<appendix id="appendix-hayes-commands">
<title>The Hayes Modem Command Set</title>

<para>Here is a description of the Hayes Command Set. Most modems follow this
command set to large extent. If you lost your modem manual or never had one in
the first place, this reference might come in handy. I for instance finally found
out how to turn my modems speaker off: <command>ATM0</command> -- Finally:
Silence!</para>

<para>The modem initialization string consists of a series of commands. It
prepares the modem for communications, setting such features as dialing mode,
waits, detection of the busy signal and many other settings. Newer modem
communications programs reset the initializations string for you according to
which menu options you select, which features you enable, &etc;.</para>

<para>For many years Hayes modems have been the standard. As the field of modem
manufactures has grown, most have adhered at least loosely to the Hayes
standard. The following is a partial list of the Hayes command set. (called the
<quote>AT</quote> commands). The Hayes Command Set can be divided into four
groups:</para>

<variablelist>
<varlistentry>
<term>Basic Command Set</term>
<listitem><para>A capital character followed by a digit. For example,
<command>M1</command>.</para></listitem>
</varlistentry>
<varlistentry>
<term>Extended Command Set</term>
<listitem><para>An <quote>&amp;</quote> (ampersand) and a capital character
followed by a digit. This is an extension of the basic command set. For example,
<command>&amp;M1</command>. Note that <command>M1</command> is different from
<command>&amp;M1</command>.</para></listitem>
</varlistentry>
<varlistentry>
<term>Proprietary Command Set</term>
<listitem><para>Usually started by either a backslash (<quote>\</quote>), or a
percent sign (<quote>&percnt;</quote>), these commands vary widely among modem
manufacturers. For that reason, only a few of these commands are listed
below.</para></listitem>
</varlistentry>
<varlistentry>
<term>Register Commands</term>
<listitem><para><command>S<replaceable>r</replaceable>=<replaceable>n</replaceable></command>
where <replaceable>r</replaceable> is the number of the register to be changed,
and <replaceable>n</replaceable> is the new value that is
assigned.</para>

<para>A <quote>register</quote> is computerese for a specific physical location
in memory. Modems have small amounts of memory onboard. This fourth set of
commands is used to enter values in a particular register (memory location). The
register will be storing a particular <quote>variable</quote> (alpha-numeric
information) which is utilized by the modem and communication software. For
example, <command>S7=60</command> instructs your computer to <quote>Set register
#7 to the value 60</quote>.</para></listitem>
</varlistentry>
</variablelist>

<note><para>Although most commands are defined by a letter-number combination
(<command>L0</command>, <command>L1</command> &etc;), the use of a zero is
optional.  In this example, <command>L0</command> is the same as a plain
<command>L</command>.  Keep this in mind when reading the table
below!</para></note>

<para>Here are some of the most important characters that may appear in the
modem initialization string.  These characters normally should not be
changed.</para>

<variablelist>
<varlistentry>
<term><command>AT</command></term>
<listitem><para>Tells the modem that modem commands follow.  This must begin
each line of commands.</para></listitem>
</varlistentry>
<varlistentry>
<term><command>Z</command></term>
<listitem><para>Resets the modem to its default state</para></listitem>
</varlistentry>
<varlistentry>
<term><command>,</command> (a comma)</term>
<listitem><para>makes your software pause for a second. You can use more than
one <command>,</command> in a row. For example, <command>,,,,</command> tells
the software to pause four seconds. (The duration of the pause is governed by
the setting of register <varname>S8</varname>.)</para></listitem>
</varlistentry>
<varlistentry>
<term><command>^M</command></term>
<listitem><para>Sends the terminating Carriage Return character to the modem.
This is a control code that most communication software translates as
<quote>Carriage Return</quote></para></listitem></varlistentry>
</variablelist>

<sect1 id="hayes-basic-commands">
<title>The Basic Hayes Command Set</title>

<para>In alphabetical order:</para>

   <table>
    <title>Basic Hayes Command Set</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Command</entry>
       <entry>Description</entry>
       <entry>Comments</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><command>A0</command> or <command>A</command></entry>
       <entry>Answer incoming call</entry>
       <entry></entry>
      </row>
      <row>
       <entry><command>A/</command></entry>
       <entry>Repeat last command</entry>
       <entry>Don't preface with <command>AT</command>.  Enter usually
       aborts.</entry>
      </row>
      <row>
       <entry><command>B0</command> or <command>B</command></entry>
       <entry>Call negotiation</entry>
       <entry>V32 Mode/CCITT Answer Seq.</entry>
      </row>
      <row>
       <entry><command>B1</command></entry>
       <entry>Call negotiation</entry>
       <entry>Bell 212A Answer Seq.</entry>
      </row>
      <row>
       <entry><command>B2</command></entry>
       <entry>Call negotiation</entry>
       <entry>Verbose/Quiet On Answer</entry>
      </row>
      <row>
       <entry><command>D</command></entry>
       <entry>Dial</entry>
       <entry><para>Dial the following number and then handshake in originate
       mode.</para><variablelist>
	 <varlistentry>
	  <term><command>P</command></term>
	  <listitem><para>Pulse Dial</para></listitem>
	 </varlistentry>
	 <varlistentry>
	  <term><command>T</command></term>
	  <listitem><para>Touch Tone Dial</para></listitem>
	 </varlistentry>
	 <varlistentry>
	  <term><command>W</command></term>
	  <listitem><para>Wait for the second dial tone</para></listitem>
	 </varlistentry>
	 <varlistentry>
	  <term><command>,</command></term>
	  <listitem><para>Pause for the time specified in register
	  <varname>S8</varname> (usually 2 seconds)</para></listitem>
	 </varlistentry>
	 <varlistentry>
	  <term><command>;</command></term>
	  <listitem><para>Remain in command mode after dialing.</para></listitem>
	 </varlistentry>
	 <varlistentry>
	  <term><command>!</command></term>
	  <listitem><para>Flash switch-hook (Hang up for a half second, as in
	  transferring a call.)</para></listitem>
	 </varlistentry>
	 <varlistentry>
	  <term><command>L</command></term>
	  <listitem><para>Dial last number</para></listitem>
	 </varlistentry>
	</variablelist></entry>
      </row>
      <row>
       <entry><command>E0</command> or <command>E</command></entry>
       <entry>No Echo</entry>
       <entry>Will not echo commands to the computer</entry>
      </row>
      <row>
       <entry><command>E1</command></entry>
       <entry>Echo</entry>
       <entry>Will echo commands to the computer (so one can see what one
       types)</entry>
      </row>
      <row>
       <entry><command>H0</command></entry>
       <entry>Hook Status</entry>
       <entry>On hook - Hang up</entry>
      </row>
      <row>
       <entry><command>H1</command></entry>
       <entry>Hook status</entry>
       <entry>Off hook - phone picked up</entry>
      </row>
      <row>
       <entry><command>I0</command> or <command>I</command></entry>
       <entry>Inquiry, Information, or Interrogation</entry>
       <entry>This command is very model specific.  <command>I0</command>
       usually returns a number or code, while higher numbers often provide much
       more useful information.</entry>
      </row>
      <row>
       <entry><command>L0</command> or <command>L</command></entry>
       <entry>Speaker Loudness.  Modems with volume control knobs will not have
       these options.</entry>
       <entry>Off or low volume</entry>
      </row>
      <row>
       <entry><command>L1</command></entry>
       <entry></entry>
       <entry>Low Volume</entry>
      </row>
      <row>
       <entry><command>L2</command></entry>
       <entry></entry>
       <entry>Medium Volume</entry>
      </row>
      <row>
       <entry><command>L3</command></entry>
       <entry></entry>
       <entry>Loud or High Volume</entry>
      </row>
      <row>
       <entry><command>M0</command> or <command>M</command></entry>
       <entry>Speaker off</entry>
       <entry><command>M3</command> is also common, but different on many
       brands</entry>
      </row>
      <row>
       <entry><command>M1</command></entry>
       <entry></entry>
       <entry>Speaker on until remote carrier detected (&ie; until the other
       modem is heard)</entry>
      </row>
      <row>
       <entry><command>M2</command></entry>
       <entry></entry>
       <entry>Speaker is always on (data sounds are heard after CONNECT)</entry>
      </row>
      <row>
       <entry><command>N0</command> or <command>N</command></entry>
       <entry>Handshake Speed</entry>
       <entry>Handshake only at speed in <link linkend="hayes-s37"><varname>S37</varname></link></entry>
      </row>
      <row>
       <entry><command>N1</command></entry>
       <entry></entry>
       <entry>Handshake at highest speed larger than <link linkend="hayes-s37"><varname>S37</varname></link></entry>
      </row>
      <row>
       <entry><command>O0</command> or <command>O</command></entry>
       <entry>Return Online</entry>
       <entry>See also <link linkend="hayes-basic-x1"><command>X1</command></link> as dial tone
       detection may be active.</entry>
      </row>
      <row>
       <entry><command>O1</command></entry>
       <entry></entry>
       <entry>Return Online after an equalizer retrain sequence</entry>
      </row>
      <row>
       <entry><command>Q0</command> or <command>Q1</command></entry>
       <entry>Quiet Mode</entry>
       <entry>Off - Displays result codes, user sees command responses (&eg;
       <computeroutput>OK</computeroutput>)</entry>
      </row>
      <row>
       <entry><command>Q1</command></entry>
       <entry>Quiet Mode</entry>
       <entry>On - Result codes are suppressed, user does not see
       responses.</entry>
      </row>
      <row>
       <entry><command>S<replaceable>n</replaceable>?</command></entry>
       <entry></entry>
       <entry>Query the contents of S-register
       <replaceable>n</replaceable></entry>
      </row>
      <row>
       <entry><command>S<replaceable>n</replaceable>=<replaceable>r</replaceable></command></entry>
       <entry>Store</entry>
       <entry>Store the value of <replaceable>r</replaceable> in S-register
       <replaceable>n</replaceable></entry>
      </row>
      <row>
       <entry><command>V0</command> or <command>V</command></entry>
       <entry>Verbose</entry>
       <entry>Numeric result codes</entry>
      </row>
      <row>
       <entry><command>V1</command></entry>
       <entry></entry>
       <entry>English result codes (&eg;
       <computeroutput>CONNECT</computeroutput>,
       <computeroutput>BUSY</computeroutput>, <computeroutput>NO
       CARRIER</computeroutput> &etc;)</entry>
      </row>
      <row>
       <entry><command>X0</command> or <command>X</command></entry>
       <entry>Smartmodem</entry>
       <entry>Hayes Smartmodem 300 compatible result codes</entry>
      </row>
      <row>
       <entry><anchor id="hayes-basic-x1"/><command>X1</command></entry>
       <entry></entry>
       <entry>Usually adds connection speed to basic result codes (&eg;
       <computeroutput>CONNECT 1200</computeroutput>)</entry>
      </row>
      <row>
       <entry><command>X2</command></entry>
       <entry></entry>
       <entry>Usually adds dial tone detection (preventing blind dial, and
       sometimes preventing <command>AT0</command>)</entry>
      </row>
      <row>
       <entry><command>X3</command></entry>
       <entry></entry>
       <entry>Usually adds busy signal detection</entry>
      </row>
      <row>
       <entry><command>X4</command></entry>
       <entry></entry>
       <entry>Usually adds both busy signal and dial tone detection</entry>
      </row>
      <row>
       <entry><command>Z0</command> or <command>Z</command></entry>
       <entry>Reset</entry>
       <entry>Reset modem to stored configuration. Use <command>Z0</command>,
	<command>Z1</command> &etc; for multiple profiles.  This is the same as
	<command>&amp;F</command> for factory default on modems without
       <acronym>NVRAM</acronym> (non-volatile memory)</entry>
      </row>
     </tbody>
    </tgroup>
   </table>
  </sect1>

  <sect1 id="hayes-extended-commands">
   <title>The Extended Hayes Command Set</title><subtitle>Ampersand Commands</subtitle>

   <table>
    <title>The Extended Hayes Command Set</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Command</entry>
       <entry>Description</entry>
       <entry>Comments</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><command>&amp;B0</command> or <command>&amp;B</command></entry>
       <entry>Retrain Parameters</entry>
       <entry>Disable auto retrain function</entry>
      </row>
      <row>
       <entry><command>&amp;B1</command></entry>
       <entry>Retrain Parameters</entry>
       <entry>Enable auto retrain function</entry>
      </row>
      <row>
       <entry><command>&amp;B2</command></entry>
       <entry>Retrain Parameters</entry>
       <entry>Enable auto retrain, but disconnect if no line improvement over
       the period dictated by <link
       linkend="hayes-s7"><varname>S7</varname></link></entry>
</row>
      <row>
       <entry><command>&amp;C0</command> or <command>&amp;C1</command></entry>
       <entry>Carrier detect</entry>
       <entry>Signal always on</entry>
      </row>
      <row>
       <entry><command>&amp;C1</command></entry>
       <entry>Carrier detect</entry>
       <entry>Indicates remote carrier (usual preferred default)</entry>
      </row>
      <row>
       <entry><command>&amp;D0</command> or <command>&amp;D</command></entry>
       <entry>Data Terminal Ready (<acronym>DTR</acronym>)</entry>
       <entry>Signal ignored (This is modem specific, you must see your manual
       for information on this one!)</entry>
      </row>
      <row>
       <entry><command>&amp;D1</command></entry>
       <entry>Data Terminal Ready (<acronym>DTR</acronym>)</entry>
       <entry>If <acronym>DTR</acronym> goes from On to Off the modem goes into
       command mode (Some modems only)</entry>
      </row>
      <row>
       <entry><command>&amp;D2</command></entry>
       <entry>Data Terminal Ready (<acronym>DTR</acronym>)</entry>
       <entry>Some modems hang up on <acronym>DTR</acronym> On to Off transition
       (This is the usual preferred default)</entry>
      </row>
      <row>
       <entry><command>&amp;D3</command></entry>
       <entry>Data Terminal Ready (<acronym>DTR</acronym>)</entry>
       <entry>Hang up, reset the modem, and return to command mode upon
       <acronym>DTR</acronym></entry>
      </row>
      <row>
       <entry><command>&amp;F0</command> or <command>&amp;F</command></entry>
       <entry>Factory defaults</entry>
       <entry><para>Generic Hayes-compatible defaults. </para><para>This is
       usually a good thing to use in your init string, since the
       <command>&amp;F1</command>-<command>&amp;F3</command> settings can vary
       among modems, and they may actually be the cause of connection
       problems. (Since you never know exactly what Brand X's
       <command>&amp;F2</command> really changes.)</para><para>On the other hand,
       it pays to try out the other options below; many people's problems can be
       solved by replacing a complicated init string with a simple
       <command>&amp;F2</command> or the like. However, if you're building an
       init string, it's best to start with a simple <command>&amp;F</command>,
       and not use the <quote>customized</quote> form of
       defaults.</para></entry>
      </row>
      <row>
       <entry><command>&amp;F1</command></entry>
       <entry>Factory Defaults</entry>
       <entry>Factory Defaults tailored to an IBM <acronym>PC</acronym>
       compatible user</entry>
      </row>
      <row>
       <entry><command>&amp;F2</command></entry>
       <entry>Factory Defaults</entry>
       <entry>Factory defaults for a Mac w/software handshaking</entry>
      </row>
      <row>
       <entry><command>&amp;F3</command></entry>
       <entry>Factory Defaults</entry>
       <entry>Factory defaults for a Mac w/hardware handshaking</entry>
      </row>
      <row>
       <entry><command>&amp;G0</command> or <command>&amp;G</command></entry>
       <entry>Guard tones</entry>
       <entry>Disable guard tones</entry>
      </row>
      <row>
       <entry><command>&amp;K0</command> or <command>&amp;K</command></entry>
       <entry>Local flow control</entry>
       <entry>Disable local flow control</entry>
      </row>
      <row>
       <entry><command>&amp;K1</command></entry>
       <entry>Local flow control</entry>
       <entry>Enable RTS/CTS hardware local flow control</entry>
      </row>
      <row>
       <entry><command>&amp;K2</command></entry>
       <entry>Local flow control</entry>
       <entry>Enable XON/XOFF software local flow control</entry>
      </row>
      <row>
       <entry><command>&amp;K3</command></entry>
       <entry>Local flow control</entry>
       <entry>Enable RTS/CTS hardware local flow control</entry>
      </row>
      <row>
       <entry><command>&amp;K4</command></entry>
       <entry>Local flow control</entry>
       <entry>Enable XON/XOFF software local flow control</entry>
      </row>
      <row>
       <entry><command>&amp;L0</command> or <command>&amp;L</command></entry>
       <entry>Dial mode</entry>
       <entry>Select dial-up mode</entry>
      </row>
      <row>
       <entry><command>&amp;M0</command> or <command>&amp;M</command></entry>
       <entry>Error control mode</entry>
       <entry>Select asynchronous non-<acronym>EC</acronym> mode (the same as
       <command>&amp;Q0</command>)</entry>
      </row>
      <row>
       <entry><command>&amp;P0</command> or <command>&amp;P</command></entry>
       <entry>Pulse dialing ratio</entry>
       <entry>U.S./Canada pulse dialing 39% make / 61% break ratio</entry>
      </row>
      <row>
       <entry><command>&amp;P1</command></entry>
       <entry>Pulse dialing ratio</entry>
       <entry>U.K./Hong Kong pulse dialing 33% make / 67% break ratio</entry>
      </row>
      <row>
       <entry><command>&amp;Q0</command> or <command>&amp;Q</command></entry>
       <entry>Error control mode</entry>
       <entry>Asynchronous non-<acronym>EC</acronym> more. No data
       buffering. <acronym>ASB</acronym> disabled.</entry>
      </row>
      <row>
       <entry><command>&amp;Q5</command></entry>
       <entry>Error control mode</entry>
       <entry>Select V.42 <acronym>EC</acronym> operation (requires flow
       control)</entry>
      </row>
      <row>
       <entry><command>&amp;Q6</command></entry>
       <entry>Error control mode</entry>
       <entry>Asynchronous mode with <acronym>ASB</acronym> (requires flow
       control)</entry>
      </row>
      <row>
       <entry><command>&amp;Q8</command></entry>
       <entry>Error control mode</entry>
       <entry>Select alternate <acronym>EC</acronym> protocol
       (<acronym>MNP</acronym>)</entry>
      </row>
      <row>
       <entry><command>&amp;Q9</command></entry>
       <entry>Error control mode</entry>
       <entry>Conditional data compression: V.42bis = yes, MNP5 = no.</entry>
      </row>
      <row>
       <entry><command>&amp;S0</command> or <command>&amp;S</command></entry>
       <entry><acronym>DSR</acronym> action select</entry>
       <entry>Always on (default)</entry>
      </row>
      <row>
       <entry><command>&amp;S1</command></entry>
       <entry><acronym>DSR</acronym> action select</entry>
       <entry>Follows <acronym>EIA</acronym> specification (Active following
       carrier tone, and until carrier is lost.)</entry>
      </row>
      <row>
       <entry><command>&amp;T0</command> or <command>&amp;T</command></entry>
       <entry>Self test</entry>
       <entry>Model specific self test on some modems</entry>
      </row>
      <row>
       <entry><command>&amp;U0</command> or <command>&amp;U</command></entry>
       <entry>Trellis code modulation</entry>
       <entry>Enable V.32 <acronym>TCM</acronym></entry>
      </row>
      <row>
       <entry><command>&amp;U1</command></entry>
       <entry>Trellis code modulation</entry>
       <entry>Disable V.32 <acronym>TCM</acronym></entry>
      </row>
      <row>
       <entry><command>&amp;V0</command> or <command>&amp;V1</command></entry>
       <entry>View active</entry>
       <entry>(and often stored) configuration profile settings (or
       <command>ATI4</command>)</entry>
      </row>
      <row>
       <entry><command>&amp;W0</command> or <command>&amp;W</command></entry>
       <entry>Store profile</entry>
       <entry>In <acronym>NVRAM</acronym> (<command>&amp;W0</command>,
       <command>&amp;W1</command> etc. for multiple profiles) Some settings
       cannot be stored. These often don't show on <command>&amp;V</command> or
       <command>ATI4</command></entry>
      </row>
      <row>
       <entry><command>&amp;Y0</command> or <command>&amp;Y</command></entry>
       <entry>Select configuration loaded at power-up</entry>
       <entry>Load profile 0 (default)</entry>
      </row>
      <row>
       <entry><command>&amp;Y1</command></entry>
       <entry>Select configuration loaded at power-up</entry>
       <entry>Load profile 1</entry>
      </row>
      <row>
       <entry><command>&amp;Z<replaceable>n</replaceable>=<replaceable>x</replaceable></command></entry>
       <entry>Soft reset and load stored profile number
       <replaceable>n</replaceable></entry>
       <entry>Note that all items after the <command>&amp;Z</command> on the
       command line are ignored</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

  </sect1>

  <sect1 id="hayes-backslash-commands">
   <title>Backslash and Percent Commands</title>

   <table>
    <title>Backslash and Percent Commands</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Command</entry>
       <entry>Description</entry>
       <entry>Comments</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><command>\A0</command> or <command>\A</command></entry>
       <entry>Character maximum <acronym>MNP</acronym> block size</entry>
       <entry>64 character maximum</entry>
      </row>
      <row>
       <entry><command>\A1</command></entry>
       <entry>Character maximum <acronym>MNP</acronym> block size</entry>
       <entry>128 character maximum</entry>
      </row>
      <row>
       <entry><command>\A2</command></entry>
       <entry>Character maximum <acronym>MNP</acronym> block size</entry>
       <entry>192 character maximum</entry>
      </row>
      <row>
       <entry><command>\A3</command></entry>
       <entry>Character maximum <acronym>MNP</acronym> block size</entry>
       <entry>256 character maximum</entry>
      </row>
      <row>
       <entry><command>&percnt;C0</command> or
       <command>&percnt;C</command></entry>
       <entry>Data Compression Enable/Disable</entry>
       <entry>Disabled</entry>
      </row>
      <row>
       <entry><command>&percnt;C1</command></entry>
       <entry>Data Compression Enable/Disable</entry>
       <entry>MNP5 enabled</entry>
      </row>
      <row>
       <entry><command>&percnt;C2</command></entry>
       <entry>Data Compression Enable/Disable</entry>
       <entry>V.42bis (<acronym>BTLZ</acronym>) Enabled</entry>
      </row>
      <row>
       <entry><command>&percnt;C3</command></entry>
       <entry>Data Compression Enable/Disable</entry>
       <entry>MNP5 &amp; V.42bis (<acronym>BTLZ</acronym>) Enabled</entry>
      </row>
      <row>
       <entry><command>&percnt;D0</command> or
       <command>&percnt;D</command></entry>
       <entry>Data compression</entry>
       <entry>512 BLTZ dictionary size</entry>
      </row>
      <row>
       <entry><command>&percnt;D1</command></entry>
       <entry>Data compression</entry>
       <entry>1024 BLTZ dictionary size</entry>
      </row>
     <row>
       <entry><command>&percnt;D2</command></entry>
       <entry>Data compression</entry>
       <entry>2048 BLTZ dictionary size</entry>
      </row>
     <row>
       <entry><command>&percnt;D3</command></entry>
       <entry>Data compression</entry>
       <entry>4096 BLTZ dictionary size</entry>
      </row>
      <row>
       <entry><command>&percnt;E0</command> or
       <command>&percnt;E1</command></entry>
       <entry>Escape method</entry>
       <entry>ESCAPE DISABLED</entry>
      </row>
      <row>
       <entry><command>&percnt;E1</command></entry>
       <entry>Escape method</entry>
       <entry><command>+++AT</command> method (default)</entry>
      </row>
      <row>
       <entry><command>&percnt;E2</command></entry>
       <entry>Escape method</entry>
       <entry><computeroutput>Break</computeroutput> <command>AT</command>
       method</entry>
      </row>
      <row>
       <entry><command>&percnt;E3</command></entry>
       <entry>Escape method</entry>
       <entry>BOTH methods enabled</entry>
      </row>
      <row>
       <entry><command>&percnt;E4</command></entry>
       <entry>Escape method</entry>
       <entry>Disable <computeroutput>OK</computeroutput> to
       <command>+++</command></entry>
      </row>
      <row>
       <entry><command>&percnt;E5</command></entry>
       <entry>Escape method</entry>
       <entry>Enable <computeroutput>OK</computeroutput> to
       <command>+++</command></entry>
      </row>
      <row>
       <entry><command>\J0</command> or <command>\J</command></entry>
       <entry><acronym>DTE</acronym> Auto Rate Adjustment</entry>
       <entry>Disabled</entry>
      </row>
      <row>
       <entry><command>\J1</command></entry>
       <entry><acronym>DTE</acronym> Auto Rate Adjustment</entry>
       <entry><acronym>DTE</acronym> rate is adjusted to match carrier rate.</entry>
      </row>
      <row>
       <entry><command>\N0</command> or <command>\N</command></entry>
       <entry>Connection type</entry>
       <entry>Normal connection (see below for definitions)</entry>
      </row>
      <row>
       <entry><command>\N1</command></entry>
       <entry>Connection type</entry>
       <entry>Direction connection</entry>
      </row>
      <row>
       <entry><command>\N2</command></entry>
       <entry>Connection type</entry>
       <entry><acronym>MNP</acronym> Auto-reliable connection</entry>
      </row>
      <row>
       <entry><command>\N3</command></entry>
       <entry>Connection type</entry>
       <entry>Auto-reliable connection</entry>
      </row>
      <row>
       <entry><command>\N4</command></entry>
       <entry>Connection type</entry>
       <entry>V.42bis reliable link with phase detection</entry>
      </row>
      <row>
       <entry><command>\N5</command></entry>
       <entry>Connection type</entry>
       <entry>V.42bis auto-reliable link with phase detection</entry>
      </row>     
      <row>
       <entry><command>\N6</command></entry>
       <entry>Connection type</entry>
       <entry>V.42 reliable link with phase detection</entry>
      </row>
        <row>
       <entry><command>\N7</command></entry>
       <entry>Connection type</entry>
       <entry>V.42 auto-reliable link with phase detection</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

<para>A direct connection is a simple straight-through connection without any
error connection or data compression.  In this case, the computer-to-modem and
modem-to-modem speeds must be identical.</para>

<para>A normal connection uses flow control (either software or hardware) to
buffer the data being sent or received, so that the modem can transmit data at a
different rate than the computer is actually sending or receiving it. For
example, a computer may send actual data at 57kbps, but using compression, the
modem only actually sends 28.8kbps. This is the mode use by most modems.</para>

<para>A reliable connection is a type of normal connection; if, for some reason,
data compression or error correction cannot be established or maintained, the
connection will hang up. (In essence, such a modem ensures that all connections
are reliable, for it will hang up if the connection isn't.)</para>

<para>Likewise, an auto-reliable connection is virtually the same, except that
the modem will try to renegotiate the connection in order to establish a
reliable connection. Again, this is the mode that most modems use.</para>

</sect1>

<sect1 id="hayes-sregisters">
<title>S-Registers</title>

   <table>
    <title>S Registers</title>
    <tgroup cols="4">
     <thead>
      <row>
       <entry>Register</entry>
       <entry>Range</entry>
       <entry>Default</entry>
       <entry>Function</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><varname>S0</varname></entry>
       <entry>0-255 rings</entry>
       <entry>1-2</entry>
       <entry>Answer on ring number. Don't answer if 0</entry>
      </row>
      <row>
       <entry><varname>S1</varname></entry>
       <entry>0-255 rings</entry>
       <entry>0</entry>
       <entry>if <varname>S0</varname> is greater than
       <returnvalue>0</returnvalue> this register counts the incoming
       rings.</entry>
      </row>
     <row>
       <entry><varname>S2</varname></entry>
       <entry>0-127 <acronym>ASCII</acronym></entry>
       <entry>43 +</entry>
       <entry>Escape to command mode character</entry>
      </row>
     <row>
       <entry><varname>S2</varname></entry>
       <entry>&gt;127</entry>
       <entry></entry>
       <entry>no ESC</entry>
      </row>
     <row>
       <entry><varname>S3</varname></entry>
       <entry>0-127 <acronym>ASCII</acronym></entry>
       <entry>13 CR</entry>
       <entry>Carriage return character</entry>
      </row>
     <row>
       <entry><varname>S4</varname></entry>
       <entry>0-127 <acronym>ASCII</acronym></entry>
       <entry>10 LF</entry>
       <entry>Line feed character</entry>
      </row>
     <row>
       <entry><varname>S5</varname></entry>
       <entry>0-32, 127 <acronym>ASCII</acronym></entry>
       <entry>8 BS</entry>
       <entry>Backspace character</entry>
      </row>
     <row>
       <entry><varname>S6</varname></entry>
       <entry>2-255 seconds</entry>
       <entry>2</entry>
       <entry>Dial tone wait time (blind dialing, see <link
       linkend="hayes-basic-x1">X<replaceable>n</replaceable></link>)</entry>
      </row>
     <row>
       <entry><anchor id="hayes-s7"/><varname>S7</varname></entry>
       <entry>1-255 seconds</entry>
       <entry>30-60</entry>
       <entry>Wait time for remote carrier</entry>
      </row>
     <row>
       <entry><varname>S8</varname></entry>
       <entry>0-255 seconds</entry>
       <entry>2</entry>
       <entry>Comma pause time used in dialing</entry>
      </row>
     <row>
       <entry><varname>S9</varname></entry>
       <entry>1-255 1/10ths second</entry>
       <entry>6</entry>
       <entry>Carrier detect time required for recognition</entry>
      </row>
     <row>
       <entry><varname>S10</varname></entry>
       <entry>1-255 1/10ths second</entry>
       <entry>7-14</entry>
       <entry>Time between loss of carrier and hangup</entry>
      </row>
     <row>
       <entry><varname>S11</varname></entry>
       <entry>50-255 milliseconds</entry>
       <entry>70-95</entry>
       <entry>Duration and spacing of tones when tone dialing</entry>
      </row>
     <row>
       <entry><varname>S12</varname></entry>
       <entry>0-255 1/50th seconds</entry>
       <entry>50</entry>
       <entry>Guard time for pause around <command>+++</command> command
       sequence</entry>
      </row>
     <row>
       <entry><varname>S36</varname></entry>
       <entry><para>Fallback options when error correction link
       fails:</para><itemizedlist>
	 <listitem><para>0 - Disconnect</para>
	 </listitem>
	 <listitem><para>1 - Establish Direct Connection</para>
	 </listitem>
	 <listitem><para>3 - Establish Normal Connection</para>
	 </listitem>
	 <listitem><para>4 - Establish an <acronym>MNP</acronym> connection if
	 possible, else disconnect</para>
	 </listitem>
	 <listitem><para>5 - Establish an <acronym>MNP</acronym> connection if
	 possible, else Direct Connection.</para>
	 </listitem>
	 <listitem><para>7 - Establish an <acronym>MNP</acronym> connection if
	 possible, else Normal connection</para>
	 </listitem>
	</itemizedlist></entry>       
       <entry>7</entry>
       <entry>Negotiation Failure Treatment</entry>
      </row>
      <row>
       <entry><anchor id="hayes-s37"/><varname>S37</varname></entry>
       <entry><itemizedlist>
	 <listitem><para><returnvalue>1</returnvalue> = 300 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>5</returnvalue> = 1200 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>6</returnvalue> = 2400 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>7</returnvalue> = 1200/75 bps (v.23
	 mode)</para>
	 </listitem>
	 <listitem><para><returnvalue>8</returnvalue> = 4800 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>9</returnvalue> = 9600 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>10</returnvalue> = 12000 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>11</returnvalue> = 14400 bps</para>
	 </listitem>
	 <listitem><para><returnvalue>12</returnvalue> = 7200 bps</para>
	 </listitem>
	</itemizedlist></entry>
       <entry>0</entry>
       <entry>Negotiation Speed (Initial handshake)</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

<para>Many modems have dozens, even hundreds, of S registers, but only the first
dozen or so are fairly standard. They are changed with a command like
<command>ATS<replaceable>n</replaceable>=<replaceable>N</replaceable></command>,
and examined with <command>ATS<replaceable>n</replaceable>?</command> (&eg;
<userinput><command>AT</command> <command>S10</command><option>=70</option>
<command>S1?</command></userinput> would tell the modem not to hang up for seven
seconds should it not hear the answering modem, and return the number of times
the phone last rang.)</para>

</sect1>
</appendix>
