          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
      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.
      
      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 recommended settings will
      yield the best data reliability and
      throughput.  Check the quick-
      configuration guide that came with the
      modem to see if your communications
      program is listed and whether it
      supports both a fixed data rate between
      the computer and modem (setting the
      modem to &B1) and hardware flow control
      (setting the modem to &H1).  If a
      program only supports software flow
      control (XON/XOFF characters), use the
      Alternative Settings after this section.

      If necessary, review your
      computer/terminal and software
      documentation as well to see if your
      system supports the recommended
      settings.  If either your software or
      machine do not, use the Alternative
      Settings after this section.

      1. Set your software to the highest rate
         your equipment and software support:
         19.2K, 38.4K, or 57.6K bits per
         second.  This sets a high serial port
         rate, that is, the rate of data
         transmission between the computer and
         the modem.

      2. Set the software to a fixed serial
         port rate so that the computer and
         modem maintain the high rate,
         regardless of the rate on the phone
         line.  Some programs call this
         locking the serial port.

      3. Set the software for hardware flow
         control.  This may be called RTS/CTS.
         The modem uses Clear to Send (CTS) to
         control the data flow from the
         computer.  This is the most efficient
         and reliable form of flow control.

      4. Set the modem to a fixed serial port
         rate (&B1) and hardware flow control
         (&H1).  Have your software put the
         computer in Terminal mode, as shown
         in Step 3 of Chapter 2, Testing the
         Installation, and type the following
         command.  (<Enter> indicates pressing
         the Enter key.)

             AT &B1 &H1 <Enter>

         To have the modem always use these
         settings as defaults, type &W at the
         end of the command string, before you
         press the Enter key.  The &W command
         causes the modem to write the
         settings to nonvolatile memory.


Alternative Settings

      Use the above command format to set the
      modem.
      1. Your machine or software doesn't
         support hardware flow control, but
         your software supports XON/XOFF
         signaling:

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

         &B1   As in Step 2 under Recommended
               Settings, if your software
               supports a fixed serial port
               rate.

      2. Your machine or your software does
         not support hardware flow control,
         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 serial port rate switches
               to match the connection rate of
               each call.  Required because
               there is no flow control.

      3. Your system doesn't support either
         Clear to Send or XON/XOFF signaling.
         In this case, we recommend that the
         following configuration not be used
         for calls above 2400 bps.

         &H0   Flow control disabled (factory
               setting).
         &B0   The serial port 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.
      4. Your software does not support a
         fixed rate:

         &B0   Factory setting.  The serial
               port 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
               with a serial port rate fixed
               higher than the link rate.  In
               addition, at the factory
               setting for data compression,
               &K1, the modem disables com-
               pression 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 serial port 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 serial port 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
               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
      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 pressing the Enter key.

             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:
             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:  16.8K (HST-to-HST only), 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.
