                     COURIER HIGH SPEED MODEMS



          CHAPTER 3.  QUICK STARTASYNCHRONOUS
                                    OPERATIONS

ASYNCHRONOUS/SYNCHRONOUS COMMUNICATIONS

      Courier high speed desktop and rack
      modems are capable of asynchronous and
      synchronous communications.  Terminals
      are usually capable of one type of
      communications or the other.  The
      typical personal computer, however, is
      equipped to communicate only asynchron-
      ously.

      Most of the operational information in
      this manual is organized around
      asynchronous communications.  During
      asynchronous communications, the
      computer adds Start and Stop bits to
      each data character before sending it to
      the modem over the serial port, and
      expects received data to be in the same
      format.

      Synchronous data transfer is based on
      synchronized timing between two
      communicating devices, not on data
      format, and is usually required by
      mainframes and other large computers.
      To exchange data synchronously, a
      personal computer requires specialized
      equipment, including the installation of
      a synchronous communications card with a
      synchronous port.  If you will be making
      synchronous connections, you'll find the
      modem easy to use.  Just follow the
      instructions in Chapter 7.


INITIAL MODEM SETTINGS

      This manual covers the operation of
      three Courier models, the Courier HST

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      Dual Standard, Courier V.32 bis, and
      Courier HST.  The modems are factory-set
      for compatibility with their own type of
      modem.

          Dual Standard modemsB0so that
           they connect at high speeds with
           both V.32 bis and HST modems
          V.32 bis modemsB0to connect at
           high speeds with V.32 bis and V.32
           modems
          HST modemsB1to connect at high
           speeds with HST modems

      The above settings do not affect how the
      modems connect at speeds of 2400 bps and
      below.  Leave the modems set to their
      factory defaults, B0 or B1.  The
      exception is when an HST modem is to
      answer overseas calls, in which case you
      should set it to B0.























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      The modem is shipped ready for use
      except for Transmit Data flow control.
      This type of flow control is required
      for calls under error control, and when
      the computer sends data to the modem
      faster than the modem can send it over
      the phone link.
      
      How you set your modem depends on
      whether or not your machine or software
      support flow control, and what type of
      flow control they allow.  Use the
      guidelines that follow.
      


Recommended Settings

      The following modem settings, plus the
      factory defaults, are recommended for
      data reliability and for the best
      throughput.  Check the Settings
      Supplement that came with the modem to
      see if your communications program is
      listed and what it supports.  Check your
      computer or terminal documentation as
      well to see if your machine supports the
      recommended settings.  If not, see
      Alternative Settings, after this
      section.

      Modem  
      Setting  Result
      
      &H1    Hardware flow control:  the modem
             uses Clear to Send (CTS) at the
             RS-232 interface to control the
             data flow from the computer.
             This is the more reliable type of
             flow control.
      &B1    The data rate between the modem
             and the computer (Data Terminal

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             Equipment, or DTE) is fixed; it
             does not switch to match the
             phone link rate when the modem
             connects with a remote modem.

      Software
      Setting  Result
      
      19.2K bps     The rate between the modem
             and the computer (DTE)
      Fixed rate    is always 38.4K or 19.2K
             bps.  This allows calls at the
             maximum speed of 14.4K bps on the
             phone line.  The 38.4K rate
             provides the greatest efficiency
             when a call is under error
             control and data compression.

      NOTE:  If you use an IBM PC or
      compatible and your software supports
      the above settings, we recommend that
      you write them to nonvolatile memory
      with the &W command as defaults.  If





















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      one of these conditions isn't true,
      review the following alternatives and
      guidelines.


Alternative Settings

      1. Your machine or software doesn't
         support Clear to Send, but your
         software supports XON/XOFF signaling:

         &H2   The modem sends the DTE Ctrl-S
               and Ctrl-Q (XOFF/XON)
               characters to control the flow
               of data.  But see Guideline #2
               below.

         &B1   As above, under Recommended
               Settings, providing your
               software supports a fixed DTE
               rate.

      2. Your machine or your software does
         not support Clear to Send, and you're
         using an Xmodem-type file transfer
         protocol:

         &H0   Flow control disabled (factory
               setting).  Do not use software
               flow control (&H2) because the
               protocol control characters
               will be misinterpreted as flow
               control characters, and you
               risk losing data.

         &B0   The DTE rate switches to match
               the connection rate of each
               call.  Required because there
               is no flow control.

         NOTE:  We do not recommend the use of
         Ymodem-G or Zmodem without the use of
         error control and flow control.

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      3. Your system doesn't support either
         Clear to Send or XON/XOFF signaling:

         &H0   Flow control disabled (factory
               setting).

         &B0   The DTE rate switches to match
               the connection rate of each
               call.  Required because there
               is no flow control.

         &M0   Error control disabled;
               recommended because there is no
               flow control.

         NOTE:  We recommend that this
         configuration not be used for calls
         above 2400 bps.


























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      4. Your software does not support a
         fixed rate:

         &B0   Factory setting.  The DTE rate
               switches to match the
               connection rate of each call.
               This does not affect error
               control or flow control.  You
               will not, however, gain the
               throughput efficiency possible
               when the DTE rate can be fixed
               at a higher rate than the link
               rate.  In addition, at the
               factory setting for data
               compression, &K1, the modem
               disables compression when the
               link rate is set to &B0.

      5. Your maximum computer rate is 9600
         bps:

         S34=3 This disables higher-speed V.32
               bis modulation.  The modem
               operates as a V.32 modem, with
               a maximum DTE and link rate of
               9600 bps.  (Does not apply to
               HST modems.)

         &H1   Hardware flow control (CTS), or
               &H2 for software flow control.

         &B1   Fixed DTE rate of 9600 bps,
               recommended for HST modems.

         &M4   Error control factory setting
               required, or &M5.

      6. You are calling a non-MNP modem:

         S27=16     Disable MNP handshaking.
               A non-MNP modem may
               misinterpret the MNP link
               request and prevent a

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               successful connection.  This
               does not apply for remote V.42
               modems with error control
               enabled.

      7. You are transferring compressed
         files:

         &K3   This disables MNP compression,
               which does not work
               successfully with files that
               are already compressed.  If
               V.42 bis compression is
               negotiated, you will gain
               optimal throughput.


DATA FORMAT

      The Courier and the remote modem must
      use the same ten-bit data format.  The
      following table lists the allowable word
      lengths, parity and Stop bits.  If a
      remote modem requires 2 Stop




















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      bits and the format totals 11 bits, the
      second Stop bit is not counted.

           StartWord    Parity     Stop
           BitsLength  (1 Bit)     Bits
           1     7    Even, Odd     1
                     Mark, Space
           1     7 None, Even, Odd, 2
                     Mark, Space     
           1     8       None      1, 2


STORING DEFAULTS IN NONVOLATILE MEMORY

      Write your default configuration to
      nonvolatile memory (NVRAM) with the &W
      command.  The following example sets the
      modem for the current session and also
      stores the settings as defaults.  The
      defaults are loaded from NVRAM when the
      modem is powered on if DIP switch 10 is
      UP.

      Be sure to insert &W last in the string,
      before the Carriage Return.

             AT &B1 &H1 &W <Enter>


DIALING

      Use the following command format.  The
      modem defaults to pulse dialing:  the T
      in the command enforces tone dialing.
      The maximum number of characters in a
      command string is 40.

             ATDT phonenumber <Enter>

      To redial the number, which the modem
      stores in a last-dialed number buffer,
      use the following command:


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             ATDL <Enter>


LINK RATE NEGOTIATION

      During link negotiation (handshaking),
      the modems negotiate the highest
      possible rate, depending on each modem's
      capabilities.  The Courier automatically
      switches rates to match the rate of the
      remote modem, within the following rate
      ranges:  14.4K, 12K, 9600, 7200, 4800,
      2400, 1200 bps and, without error
      control, 300 bps.  For more detailed
      information on handshaking, see Appendix
      A.

ONLINE FALLBACK

      When online at high speeds, the modems
      perform Adaptive Speed Leveling.  They
      monitor the condition of the phone line.
      If they sense disturbances that threaten
      data integrity, they fall back and
      retrain (resynchronize) at the next
      lower speed in their fallback range:
      14.4K, 12K, 9600, 7200, 4800 bps.

      If conditions remain poor, Courier
      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.










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