               CHAPTER 4.  INTERFACE CONTROLS
                       ASYNCHRONOUS OPERATIONS

      

      Use the commands explained in this
      chapter to select modem settings at the
      terminal or computer and phone link
      interfaces.  These settings apply only
      to asynchronous operations.  For syn-
      chronous calls, see Chapter 7.


TERMINOLOGY

      ARQ (automatic repeat request, or
      retransmission) is the term used by
      USRobotics in error control commands and
      response codes.  An ARQ connection
      indicates a call under error control.

      The terminal or computer is referred to
      as the DTE (Data Terminal Equipment).
      The DTE rate is the rate at which your
      DTE and Courier modem communicate, for
      example, 19.2K bps.  The Courier modem
      is referred to as the DCE (Data
      Communications Equipment).

      Figure 4.1 indicates which commands
      control modem operations at the DTE/DCE
      (computer-to-modem) and phone link
      (modem-to-modem) interfaces.















                       Figure 4.1Interface
               Control Commands


      Detailed command descriptions are in
      this chapter.  If you're familiar with
      modem operations, you may want to review
      the Quick-Start instructions in Chapter
      3 instead.
CHANGING SETTINGS

      When you change a setting, the modem
      retains it until you do one of the
      following:

          Change the setting.
          Issue the ATZ command to reset the
           modem.  If DIP switch 10 is UP, the
           modem resets to the defaults stored
           in nonvolatile memory (NVRAM).  If
           DIP switch 10 is DOWN, the modem
           resets to the factory settings
           stored in read-only memory (ROM).
          Issue the AT&F command to reset the
           modem to its factory settings.
          Turn the modem off, and power it on
           again.  The modem then loads either
           the nonvolatile memory defaults
           (DIP switch 10 UP) or the factory
           settings (DIP switch 10 DOWN).


HIGH SPEED MODULATION:  V.32 BIS/HST

      At high speeds, USRobotics modems use
      either V.32 bis modulation or HST
      modulation, unless they are Dual
      Standard modems that have both
      capabilities.  By high speeds we mean
      calls at 16.8K (HST-to-HST only), 14.4K,
      12K, 9600, 7200 or 4800 bps.  (The
      modems use identical, standard
      modulation at 2400 bps and below.)

   Modulation SettingsBn

      The modems are set at the factory as
      follows:

      V.32 bis modulationB0.  V.32 bis modems
      are always set to B0.  Dual Standard
      modems are factory set to B0 so that
      they connect with both V.32 bis modems
      and HST modems.  If a Dual Standard
      modem is set to B1, it can call but
      cannot answer V.32/V.32 bis modems.

      HST modulationB1.  HST modems are
      usually set to B1.  However, they can be
      set to B0 to answer international calls.
      A Dual Standard modem set to B1 can
      successfully call V.32/V.32 bis or HST
      modems but, as mentioned above, cannot
      answer V.32/
      V.32 bis calls.

      Under V.32 bis modulation, data flows at
      the same rate in both directions.  Under
      HST modulation, data flow is
      asymmetrical:
      the high speed in one direction and 450
      bps in the other.  The modems switch the
      fast and slow channels when the volume
      of data flow changes.  Interactive
      applications appear faster under V.32
      modulation, while connection times are
      faster under HST modulation.

      WARNING:  To connect above 9600 bps, the
      DTE rate must be 19.2K, 38.4K or 57.6K
      bps.  If the local computer is limited
      to 9600 bps, V.32 bis modems are limited
      to 9600 bps maximum, that is V.32.
      Disable V.32 bis modulation so that the
      modem does not switch its DTE rate up
      higher than 9600 bps.  Do this by
      setting Register S34 to 3 (S34=3) and
      including that setting in the defaults
      you write to nonvolatile memory, as
      shown in the next chapter.


MODULATION AT 2400 BPS AND BELOW

      The Bn setting is not significant at
      speeds of 2400 bps and lower except for
      answering calls from overseas.  (See
      International Calls at the end of
      Chapter 6.)  The only difference you
      might notice is that the B1 setting
      (Bell answer tone) allows the modems to
      connect slightly faster.  Error control,
      DTE rates, flow control and other
      settings are the same for both HST and
      V.32 bis modems.


THROUGHPUT GUIDELINES

      The following guidelines should help you
      to make the most of your modem's
      advanced performance features.  In many
      instances, experimentation and
      experience will indicate what works best
      for your applications.

      You'll obtain optimal throughput under
      the following conditions.

      1. Your communications software supports
         a fixed DTE rate higher than the link
         rate, for example, setting your
         software to 57.6K, 38.4K or 19.2K bps
         and setting the modem to &B1.  Check
         the quick-configuration guide that
         came with the modem or your software
         manual, and see Data Rate Commands in
         this chapter.

      2. The call is under data compression.
         See Data Compression (&Kn) in this
         chapter.
      3. You're transmitting text files.
         Throughput is higher for text files
         than for other types of files, such
         as .EXE or .COM binary files.  For a
         comparative table, see the end of
         Appendix A.

      4. File transfer is not slowed down by a
         file-transfer protocol.  Many non-
         text files require a file transfer
         protocol, but throughput result vary.
         Certain public domain file transfer
         protocols, for example, have the
         following effects:

         Kermit     Throughput is severely
               reduced due to Kermit's short
               block lengths (under 128 bytes)
               and acknowledgment turn-around
               time.

         Xmodem     Throughput may be reduced
               if your version uses short
               block lengths, for example 128
               bytes.  Some versions use
               blocks of 1K bytes, which is
               much better, although overhead
               (error control protocol
               information) still affects
               overall throughput.

         Ymodem     This protocol is better
               than some because of larger
               block lengths (1K bytes),
               although overhead (error
               control protocol information)
               still affects overall
               throughput.

         The above protocols further reduce
         throughput during error control (ARQ)
         connections.  The accuracy of the
         data is checked twice, by the file
         transfer protocol and the modem.  To
         avoid redundancy, use the above
         protocols only for non-ARQ
         connections, and only at speeds of
         2400 bps and lower.

         For the best throughput, but on ARQ
         connections only and with hardware
         flow control, we recommend the most
         current version of Zmodem.  Overhead
         is minimal with this protocol, with
         throughput almost equal to that
         obtained with no file-transfer
         protocol.  Leave the modem at its &M4
         and &K1 settings for both error
         control and data compression.
         Ymodem-G is another good choice, but
         only on error-controlled calls,
         because Ymodem-G aborts the
         connection if it detects an error.

         WARNING:  If you are using an X, Y or
         Zmodem-type protocol, do not use the
         modem's software flow control.  See
         the more detailed warnings in this
         chapter under Transmit Data Flow
         Control (&Hn) and Received Data
         Software Flow Control (&In).
ERROR CONTROL (&Mn)

      The Courier first attempts a connection
      using V.42 (LAPM) error control and, if
      that doesn't succeed, it attempts an MNP
      connection.  If that doesn't succeed,
      the Courier tries to connect without
      error control.  (The process takes much
      less time than it takes to key in this
      paragraph.)

      The exception to the above is that HST
      modems, at high speeds, do not use V.42
      error control.  They use USRobotics'
      proprietary HST technique, an MNP-type
      of protocol adapted for its high
      speed/low speed asymmetrical modulation.

      Error control is possible at speeds of
      1200 bps and higher.  For more detailed
      information, see Appendix A.  The
      following options are available.

      &M0 Normal mode, no error control.
          Because of the nature of phone line
          channels, this is never recommended
          for calls above 2400 bps.

      &M1 This setting is exclusive of the
          modems' error control and is used
          only for Synchronous mode.  See
          Chapter 7,  Synchronous Operations,
          for instructions.

      &M2 Reserved.

      &M3 Reserved.

      &M4 Normal/ARQ mode.  This setting is
          the default.  If the remote modem
          doesn't recognize the Courier's
          error control signalsV.42, HST or
          MNPthe modem operates in Normal
          mode, as though it were set to &M0.

          NOTE:  V.32 bis modems reverting to
          Normal mode transfer data at high
          speeds without the reliability of
          error control.  To avoid this, V.32
          bis and V.32 modems, local and
          remote, should always be set for
          error control.  HST modems, if
          unable to establish an error control
          connection, drop to 2400 bps.

      &M5 ARQ mode.  If the remote modem
          doesn't recognize the error-control
          requestV.42, HST or MNPthe Courier
          hangs up.
   With Auto Answer

      When set to &M4 or &M5 and a call comes
      in, the modem goes off hook and responds
      to received error control signals.  If
      the modem doesn't receive those signals
      and is set to Normal/
      ARQ mode (&M4), it answers the call in
      Normal mode (&M0).  If it doesn't
      receive the signals and is set to ARQ
      mode (&M5), it hangs up.


FLOW CONTROL OVERVIEW

      This feature controls the flow of data
      into and out of the modem's Transmit and
      Receive data buffers.  Due to variations
      in computer and terminal equipment and
      in software requirements, the Courier is
      shipped with all flow control options
      disabled.

      Users at Hewlett Packard installations
      that use the ENQ/ACK protocol should
      note the HP settings, &I3 and &I4,
      described under Received Data Software
      Control (&In).

      NOTE:  Flow control of data from the DTE
      to the modem is required under the
      circumstances described under Transmit
      Data Flow Control (&Hn).  Received Data
      flow control is not as critical unless,
      for example, you are writing incoming
      files to disk on a very slow computer.

   Transmit Data Buffer Sizes

      The size of the Transmit data buffer
      depends on whether the connection is
      under error control or not, as follows.
          ARQ connections:  3.25K bytes.
          Non-ARQ connections:  1.5K bytes,
           allowing use of error control file
           transfer protocols such as Xmodem
           and Ymodem without flow control.
      If bit 3 of Register S15 is turned on,
      the non-ARQ buffer size is reduced to
      128 bytes, for the convenience of some
      remote users of slower modems.  See S-
      Register Summary, S15, in Appendix B.

   Received Data Buffer Size

      The size of this buffer remains constant
      at 2K bytes.
TRANSMIT DATA FLOW CONTROL (&Hn)

      This type of flow control is for data
      transmitted to the modem by its attached
      DTE, that is, by your computer or
      terminal.  The modem monitors its buffer
      as data comes from the DTE.  If the
      buffer approaches 90% capacity, the
      modem signals the DTE to stop
      transmitting.  When the modem has sent
      enough data over the link to half empty
      the buffer, it signals the DTE to resume
      transmitting.

   When it is Required

      Transmit Data flow control should be
      enabled in the following situations.

          You're using error control (any
           allowable speed above 300 bps),
           with or without data compression.
          The DTE rate is higher than the
           link rate, for example, the DTE is
           sending data to the modem at 38.4K
           or 19.2K bps and the link rate is
           14.4K bps.  Another example is a
           DTE rate of 19.2K bps and a link
           rate of 2400 bps.

      NOTE:  The modem uses either hardware or
      software flow control.  Your software
      and machine must support whichever type
      you select, although we recommend
      hardware flow control, if possible.
      Review the quick-configuration guide
      that came with the modem and, if
      necessary, your software and computer
      manuals.
   Hardware Control

      The modem drops the Clear to Send (CTS)
      signal it's been sending to the DTE when
      the modem's buffer nears 90% capacity.
      It starts sending CTS again when the
      buffer is about half full.

   Software Control

      The modem sends the DTE the standard
      ASCII Transmit OFF (XOFF) character,
      <CTRL-S>, when its buffer nears 90%
      capacity.  The  modem sends the ASCII
      Transmit ON character, <Ctrl>-Q, when
      the buffer is about half full.  ASCII
      definitions are as follows:

      XON      <Ctrl>-Q       (ASCII 17
         Decimal, 11 Hex)
      XOFF     <Ctrl>-S       (ASCII 19
         Decimal, 13 Hex)
      NOTE:  You may have to set your software
      as well to either hardware or software
      flow control.  Some programs also
      require that you turn off the type you
      are not using.

      &H0 Transmit Data flow control disabled.
          Default.

      &H1 Hardware flow control.  Recommended
          setting.  Requires that your DTE and
          software support Clear to Send (CTS)
          at the RS-232 interface.

      &H2 Software flow control.  Requires
          that your software support XON/XOFF
          signaling.  See the guidelines that
          follow.  The ASCII characters may be
          user-defined.  See Registers S22 and
          S23 in Appendix B.  That appendix
          also includes an ASCCI chart.

      &H3 Use both hardware and software flow
          control.  If you are unsure about
          what your equipment supports, select
          this option.  But keep the warning,
          above, in mind about software flow
          control.

      WARNING:  If possible, always use
      hardware flow control.  You may lose
      data if XON/XOFF characters occur in the
      data stream from other sources.  They
      may, for example, come from the remote
      system:  an XON from the remote system,
      after your modem has sent an XOFF, can
      result in buffer overflow.

      Ctrl-S (XOFF) and Ctrl-Q (XON)
      characters also occur in binary files,
      and are used by Xmodem-type protocols.
      You risk having these characters
      misinterpreted as modem flow control
      characters and dropped from the data
      stream.

   Guidelines

      If your terminal or software does not
      support Clear to Send, use of software
      flow control may prove satisfactory if
      you're only transferring text files.

      However, if you're transferring non-text
      (binary) files, or using an Xmodem-type
      protocol, disable flow control entirely
      (&H0).  In addition, be sure the modem
      is set to &B0 and &N0, so that the DTE
      and link rates are equal.
RECEIVED DATA FLOW CONTROL

      Separate commands, &Rn (hardware) and
      &In (software), control the flow of
      Received Data passed by the Courier to
      your DTE.  Because most computers handle
      incoming data at a much faster rate than
      the modem receives it over the phone
      line, you may want to leave this type of
      flow control disabled.

      Your software and machine must support
      whichever type you select, although we
      recommend hardware flow control, if
      possible.  Review the quick-
      configuration guide that came with the
      modem and, if necessary, your software
      and computer manuals.

   Hardware Control (&Rn)

      When the DTE drops its Request to Send
      (RTS) signal, the modem stops passing
      along received data.  The DTE sends RTS
      again when it is ready to receive.

      Your DTE and software must support RTS.
      You cannot use this type of flow con-
      trol, however, if your software requires
      a constant RTS signal.

      WARNING:  Use this type of flow control
      only for ARQ (error-control) calls,
      because under error control the modems
      use flow control across the phone link.
      During non-ARQ connections, there is no
      way to signal the remote modem to stop
      sending data.  If your modem stops
      passing data to the DTE and the remote
      modem keeps sending across the link, the
      Received Data buffer will overflow.
      &R0 Delay Clear to Send Response after
          Request to Send signal (RTS/CTS
          delay).  The delay is required by
          some synchronous mainframes and does
          not apply to asynchronous calls.
          See Chapter 7.

      &R1 The modem ignores RTS.  Default.
          This setting is required if your DTE
          or software does not support RTS.

      &R2 Hardware flow control of received
          data enabled.  The modem sends data
          to the DTE only on receipt of the
          RTS signal.

   Software Control (&In)

      When you send the modem a <Ctrl>-S
      (XOFF) command from the keyboard, the
      modem stops passing received data to the
      DTE.  When you send a <Ctrl>-Q (XON)
      command, the modem resumes.  (Hold down
      the Ctrl key and press the letter.
      Don't type the angle brackets.  The
      brackets indicate a special, named key
      on the keyboard.)

      WARNING:  In ordinary operation, the
      only characters the modem recognizes
      during a call are the three pluses (+++)
      of the escape code.  But when software
      flow control is enabled, the modem also
      looks for <Ctrl>-S or <Ctrl>-Q
      characters.  If these characters occur
      in a file or as part of a protocol, the
      modem reads them as XON/XOFF characters
      and acts on them.  In some cases, the
      modem drops them from the data stream.

      &I0 Disables XON/XOFF flow control of
          received data.  Default.
          Recommended for non-ARQ (Normal
          mode) calls, but see &I5.  The I0
          option provides transparency for all
          characters except the escape code
          sequence (+++), because at this
          setting the modem does not look for
          control characters.

          NOTE:  Because of the risk described
          in the above warning, the settings
          that follow are only recommended for
          users whose data does not include
          XON/XOFF control characters.

      &I1 The Courier acts on your typed
          XON/XOFF commands, and passes them
          to the remote computer.  Use in ARQ
          mode only, but keep in mind that the
          XON/XOFF characters sent to the
          remote computer may interfere with
          XON/XOFF signaling between the
          remote computer and remote modem.
          See &I2.

      &I2 The Courier acts on your XON/XOFF
          commands, but removes them from the
          data stream instead of passing them
          to the remote computer.  This
          ensures that the remote computer
          does not confuse your XON/XOFF
          characters with those from its
          attached modem.  This is the
          recommended setting for ARQ mode.

          However, if the call is not in ARQ
          mode, there is no flow control on
          the phone link.  If you send an XOFF
          to your modem and it stops passing
          data, it has no way to tell the
          remote computer and modem to stop
          sending for a while, and your
          modem's buffer may overflow.  For
          more reliable control in non-ARQ
          mode, see &I5.
      &I3 Hewlett PackardHost Mode.  Applies
          only to modems attached to an HP
          mainframe that uses the ENQ/ACK
          protocol.  See Appendix E.

      &I4 Hewlett PackardTerminal Mode.
          Applies only to modems attached to
          terminals in an HP system that uses
          the ENQ/ACK protocol.  See Appendix
          E.

      &I5 This setting is designed to enable
          flow control on the phone link when
          the connection is not under error
          control.  For this to work for you,
          the remote modem must have &I5
          capability.

          In ARQ mode, a Courier set to &I5
          operates the same as it does when
          set to &I2.  It acts on your
          XON/XOFF commands, but does not pass
          them to the remote system.  The
          error control protocol enables the
          modems to control the flow of data
          on the phone link.

          In non-ARQ mode, a Courier set to
          &I5 operates as though flow control
          were disabled (&I0):  it does not
          look for your typed XON/XOFF
          commands.  However, it looks for
          XON/XOFF characters coming in over
          the phone link.  When the remote
          operator sends XON/XOFF commands,
          the Courier either resumes or stops
          transmitting data over the link and
          drops the characters from the data
          stream.

          If both modems are set to &I5,
          operators at each end can signal the
          remote modem to stop sending,
          thereby controlling the data flow on
          the phone link and preventing their
          own modem's buffer from overflowing.
          At the DTE/DCE interfaces, the
          modems independently control the
          flow of data through their Transmit
          Data (&H) settings.

   Guidelines

      Use of software flow control may prove
      satisfactory if you're only transferring
      text files.  However, if you're
      transferring non-text (binary) files, or
      using an Xmodem-type protocol, disable
      flow control entirely (&R1, &I0).  In
      addition, set the modem to &B0 and &N0,
      so that the DTE and link rates are
      equal.
DATA COMPRESSION (&Kn)

      When data compression is enabled, the
      transmitting modem detects redundant
      bits of data and recodes them into
      shorter units of fewer bits.  The
      receiving modem decompresses the
      redundant data units before passing them
      to the receiving DTE.

      Compression does not occur unless the
      modems are able to establish an error
      control connection.  In addition, the
      modem should be set for Transmit Data
      flow control (&H1, 2 or 3), as described
      earlier in this chapter.

      &K0 Data compression disabled.

      &K1 Auto enable/disable.  This is the
          default.  The modem enables
          compression if the DTE rate is
          fixed, &B1, and disables compression
          if the DTE rate follows the link
          rate, &B0.  The reason is that
          compression offers no throughput
          advantage when the DTE and link
          rates are equal:  compression may
          even degrade throughput.

      &K2 Data compression enabled.  Use this
          setting to keep the modem from
          disabling compression.

      &K3 Selective data compression.  The
          modem negotiates only for V.42 bis
          compression, and disables MNP Level
          5 (MNP5) compression.  Use this
          setting to transfer 8-bit binary
          files, .ARC files, and other files
          that are already compressed.  See
          the note below.
      If the Courier is connecting with a V.42
      or HST modem, it negotiates V.42 bis
      compression.  If V.42 bis is not feasi-
      ble, the connection remains under error
      control, but without data compression.

      If the Courier is connecting with an MNP
      modem, it negotiates for MNP Level 5
      (MNP5) data compression.  If the remote
      modem does not have MNP5, the connection
      remains under MNP error control, but
      without compression.

      NOTE:  MNP5 compression is not useful
      when transferring files that are already
      compressed, such as the .ARC files
      downloaded from many Bulletin Boards and
      8-bit binary files, which appear to the
      modem to be compressed.  MNP5 tends to
      add data to the transmission so that
      throughput over the link degrades.
      V.42 bis compression dynamically detects
      when data is already com-
      pressed and turns off until it detects
      that compression will work to advantage.
      The special &K3 setting enables the best
      throughput for already-compressed files.

      See Throughput Guidelines and Data
      Compression in Appendix A for more
      information, including throughput to
      expect for different kinds of files.


DATA RATE COMMANDS

      The &Bn and &Nn commands allow you to
      select variable or fixed data rates at
      the DTE and link (modem-to-modem) inter-
      faces.  Throughout this manual we
      recommend that, if your software allows,
      you select a fixed rate at the DTE
      interface (&B1) so that it can remain
      higher than the rate on the phone link.

      You'll get greater throughput regardless
      of the call's rate on the phone link.
      Just remember to also enable Transmit
      Data flow control, preferably hardware
      (&H1).

      For the phone link we recommend variable
      rates (&N0).  This enables the modem to
      switch its link rate to match the rate
      of a remote modem, no matter what the
      remote modem's rate is.  If the link
      rate is fixed, for example at 9600 bps
      (&N6), the modem only connects with
      modems operating at that rate.  Of
      course, if your application requires
      connections with modems at only one
      rate, you may wish to set the modem to a
      fixed link rate.
   Software Requirements

      Both variable and fixed rates require
      communications software support.  Most
      communications programs support variable
      rates.  Not all software supports fixed
      rates, although this support is
      increasing in program updates.  Check
      the quick-configuration guide that came
      with your modem and/or your software
      documentation if your software is a
      later version than the version in the
      quick-configuration guide.

      
DTE RATE SELECT (&Bn)

      Use this command to select variable or
      fixed rates at the DTE interface.
      Initially, the modem's DTE interface
      rate depends on your terminal or soft-
      ware setting.  The modem detects this
      rate from the rate at which it receives
      the AT command.  After that, the DTE
      rate depends on the modem's &B setting,
      as follows.

      &B0 Variable rates.  Default.  When the
          modem switches its link rate to
          connect with a modem operating at a
          different rate, it also switches its
          DTE rate.  The software or terminal
          also switches DTE rates to match the
          link rate.  This setting requires
          variable rates on the phone link,
          &N0, so that the Courier can switch
          link rates to match the rate of the
          remote modem.

      &B1 Fixed rate.  The modem always
          communicates with the DTE at the
          rate at which you have set the
          terminal or software, regardless of
          the link rate.  For the greatest
          throughput, set the DTE to 57.6K,
          38.4K or 19.2K bps for high speed
          calls and to at least 9600 bps for
          2400-bps calls.

          This setting is not affected by the
          &N setting.  However, the DTE rate
          must be equal to or higher than the
          link rate.

          In addition, this setting requires
          Transmit Data flow control enabled,
          preferably hardware control (&H1),
          so that the modem's buffer doesn't
          overflow.

      &B2 Fixed for ARQ calls/Variable for
          non-ARQ calls.  Answer mode only.
          When the modem goes off hook and
          connects in ARQ mode, it shifts its
          DTE rate up to a user-specified
          rate, for example, 19.2K bps.  If
          the connection is not under error
          control, the modem behaves as if it
          were set to &B0 and switches its DTE
          rate to match the connection rate of
          each call.

          This option is designed for
          installations such as Bulletin
          Boards that receive calls from a
          wide variety of modems, ranging from
          the very slow to those with the
          Courier's advanced design.

          To implement this feature, set the
          modem to return ARQ CONNECT result
          codes, &A1 or &A2, and to Transmit
          Data flow control, preferably &H1.
          Set your software to the desired
          rate, and send the modem an AT &B2
          [other settings] &W command string.

          The modem stores the rate of the
          command in NVRAM along with the
          settings.  Each time it makes an ARQ
          connection, the modem checks NVRAM
          for the specified DTE rate.

          When sending subsequent
          configurations to NVRAM, be sure
          your software is set to your
          selected DTE rate, so that the
          correct rate is maintained.

Link Rate Select (&Nn)

      Use this command to select variable or
      fixed rates at the link interface.
      Variable rates allow the modem to
      connect with a variety of remote modems.

      &N0 Variable rates.  Default.  The
          Courier negotiates with the remote
          modem for the highest possible link
          rate, depending on the capabilities
          of the remote modem.  This is the
          recommended setting.

      &N1-8    Fixed rate.  The modem only
          connects if the remote modem is
          operating at the same rate.  If not,
          the modem hangs up.

          The link rate must always be lower
          than, or equal to, the DTE rate,
          never higher.  If the link rate is
          lower than the DTE rate, enable
          Transmit Data flow control,
          preferably hardware control (&H1),
          so that the modem's buffer doesn't
          overflow.

          If you wish, you can filter out
          calls at other than a specific
          speed, for security or other
          reasons, by fixing the link rate.

          The options are as follows.  Rates
          above 9600 bps are valid only for
          V.32 bis and HST operations, not
          V.32 operations.

           &N1            300 bps            &
           N2             1200 bps
           &N3            2400 bps           &
           N4             4800 bps
           &N5            7200 bps           &
           N6             9600 bps
           &N7            12.K bps           &
           N8             14.4K bps
           &N9  16.8K bps (HST-to-HST only)
Guidelines For Low-speed Calls

      Calls at 300 bps:  These guidelines
      apply to all Courier high speed modems
      for standard 300-bps calls.  The modem
      must be set to &B1.  If your software
      rate is higher than 300 bps, set the
      modem for Transmit Data flow control,
      &H1 or higher.  Set a variable link rate
      (&N0) or, if you know the remote modem
      is operating at 300 bps, set a fixed
      link rate of 300 bps (&N1).

      NOTE:  Substitute the &B0 setting only
      if the remote modem is operating at 300
      bps and the system it is attached to
      (usually older equipment) operates at
      110-300 bps.  In addition, set your
      software to 300 bps if you plan to use
      the escape code sequence (+++) to hang
      the modem up or bring it back to Command
      mode.  When set to &B0 and operating at
      300 bps, the modem ignores all flow
      control settings.

      Calls at 110 bps:  Set the modem to &B0
      and &N0, and your software to 110 bps.
      The Courier then will only connect with
      a modem that is operating at 110 bps.
      The modem ignores all flow control
      settings.

      If the Courier is set to &B1 for a 110
      bps call, the call will be unsuccessful.
      When the modem is set to &B1, even if
      the DTE transmits to the modem at 110
      bps, the modem transmits the data over
      the phone link at 300 bps using Bell 103
      modulation.


Data Set Ready (&Sn)
      The modem sends the DTE a Data Set Ready
      (DSR) signal via the RS-232 interface.
      (Data Set is industry jargon for modem.)
      Few, if any, commercial communications
      programs require the modem to control
      DSR, &S1.  Leave the modem set for DSR
      overridden, &S0, unless you know that
      your installation requires a different
      setting.

      &S0 DSR is always ON (override).
          Default.

      &S1 In Originate mode, the modem sends
          DSR after it dials, when it detects
          the remote modem's answer tone.  In
          Answer mode, the modem sends DSR
          after it sends an answer tone.

      &S2 This option is for specialized
          equipment such as automatic callback
          units.  On loss of carrier, the
          modem
          sends a pulsed DSR signal with Clear
          to Send (CTS) following Carrier
          Detect (CD).

      &S3 This is the same as &S2, but without
          CTS following CD.


DATA TERMINAL READY (&Dn)

      This command, like DIP switch 1,
      controls Data Terminal Ready (DTR)
      signaling from the DTE to the modem.
      The modem is factory set with DIP switch
      1 UP, for normal operations and the
      override disabled.

      If you want to change the modem's DTR
      operations during a session, you can
      either change the DIP switch position or
      use the &D command.  Whichever method
      you choose, check either the quick-
      configuration guide that came with the
      modem or your communications software
      manual to see which setting is required.

      NOTE:  At power-on and reset, the modem
      operates according to the DIP switch
      setting.  This command is not stored in
      nonvolatile memory as a power-on/reset
      default.

      &D0 DTR override.  The modem operates as
          though DTR is always ON.

      &D1 Reserved.

      &D2 Normal DTR operations.  The terminal
          or computer must send a DTR signal
          for the modem to accept commands.
          Dropping DTR terminates a call.

CARRIER DETECT (&Cn)

      This command, like DIP switch 6,
      controls Carrier Detect (CD) signaling
      from the modem to the DTE.  The modem is
      factory set with DIP switch 6 UP, normal
      CD operations and the override disabled.

      If you want to change the modem's
      Carrier Detect operations during a
      session, you can either change the DIP
      switch position or use the &C command.
      Whichever method you choose, check
      either the quick-configuration guide
      that came with the modem or your
      communications software manual to see
      which setting is required.

      NOTE:  At power-on and reset, the modem
      operates according to the DIP switch
      setting.  This command is not stored in
      nonvolatile memory as a power-on/reset
      default.

      &C0 CD override, CD always ON.

      &C1 Normal CD operations.  The Courier
          sends a CD signal when it connects
          with another modem and drops CD on
          disconnect.


RACK CONTROLLER UNIT ACCESS (%Rn)

      If the Courier modem is part of a
      USRobotics Total Control Modem
      Management System, setting the modem to
      %R1 allows an operator to use the modem
      to call a Rack Controller Unit (RCU) at
      a remote site.

      %R0 Normal operations, RCU access
          disabled.  This is the default.

      %R1 RCU access enabled.


TOUCH-TONE RECOGNITION MODE (%T)

      This command enables the modem, when off
      hook, to detect the Touch-Tone
      frequencies of dialing modems.  %T is
      meant primarily for use with network
      applications, but may also be integrated
      into certain software programs.  For
      example, %T could be used in a security
      program to identify incoming Touch-Tone
      security codes.

      To enable this feature, type ATH1
      <Enter> to force the modem off hook.
      Then type AT%T <Enter>.

      To return the modem to Command mode,
      press any key or drop the DTE's DTR
      signal.  The modem responds OK.
