267 lines
5.2 KiB
HTML
267 lines
5.2 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Configure dumb modem</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="Modem configuration"
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HREF="modem.html"><LINK
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REL="PREVIOUS"
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TITLE="Using Minicom to give commands to
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a modem"
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HREF="modem-minicom.html"><LINK
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REL="NEXT"
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TITLE="Configure modem with AT commands"
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HREF="modem-hayes.html"></HEAD
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><BODY
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CLASS="SECTION"
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BGCOLOR="#FFFFFF"
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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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><A
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HREF="modem-minicom.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 12. Modem configuration</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="modem-hayes.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="MODEM-DUMB"
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></A
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>12.2. Configure dumb modem</H1
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><P
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>Linux, like most <SPAN
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CLASS="PRODUCTNAME"
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>UNIX</SPAN
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>-like
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operating systems, expects a serial console to be connected to a
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dumb modem. Dumb modems are not seen much these days, perhaps only
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on exotic hardware such as <SPAN
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CLASS="ACRONYM"
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>ISDN</SPAN
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> terminal adapters
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or satellite ground terminals.</P
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><P
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>A dumb modem is configured using hardware. <A
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HREF="modem-dumb.html#MODEM-DUMB-FRONT"
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>Figure 12-1</A
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> shows the front panel of a fanciful
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dumb modem. In reality the speed and mode settings are likely to
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be done using jumpers or DIP switches.</P
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><DIV
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CLASS="FIGURE"
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><A
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NAME="MODEM-DUMB-FRONT"
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></A
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><P
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><B
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>Figure 12-1. Front panel of a dumb modem</B
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></P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="PROGRAMLISTING"
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>+-----------------------------+
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| |
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| SPEED MODE |
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| [ ] 300 [ ] Originate |
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| [ ] 600 [X] Answer |
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| [ ] 2400 |
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| [X] 9600 |
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+-----------------------------+</PRE
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></FONT
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></TD
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></TR
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></TABLE
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></DIV
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><P
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>The modem's speed is set to the desired bit rate, in our case
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9600<SPAN
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CLASS="ABBREV"
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>bps</SPAN
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>. The modem's mode is set to Answer,
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that is, to wait for incoming calls and to answer them.</P
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><P
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>If the <SPAN
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CLASS="ACRONYM"
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>RS-232</SPAN
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> control line Data Terminal
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Ready is low, the modem will not answer a call. The computer is
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off or the computer's serial interface is not yet initialized.
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Once <SPAN
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CLASS="ACRONYM"
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>DTR</SPAN
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> is high the modem will answer incoming
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calls.</P
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><P
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>Once an incoming call is established the modem raises the
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Data Carrier Detect control line. Only when DCD is high is
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received data valid (data receieved from a dumb modem when DCD is
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not asserted is probably line noise). Only when DCD is high is
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transmitted data passed through the link.</P
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><P
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><SPAN
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CLASS="APPLICATION"
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>getty</SPAN
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> on the Linux computer has
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been waiting for <SPAN
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CLASS="ACRONYM"
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>DCD</SPAN
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> to come high, and
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<SPAN
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CLASS="APPLICATION"
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>getty</SPAN
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> welcomes the user and requests
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them to log in.</P
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><P
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>Whilst the user is logged in and data is flowing, Clear to
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Send and Ready to Send are used between the modem and the computer
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to prevent data being sent too soon. The computer lowers Ready to
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Send when it is too busy to receive a character. The modem lowers
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Clear to Send when it is too busy to receive a character.</P
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><P
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>When the user hangs up, Data Carrier Detect falls and the
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hang up signal is sent to all processes associated with the dial in
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session.</P
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><P
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>Alternatively, the user can log out. When the shell dies,
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the computer pulls Data Terminal Ready low, causing the modem to
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hang up. When the <SPAN
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CLASS="APPLICATION"
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>getty</SPAN
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> brings Data
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Terminal Ready high again, the modem will accept more incoming
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calls.</P
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><P
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>We have not yet described Data Set Ready. This line is low
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if the modem is off or if the modem has not yet initialized. When
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DSR is low all other signals from the modem are undefined. For
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example, if DSR is low but DCD "floats" to the high voltage then
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software should behave as if DCD is not asserted.</P
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></DIV
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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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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="modem-minicom.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="modem-hayes.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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>Using <SPAN
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CLASS="PRODUCTNAME"
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>Minicom</SPAN
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> to give commands to
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a modem</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="modem.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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>Configure modem with <SPAN
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CLASS="ACRONYM"
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>AT</SPAN
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> commands</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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> |