SPORTSTER 14,400
                                   CHAPTER 4.  INTERFACE CONTROLS
                                                                 
      
      Use the commands explained in this
      chapter to select modem settings at the
      serial port and phone line interfaces.
      The operations affected include flow and
      error control, data rate shifting, and
      data compression.
      
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 serial port rate is the rate at
      which the Sportster 14,400 communicates
      with its attached terminal (at the
      serial port interface).
      Terminal refers to the machine
      (terminal or personal computer) to
      which the Sportster 14,400 is attached.
      
      The connection rate is the rate at
      which the Sportster 14,400 communicates
      with remote modems over a phone link.
      
MODEM INTERFACES

      Figure 4.1 shows which commands control
      modem operations at the serial port and
      phone line interfaces.
      
                       
       Serial Port Interface    Phone Line
        Interface
          Serial port rate:  &Bn   CCITT/Bell
          answer sequence:  Bn
          Flow control   Connection rate:  &Nn
           Transmit Data:  &Hn Error control
           (ARQ):  &Mn
           Received Data:  &Rn, &In Data
           Compression:  &Kn
          DSR override:  &Sn
     Figure 4.1_Interface Control Commands
      Detailed command descriptions are in
      this chapter.  If you're familiar with
      modem operations, you may want to
      review Chapter 3, Quick Start, and then
      use this chapter for reference.
      
CHANGING SETTINGS

      When you change a setting, the modem
      retains it until you do one of the
      following.
          Change the setting again.
          Issue the ATZ command to reset the
           modem.  If DIP switch 7 is UP, the
           modem resets to the defaults stored
           in nonvolatile memory (NVRAM).  If
           DIP switch 7 is DOWN, the modem
           resets to the factory settings
           permanently 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 loads either the
           nonvolatile memory defaults (DIP
           switch 7 UP) or the factory
           settings (DIP switch 7 DOWN).
           
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.
      Throughput is the volume of user
      information transferred per second,
      without Start and Stop bits and other
      overhead information.  You'll obtain
      optimal throughput under the following
      conditions.
      1.Your communications software supports
         a fixed serial port rate higher than
         the connection rate, for example,
         setting your software to lock into
         the 38.4K bps rate, 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.  See Recommended Settings
         and Alternative Settings in Chapter 3
         for quick-start guidelines.
      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 other types of files, such as
         .EXE or .COM binary files.  For a
         comparative table, see the end of
         Appendix A.
      4.File transfer may be slowed down by a
         file-transfer protocol.  Many non-
         text files require a file-transfer
         protocol, but throughput results
         vary.  Certain public domain file-
         transfer protocols, for example, have
         the following effects.
         KermitWith the basic Kermit,
               throughput is severely reduced
               due to short block lengths
               (under 128 bytes) and
               acknowledgment turnaround
               time.  Later enhancements to
               Kermit permit larger data
               block lengths.
         XmodemThroughput may be reduced if
               your version uses short block
               lengths, for example 128
               bytes.  Some versions use
               blocks of 1 Kbyte, which is
               much better, although overhead
               (error-control protocol
               information) still affects
               overall throughput.
         YmodemThis protocol is similar to
               Xmodem with 1-Kbyte block
               lengths, and allows multiple
               files to be sent in one
               transfer.
         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, disable modem error
         control by setting the modem to &M0.
         For better throughput, we recommend
         Ymodem-G, with the modem left at its
         error-control default, &M4.  Ymodem-G
         assumes the modems are handling error
         control:  overhead is minimal, with
         throughput almost equal to that
         obtained with no file-transfer
         protocol.  However, keep in mind that
         Ymodem-G is only useful if the modems
         are using error control.  In
         addition, follow this recommendation
         only if your machine and software
         support hardware flow control.
         On the other hand, the most current
         version of Zmodem can yield even
         greater efficiency.  Leave the modem
         at its error-control default (&M4)
         and data compression default, &K1.
         Zmodem performs the same kind of com
         pression as V.42 bis; it turns off
         its compression if files are already
         compressed.
         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 warning in this
         chapter under Warning on Software
         Flow Control.
         
CCITT/BELL SETTINGS (BN)

      B0 The CCITT answer sequence.  This
          setting is the default and is
          required for the modem to answer
          V.32 and V.32 bis calls.  This
          setting is also used to answer all
          calls from overseas, but does not
          affect U.S./ Canada calls.  It
          takes slightly longer to connect at
          2400 bps and below, however.
      B1 The Bell answer tone, commonly used
          in the U.S. and Canada for
          300/1200/2400 bps connections.  If
          you're only using the modem to call
          or answer 2400-bps or slower
          modems, the Sportster 14,400 will
          make connec-tions more quickly at
          this setting.  Keep in mind,
          however, that a modem set to B1
          can't answer V.32 or V.32 bis
          calls.
          
ERROR CONTROL (&MN)

      The Sportster 14,400 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 modem tries to
      connect without error control.  (The
      process takes much less time than it
      takes to read this paragraph.)
      Error control is possible at rates of
      1200 bps and above.  For background
      information, see Appendix A.  The
      following options are available.
      &M0Normal mode, no error control.
          Because of the nature of phone-line
          channels, this is never recommended
          for calls above 2400 bps unless
          you're using an error-correcting
          file-transfer protocol.
      &M1Reserved.
      &M2Reserved.
      &M3Reserved.
      &M4Normal/ARQ mode.  This setting is
          the default.  If the remote modem
          doesn't recognize the Sportster
          14,400's error-control signals_V.42
          or MNP_the modem operates in Normal
          mode, as though it were set to &M0.
      &M5ARQ mode.  If the remote modem
          doesn't recognize the error-control
          request_V.42 or MNP_the Sportster
          14,400 hangs up.
          
Auto Answer with Error Control

      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 Sportster 14,400 doesn't receive
      those signals and is set to Normal/ARQ
      mode (&M4), it answers the call in Nor
      mal mode (&M0).  If it doesn't receive
      the signals and is set to ARQ mode
      (&M5), it hangs up.
      
ONLINE FALLBACK AND FALL FORWARD

      When online at 14.4K bps under error
      control, V.32 bis modems monitor the
      condition of the phone line.  If they
      sense disturbances that threaten data in
      tegrity, they fall back and retrain
      (resynchronize) at the less vulnerable
      speed of 12,000 bps.
      If conditions remain poor, the modems
      continue to fall back to the next lower
      speed.  As line improvements occur,
      they fall forward to the next higher
      speed, up to the link rate of the call.
      
      Without online fallback, disturbances
      on the line could cause the modems to
      hang up from a high-speed connection.
      The error-control protocols have a
      retry timeout (a maximum of 12 re
      transmissions) after which the modems
      hang up.  Falling back to keeps the
      modems online to complete the call.
      See Online Fallback/Fall Forward in
      Appendix A for more information.
      
FLOW CONTROL OVERVIEW

      This feature controls the flow of data
      into and out of the modem's Transmit and
      Received Data buffers.  Due to varia
      tions in terminal and communications
      software features, the Sportster 14,400
      is shipped with all flow control options
      disabled.
      NOTE:  Flow control of data from the
      terminal to the modem is required under
      the circumstances described under
      Transmit Data Flow Control (&Hn), later
      in this chapter.  Flow control of data
      received over the phone link 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:  3.25 Kbytes.
          Non-ARQ connections:  1.5 Kbytes.
           This allows the use of error-
           control file-transfer protocols
           such as Xmodem and Ymodem without
           flow control.
           
Received Data Buffer Size

      The size of this buffer remains
      constant at 2 Kbytes.
      
Hardware/Software Flow Control Options

      The modem uses either hardware or
      software flow control to manage the
      amount of data stored in the buffers, to
      prevent buffer overflow.
      For the reasons described in the next
      section, we recommend hardware flow
      control.  This type of flow control is
      performed between the modem and terminal
      with RS-232 signaling.
      Software flow control uses the standard
      ASCII Transmit OFF (XOFF) character,
      <Ctrl>-S, and the Transmit ON (XON)
      character, <Ctrl>-Q.
      ASCII definitions are as follows.
      XON  <Ctrl>-Q    ASCII 17 Decimal, 11
         Hex
      XOFF <Ctrl>-S    ASCII 19 Decimal, 13
         Hex
      The ASCII definitions of these
      characters may be user-defined; see
      Registers S22 and S23 in Appendix B.
      That appendix also includes an ASCII
      chart.
      NOTE:  Both your software and terminal
      must support the type of flow control
      you select.  Many terminals, and
      computers that are 100% IBM PC-
      compatible, support hardware flow
      control.  Apple computers, however, do
      not.  If you're using software that is
      included in the quick configuration
      guide, refer to that document to see
      what your software supports.
      Otherwise, see your software
      documentation.
      
Warning on Software Flow Control

      In ordinary operations, the only
      characters the modem recognizes during a
      call are the three pluses (+++) of the
      escape code that returns it to command
      mode.  But when software flow control is
      enabled, the modem or terminal
      (receiving device) also looks for <Ctrl>-
      S or <Ctrl>-Q characters.  If these
      characters occur in a file or as part of
      a protocol, the device reads them as
      XON/ XOFF characters and acts on them.
      For example, XON/XOFF characters occur
      in binary files, and are used by Xmodem-
      type protocols.  They may also come from
      the remote system:  an XON from the
      remote system, after your modem has sent
      an XOFF, can result in buffer overflow.
      We recommend the following settings.
      1.If possible, always use hardware flow
         control.  Software flow control may
         prove satisfactory if you're only
         transferring text files; however, you
         may lose data if XON/ XOFF characters
         occur in the data stream from other
         sources.
      2.If your equipment doesn't support
         hardware flow control and you are
         performing Xmodem-type protocol or bi
         nary file transfers, set the modem to
         the following factory defaults.
         a.&H0, to disable flow control
           entirely.
         b.&B0, so that the serial port rate
           switches to match the connection
           rate.
         c.&N0, to enable the variable
           connection rate.
        We also recommend that you select a
         protocol that performs error
         correction, and turn off the modem's
         error control, that is, set it to
         &M0.
         
TRANSMIT DATA FLOW CONTROL (&HN)

      This type of flow control regulates
      data your terminal transmits to the
      modem for transmission over the phone
      link.  The modem monitors its Transmit
      Data buffer as data comes in.  If the
      buffer approaches 90% capacity, the
      modem signals the terminal to stop
      transmitting.  When the modem has sent
      enough data over the link to half-empty
      the buffer, it signals the terminal 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 rate above 300 bps), with
           or without data compression.  If
           data blocks must be sent again due
           to errors, a continuous stream of
           data from the terminal could
           overflow the modem's buffer.
          The serial port rate is higher than
           the connection rate, for example,
           the serial port rate is 38.4K bps
           and the connection rate is 14.4K
           bps.  Another example is a serial
           port rate of 19.2K bps and a
           connection rate of 2400 bps.
      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.
      &H0Transmit Data flow control disabled.
          Default.
      &H1Hardware flow control.  Recommended
          setting.  Requires that your
          terminal and software support Clear
          to Send (CTS).  The modem drops the
          Clear to Send (CTS) signal to the
          terminal when the modem's buffer
          nears 90% capacity, and starts
          sending CTS again when the buffer
          is about half full.
      &H2Software flow control.  Requires
          that your software support XON/XOFF
          signaling.  The modem sends an XOFF
          to the terminal when its buffer
          nears 90% capacity, and sends an
          XON when the buffer is about half
          full.
      &H3Use both hardware and software flow
          control.  Select this option if
          you're not sure what your terminal
          supports.
      NOTE:  If possible, always use hardware
      flow control.  For a detailed
      explanation, see the Warning on
      Software Flow Control, earlier in this
      chapter.
      
RECEIVED DATA FLOW CONTROL

      Separate commands, &Rn (hardware) and
      &In (software), control the flow of
      Received Data passed by the Sportster
      14,400 to your terminal.  Because
      computers handle incoming data at a much
      faster rate than the modem receives it
      over the phone line, most users won't
      need this type of flow control.
      Both your terminal and software must
      support the type of Received Data flow
      control you select.  Review the quick
      configuration guide and, if necessary,
      your software and computer manuals.  We
      recommend hardware flow control, if
      possible.  See the Flow Control Overview
      and Warning on Software Flow Control
      earlier in this chapter.
Hardware Control (&Rn)

      When your terminal drops its Request to
      Send (RTS) signal, the modem stops
      passing along received data.  The
      terminal sends RTS again when it is
      ready to receive more data.
      Your terminal and software must support
      RTS.  You cannot use this type of flow
      control, however, if your software
      requires a constant RTS signal.
      NOTE:  Use this type of flow control
      only for ARQ (error-control) calls,
      because the V.42 and MNP protocols
      control the flow of data across the
      phone link.  During non-ARQ connections,
      however, there is no way to signal the
      remote modem to stop sending data.  If
      your modem stops passing data to your
      terminal and the remote modem keeps
      sending across the link, the Received
      Data buffer will overflow.
      &R0Reserved.
      &R1The modem ignores Request to Send
          (RTS).  Default.  This setting is
          required if your software does not
          support RTS.
      &R2Hardware flow control of received
          data enabled.  The modem passes
          received data to your terminal 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
      your terminal.  (Hold down the Ctrl key
      and press the letter.  Don't type the
      angle brackets.)  When you send a <Ctrl>-
      Q (XON) command, the modem resumes.
      NOTE:  Because of the risk of data loss,
      all of the options that follow, except
      &I0, are only recommended if your data
      does not include XON/XOFF control
      characters.  For a more detailed ex
      planation, see Warning on Software Flow
      Control earlier in this chapter.
      &I0Disables XON/XOFF flow control of
          received data.  Default.
          Recommended for non-ARQ (Normal
          mode) calls.
      &I1The Sportster 14,400 acts on your
          typed XON/XOFF commands, and passes
          them to the remote computer.  This
          is not recommended for non-ARQ
          calls.  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.  &I2 is
          the preferred option for ARQ calls.
      &I2The Sportster 14,400 acts on your
          XON/XOFF commands, but removes them
          from the data stream instead of
          passing them to the remote
          computer.  This is the recommended
          setting for ARQ mode, only.  It
          ensures that the remote computer
          does not confuse your XON/XOFF char
          acters with those from its attached
          modem.
         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.
          
DATA COMPRESSION (&KN)

      When data compression is enabled, the
      transmitting modem detects redundant
      units of data and recodes them into
      shorter units of fewer bits.  The
      receiving modem decompresses the redun
      dant data units before passing them to
      the receiving computer.
      Compression does not occur unless the
      modems are able to establish an error-
      control connection.  In addition, your
      modem should be set for Transmit Data
      flow control (&H1, 2 or 3), as described
      earlier in this chapter.
      If the modem makes a V.42 connection, it
      negotiates V.42 bis compression.  If
      V.42 bis is not feasible, the connection
      remains under error control, but without
      data compression.
      If the Sportster 14,400 makes an MNP
      connection, it negotiates for MNP Level
      5 (MNP5) data compression.  If the
      remote modem does not have MNP5
      capability, the connection remains under
      MNP error control, but without
      compression.
      &K0Data compression disabled.
      &K1Auto enable/disable.  This is the
          default.  The modem enables
          compression if the serial port rate
          is fixed, &B1, and disables
          compression if the serial port rate
          follows the connection rate, &B0.
          The reason is that compression of
          fers no throughput advantage when
          the serial port and connection
          rates are equal:  compression may
          even degrade throughput.
      &K2Data compression enabled.  Use this
          setting to keep the modem from
          disabling compression.
      &K3Selective data compression.  The
          modem negotiates only for V.42 bis
          compression, and disables MNP Level
          5 (MNP5) compression.  See the
          following note.
      NOTE:  MNP5 compression is not useful
      when transferring files that are already
      compressed, such as the .ZIP files
      downloaded from many Bulletin Boards,
      and 8-bit binary files, which appear to
      the modem to be already compressed.
      MNP5 tends to add data to these files,
      and throughput over the phone link
      degrades.
      V.42 bis compression dynamically detects
      when data is already compressed and
      turns off until it detects that compres
      sion will work to advantage.  The
      special &K3 setting enables the best
      throughput for already-compressed files
      and 8-bit binary 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 serial port and phone line (modem-to-
      modem) interfaces.
      Throughout this manual we recommend
      that, if your software allows, you
      select a fixed rate at the serial port
      interface (&B1) so that the serial port
      rate can remain higher than the rate on
      the phone link.  You'll get greater
      throughput regardless of the connection
      rate.  Just remember to also enable
      Transmit Data flow control, preferably
      hardware (&H1).
      For the phone line interface we
      recommend variable rates (&N0).  This
      enables the modem to switch its
      connection rate to match the rate of a
      remote modem, no matter what the remote
      modem's rate is.  If the connection
      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 connection rate.
      
Software Requirements

      Both variable and fixed serial port
      rates require communications software
      support.  Most communications programs
      support variable rates.  Not all
      software supports a fixed serial port
      rate independent of the connection
      rate, although this support is in
      creasing in program updates.  Check the
      quick configuration guide and/or your
      software manual, especially if your
      software is a later version than the
      version in the quick configuration
      guide.
      
Serial Port Rate Select (&Bn)

      Use this command to select variable or
      fixed rates at the serial port
      interface.  Initially, the modem's rate
      depends on your software setting.  The
      modem detects this rate from the rate at
      which it receives the AT command.  After
      that, the modem's serial port rate
      depends on the modem's &B setting, as
      follows.
      &B0Variable serial port rates.
          Default.  The modem switches its
          serial port rate to follow the
          connection rate it establishes with
          a remote modem.  The software also
          switches its serial port rate to
          match the connection rate.  If your
          software has an Auto Baud Detect
          feature, turn it on.
      &B1Fixed serial port rate.  The modem
          detects its serial port rate from
          your last AT command, and maintains
          that rate regardless of the
          connection rate.  For the greatest
          throughput, set your software to
          57.6K or 38.4K bps first, and then
          set the modem to &B1.
         The serial port rate must be equal
          to or higher than the connection
          rate.  In addition, this setting
          requires Transmit Data flow con
          trol, preferably hardware control
          (&H1), so that the modem's buffer
          doesn't overflow.
         Some programs require that you turn
          off Auto Baud Detect to fix the
          serial port rate.  Others use the
          term Serial Port Lock (yes or no).
      &B2Fixed for ARQ calls/Variable for non-
          ARQ calls, Answer mode only.  When
          the modem goes off hook and answers
          in ARQ mode, it shifts its serial
          port rate up to the rate you've
          written to NVRAM, for example,
          38.4K bps.  If the connection is
          not under error control, the modem
          behaves as if it were set to &B0
          when it answers, and switches its
          serial port 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 Sportster 14,400's advanced
          design.
          
Connection Rate Select (&Nn)

      Use this command to select variable or
      fixed rates at the phone line interface.
      Variable rates allow the modem to con
      nect with a variety of remote modems,
      while fixed rates limit calls to a
      single connection rate.
      &N0Variable rates.  Default.  The
          Sportster 14,400 negotiates with
          the remote modem for the highest
          possible connection rate, depending
          on the capabilities of the remote
          modem.  This is the recommended
          setting.
      &N Fixed rate.  The Sportster 14,400
          only connects if the
      1O6remote modem is operating at the
          same rate.  If not, the Sportster
          14,400 hangs up.
         The fixed rate options are as
          follows.
           &N1           300 bps        &N2
           1200 bps
           &N3           2400 bps       &N4
           4800 bps
           &N5 7200 bps  &N6       9600 bps
           &N7 12K bps   &N8  14.4K bps
         By fixing the connection rate, you
          can filter out calls at other than
          a specific rate, for security or
          other reasons.
          
DATA SET READY (&SN)

      The modem sends your terminal a Data Set
      Ready (DSR) signal.  (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.
      &S0DSR is always ON (override).
          Default.
      &S1In Originate mode, the modem sends
          DSR after dialing, when it detects
          the remote modem's answer tone.  In
          Answer mode, the modem sends DSR
          after it sends an answer tone.
          
BREAK HANDLING (&YN)

      This command allows you to send a break
      to abort data transfer without
      disconnecting from the phone line
      interface.  The following options are
      available.
      &Y0Destructive, no Break transmitted:
          the modem clears the data from its
          transmit buffer (all data is lost)
          but does not transmit the Break to
          the remote modem.
      &Y1Destructive, expedited:  the modem
          clears the buffer and immediately
          sends a Break to the remote modem.
          Default.
      &Y2Nondestructive, expedited:  the
          modem retains data in the transmit
          buffer, but immediately sends a
          Break to the remote modem.
      &Y3Nondestructive, unexpedited (send
          Break in sequence):  the modem
          transmits any buffer data received
          before the Break, sends the Break,
          and then sends any subsequent input
          from the terminal.
      NOTE:  If the call is under MNP5 data
      compression, destructive Breaks cause
      both modems to reset their data compres
      sion tables.  When transmission resumes,
      the modems build new tables, and the
      result is lower than normal throughput.

