



                                                                COURIER HST

                                                                  CHAPTER 3
                                                     OPERATIONS AT A GLANCE


          Here are some brief guidelines for immediate operation of the
          modem.

          DATA FORMAT

          Ten-bit data units:  check the table on page 6-1 or page F-1.

          ERROR CONTROL

          Both modems must be set to error control mode for error detection
          and retransmission to occur.  Always use error control for 9600-
          bps communications.

          We use the term ARQ (automatic repeat request) for error control.
          The three ARQ settings are as follows:

          &M0  

          Normal Mode.  No error control.

          &M4  

          Normal/ARQ Mode -- Default.  The Courier attempts an ARQ
          connection; if the signal isn't recognized, the modem continues
          in Normal Mode (&M0).

          &M5  

          ARQ Mode.  The Courier attempts an ARQ connection; if the signal
          isn't recognized, the modem hangs up.

          DATA COMPRESSION

          The modem is set at the factory to negotiate for data
          compression.  Leave the modem set to its default unless you are
          transferring files that are already compressed, in which case you
          should disable data compression by setting the modem to &K0.  For
          more information, see the throughput guidelines at the beginning
          of Chapter 4 and the &K command description in that chapter.

          DIALING

          Use the following settings/commands to call the specified type of
          modem at the indicated speeds.

          HST-Compatible, 9600 bps

               Terminal/software:  19.2k (preferable) or 9600 bps

               Type:  AT &H3 D phone number <Enter>

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                                                                COURIER HST

          The command includes the setting for hardware/software flow
          control (&H3).  See the flow control guidelines that follow.

          If the modem isn't set for error control, include &M4 or &M5 in
          the command line.

          MNP-Compatible, 2400 or 1200 bps

               Terminal/software:  19.2k, 9600, 2400 or 1200 bps

               Type:     AT &H3 D phone number <Enter>

          The command includes the setting for hardware/software flow
          control (&H3).  See the flow control guidelines that follow.

          Non-MNP Compatible, 2400, 1200 or 300 bps

          See the flow control guidelines that follow.

               Terminal/software:  2400, 1200 or 300 bps

               Type:  AT &M0 D phone number <Enter>

          NOTE:  &M0 suppresses the MNP signals that may be misinterpreted
          by the remote system and prevent a successful connection.

          FLOW CONTROL

          Hardware and/or software flow control can be used for transmitted
          and received data.   We recommend hardware Transmit data flow
          control (&H1) since this setting doesn't affect the data stream.
          This is especially important if you're transmitting binary data.

          Transmit Data flow control (&H) is required in the following
          situations.  Received Data flow control is optional.

               o    You're using error control (9600/2400/1200 bps), with
                    or without data compression.  Flow control prevents
                    buffer overflow in the event that line disturbances
                    cause frequent retransmissions.

               o    The rate at the local terminal interface is higher than
                    the link rate.

               o    The volume of data transfer is high.

          Flow control commands are explained in Chapter 4.

          MAXIMUM THROUGHPUT

          Use these settings for both Originate and Answer Modes.  The
          default data compression setting, &K1, is assumed.

               Terminal/software:  19.2k bps

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                                                                COURIER HST

                           Modem:  Fixed terminal interface rate (&B1)
                                   Variable link rate (&N0)
                                   Transmit Data flow control (&H1
                                   preferred, &H2 or &H3)

          HIGH-SPEED CHANNEL TURNAROUND

          As described in Chapter 1, HST modems use asymmetrical
          modulation.  The modems allocate the high-speed channel on demand
          so that the modem with the greatest amount of data in its buffer
          transmits at 9600 bps.

          Turnaround of the channel is automatic and requires no user
          intervention.  If the remote modem has a 300-bps back channel,
          the Courier automatically drops its back channel rate from 450 to
          300 bps.

          LINK NEGOTIATION

          When a Courier HST calls an HST-compatible modem and both are
          operating at 9600 bps, the modems negotiate the link connection
          (shake hands) at 2400 bps and automatically resume operation at
          9600 bps.  The lower handshaking rate maintains compatibility
          with the existing telephone network as well as existing hardware
          and software.

          It's possible for two HST-compatible modems to connect directly
          at 9600, without 2400-bps handshaking, but both modems must be
          set as follows:

               Terminal/software:  9600 or 19.2k bps
                           Modem:  Fixed link rate of 9600 bps (&N6)
                                   Error control, either &M4 or &M5
                                   Transmit Data flow control, preferably   
                                   hardware, &H1

          DATA RATE DEFAULTS

              Terminal interface:  &B0, detect rate from the AT command,
                                   then follow the connection rate.
                  Link interface:  &N0, negotiate the highest possible      
                                   rate with the remote modem in both       
                                   Originate and Answer Modes.

          The terminal interface and link rates may be set to fixed rates
          using &B1 or &N1-6, respectively.

          NONVOLATILE MEMORY (NRAM)

          The modem is factory set (DIP switch 10 OFF) to load the settings
          stored in NRAM on power up.  Write your own configuration
          defaults to NRAM with the &W command.  To review the NRAM



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                                                                COURIER HST

          settings, refer to Appendix B-4 or send the modem the I5 option
          of the Inquiry command:

                    ATI5 <Enter>

          PHONE NUMBER STORAGE

          Use &Zn=s to store four frequently called phone numbers in NRAM
          (nonvolatile memory); n = positions 0 through 3, s = the number-
          string.  The string may contain a maximum of 36 characters and
          any Dial command options, but no other operational settings.

          Example:  AT&Z=9,,1 312 5551234 <Enter> (Store at position 0)

          Example:  AT&Z1=5551234 <Enter>         (Store at position 1)

          DIALING STORED NUMBERS

          To dial, issue the DSn command, where n indicates the position of
          the number in NRAM.

          Example:  ATDS1 <Enter>  (Dial number at position 1)

          The first command in the following sequence stores the phone
          number and access code for a long distance service.  The second
          command dials the stored number/access code and continues dialing
          the long distance phone number.

                    AT&Z2 = 5551234,,9876,, <Enter>
                    ATDS2 D 1 312 5556789 <Enter>

          BUSY ANSWER

          Use A> instead of A/ and the modem redials up to 10 tries instead
          of once.  Neither A> nor A/ takes the AT prefix or a Carriage
          Return.

          Insert the Repeat Mode command (>) before or after the Dial
          string and the modem automatically redials up to 10 tries:

                    AT>D5551234 <Enter>
                    ATD5551234> <Enter>

          You may include the Repeat command when you store a phone number-
          string in NRAM:

                    AT&Z2=5551234> <Enter>

          INACTIVITY TIMER

          Set the inactivity timer (S-Register 19) if you suspect that a
          connection may be inadvertently left open without data transfer.



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