
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></DIV
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><P
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>Unlike minicomputer systems, the
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<SPAN
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CLASS="PRODUCTNAME"
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><SPAN
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CLASS="ACRONYM"
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>IBM</SPAN
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>
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<SPAN
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CLASS="ACRONYM"
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>PC</SPAN
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></SPAN
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> was not designed to use a
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serial console. This has two consequences.</P
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><P
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>Firstly, Power On Self-Test messages and Basic Input/Output
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System (<SPAN
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CLASS="ACRONYM"
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>BIOS</SPAN
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>) messages are sent to the screen
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and received from the keyboard. This makes it difficult to use the
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serial port to reconfigure the <SPAN
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CLASS="ACRONYM"
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>BIOS</SPAN
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> and
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impossible to see Power On Self-Test errors.</P
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><P
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>An increasing number of manufacturers of rackable
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<EM
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>server</EM
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> equipment are altering their
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<SPAN
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CLASS="ACRONYM"
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>BIOS</SPAN
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>s to optionally use the
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<SPAN
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CLASS="ACRONYM"
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>RS-232</SPAN
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> port for BIOS configuration and test
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messages. If you are buying a machine specifically for use with
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serial console you should seek this feature. If you have an
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existing machine that definitely requires access to the
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<SPAN
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CLASS="ACRONYM"
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>BIOS</SPAN
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> from the serial port then there are
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hardware solutions such as <A
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HREF="http://www.realweasel.com/"
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TARGET="_top"
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><SPAN
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CLASS="PRODUCTNAME"
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>PC Weasel
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2000</SPAN
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></A
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>.</P
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><P
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>Secondly, the <SPAN
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CLASS="ACRONYM"
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>RS-232</SPAN
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> port on the
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<SPAN
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CLASS="PRODUCTNAME"
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><SPAN
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CLASS="ACRONYM"
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>IBM</SPAN
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>
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<SPAN
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CLASS="ACRONYM"
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>PC</SPAN
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></SPAN
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> is designed for connecting to a
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modem. Thus a null modem cable is needed when connecting the PC's
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serial port to a terminal.</P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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BORDER="0"
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN140"
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HREF="intro-why.html#AEN140"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
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>The <SPAN
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CLASS="SYSTEMITEM"
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>Linux</SPAN
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>
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<SPAN
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CLASS="PRODUCTNUMBER"
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>2.4</SPAN
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> kernel also supports the
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output of console messages to
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<SPAN
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CLASS="PRODUCTNAME"
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>Centronics</SPAN
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> or
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<I
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CLASS="CITETITLE"
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><SPAN
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CLASS="ACRONYM"
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>IEEE</SPAN
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> 1284-2000</I
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>
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parallel printer interfaces.</P
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></TD
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></TR
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></TABLE
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="intro-what.html"
|
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ACCESSKEY="P"
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>Prev</A
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></TD
|
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><TD
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WIDTH="34%"
|
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ALIGN="center"
|
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VALIGN="top"
|
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><A
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HREF="index.html"
|
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ACCESSKEY="H"
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>Home</A
|
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></TD
|
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><TD
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WIDTH="33%"
|
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ALIGN="right"
|
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VALIGN="top"
|
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><A
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HREF="intro-word.html"
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ACCESSKEY="N"
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>Next</A
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></TD
|
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></TR
|
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><TR
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><TD
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WIDTH="33%"
|
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ALIGN="left"
|
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VALIGN="top"
|
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>What is a console?</TD
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><TD
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WIDTH="34%"
|
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ALIGN="center"
|
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VALIGN="top"
|
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><A
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HREF="intro.html"
|
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ACCESSKEY="U"
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>Up</A
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></TD
|
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><TD
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WIDTH="33%"
|
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ALIGN="right"
|
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VALIGN="top"
|
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>Alternative meanings of <SPAN
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CLASS="QUOTE"
|
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>"console"</SPAN
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></TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |