CHAPTER 6.  ASYNCHRONOUS DIALING AND ANSWERING

      

      The information in this chapter applies
      to asynchronous calls only.  For
      synchronous operations, refer to Chapter
      7.


BEFORE CONNECTING

      For two modems to connect and exchange
      data, they must use the same modulation
      standard and, for error control to be
      successful, the same error control
      protocol.  The end-to-end devices (the
      computers, or DTEs) must use the same
      word length, parity, and number of stop
      bit settings.


Modulation/Standards

      The Courier modem adheres to the
      following modulation and signaling
      standards.

       USR-HST   16.8K,
                 14.4K/12K/9600/7200/4800 bps
                 (Dual Standard and HST
                 modems)
       CCITT V.32 bis    14.4K/12K/9600/7200/4
                 800 bps (Dual Standard and
                 V.32 bis modems)
       CCITT V.32   9600/4800 bps (Dual
                 Standard and V.32 bis modems)
       V.22 bis  2400 bps
       Bell 212A/   1200 bps
       CCITT V.22   
       CCITT V.23   1200 bps with 75 bps back
                 channel (British phone
                 system)
       CCITT V.25   Answer sequence for calls
                 originating outside the U.S.
                 and Canada
       Bell 103 or  300 bps
       CCITT V.21   
       CCITT V.42   LAPM error control, 1200
                 bps and higher
       CCITT V.42 bis    Data compression,
                 1200 bps and higher
       MNP       Levels 2, 3 and 4 error
                 control, level 5 data
                 compression, 1200 bps and
                 higher
       CCITT V.54   Analog digital and remote
                 digital loopback testing
 
Setting the Transmission Rate

      Setting your terminal or software to
      57.6K, 38.4K or 19.2K bps ensures that
      the Courier can operate at its highest
      speed.  The modem detects the rate from
      the AT command and operates accordingly.
      It also detects your terminal/software
      settings for the data formats shown
      below in Table 6.1.

      Follow the guidelines in the Quick Start
      in Chapter 3 or the more detailed
      instructions in Chapter 4 before
      selecting either variable or fixed DTE
      rates (&Bn).  Setting the modem to &B1
      and your software to a high DTE rate
      will give you the best throughput.   Set
      the modem for flow control (&Hn), as
      well.

      We recommend leaving the modem at its
      link operation factory setting, &N0,
      unless your application requires that
      you specify a fixed link rate.  &N0
      allows the Courier to automatically
      negotiate with the remote modem for the
      highest possible link rate.


Data Format

      Both modems must use the same 10-bit
      data format, which you must select
      through your terminal or software.  The
      formats allowed are as follows:

          Start Data               Stop
           Bits Bits    Parity     Bits
           
            1    7    Even, Odd,    1
                     Mark, Space        
            1    7  None, Even, Odd     2    
                     Mark, Space
            1    8       None           1,2  

          Table 6.1Allowable Data Formats



Error Control

      For reliable high speed data transfer,
      always set the Courier for error
      control, &M4 (the default) or &M5, for
      calls above 2400 bps.  Most users
      communicating with V.42- or MNP-
      compatible modems will want error
      control at 2400 and 1200 bps as well.
      It is a good idea to find out if the
      remote modem is MNP-compatible.  Some
      public network services, for example,
      are not.  If the remote modem doesn't
      have MNP capability, it may misinterpret
      the error control request and block a
      successful connection.  If you know the
      remote modem doesn't support MNP,
      disable MNP handshaking by setting
      Register S27 to 16.  The modem still
      connects with V.42-compatible modems.


PLACING CALLS

      The commands discussed in this section
      are used in the following operations:

         Dialing     D (0-9 # * , ; ! P T W @
           R), DSn
         Redialing   A/, A>, >
         Canceling dialing    <any key>


Dial (D)

      When the Dial command is issued the
      modem goes off hookthe equivalent of
      picking up your phonethen enters
      Originate mode and dials the number
      sequence that follows.  The modem also
      executes any other commands or options
      included in the command line.

      The command string may include up to 40
      characters.  The modem counts, but
      ignores punctuation characters such as
      parentheses and hyphens.  It does not
      count spaces, the AT prefix or the
      Carriage Return (Enter key) required to
      execute the command.
      The following command example instructs
      the modem to stop the display of
      commands (E0, turn off the local echo),
      turn off the speaker (M0), and dial (D)
      the phone number (1234567) using Touch-
      Tone dialing (T).  The spaces shown are
      ignored by the modem and are only
      included here for readability.

             AT E0 M0 DT 1234567 <Enter>


Cancel Dialing

      To cancel Dial-command execution, press
      <any key>. If you inadvertently hit a
      key on the keyboard while the modem is
      dialing, the call is canceled.  If this
      occurs, type the A/ command explained
      under Redialing later in this chapter.
Dialing Type, Pulse or Tone

      If set to X0 or X1, the modem defaults
      to pulse (rotary) dialing.  To have the
      modem use tone dialing, which includes
      the asterisk (*) and pound sign (#), use
      the T command.   The command may be
      included in the Dial string, as in the
      previous command example, or issued
      separately:

             ATT <Enter>

      The following command resets the modem
      to pulse dialing:

             ATP <Enter>

      You can switch from one dial type to
      another within a dialing sequence.

      NOTE:  The modem remains set to the
      dialing type of the last call until it
      is reset (ATZ command) or it receives a
      different dial-type command.


Adaptive Dialing (X2X7)

      When any of the X2 through X7 options is
      in effect and you do not issue a dialing
      type in the Dial string, the Courier
      uses tone dialing, which is faster than
      the default pulse type.  However, if the
      phone company's central office does not
      have Touch-Tone detection equipment, the
      modem cannot break dial and continues to
      detect the dial tone.  If this occurs,
      the modem automatically reverts to pulse
      dialing.


Pause (,)
      A comma causes a two-second delay in the
      dial sequence.  The following example
      contains four-second delays at several
      points:

             ATDP 9,,7654321,,55555,,1 312
             1234567 <Enter>

      The first four-second pause is to access
      an outside line after dialing 9, but you
      may wish to use the W option, described
      in what follows.  The second pause is to
      make sure the remote system is ready for
      the user's account number, and the
      third, to delay before dialing the long-
      distance number.

      Such pauses, however, may not be
      necessary.  Experiment and use delays
      only as required.
Slash (/)

      A slash (/) can be used in any command
      string to have the modem pause for only
      125 milliseconds.  Some users find it
      helpful to have the shorter delay of a
      series of slashes, rather than the 2-
      second comma pause.


Wait for Another Dial Tone (W)

      This command is useful in situations
      where you must wait for a second dial
      tone before continuing dialing.  For
      example, if you need to dial for an
      outside line, as in the following
      example, the Courier continues dialing
      as soon as it detects the next dial
      tone.

             ATD9W1234567 <Enter>

      NOTE:  This command executes only if
      result code option X3 or greater has
      been issued.  If the modem is set to X2
      or lower, the modem interprets the W as
      a comma (two-second pause).


Dial and Return to Command Mode (;)     

      If your phone is plugged into the modem,
      you can use this option to have the
      modem Auto Dial a telephone rather than
      a modem.  The Courier dials, remains off
      hook and returns the OK message,
      indicating it is in Command mode.

      For example, to have the modem place a
      voice call, enter the Dial command with
      a semicolon:
             ATDT5551234; <Enter>

      When the modem returns the OK result,
      pick up your phone receiver so you can
      talk to the other party, and send the
      command that hangs up the modem:

             ATH <Enter>

      Similarly, you can call a recorded
      weather or other service.  Have the
      modem Dial, listen to the recording over
      the modem's speaker and, when you are
      finished, tell the modem to hang up.


Dialing Letters (+)

      Quotation marks are used to have the
      modem dial abbreviations and acronyms
      used as phone numbers, such as DIAL USR
      (the USRobotics Sales Department's 800
      number).  The option is
      called Quote mode.  Quotation marks are
      inserted at the beginning of the
      alphabetic string:

             ATDT+BBS NEWS <Enter>

      NOTE:  If you are including another
      command after the dial string, use
      closing quotation marks before the
      additional command.


Transferring Calls (!)

      This command applies to modems in
      installations where other modems share
      the phone line.  The modem flashes the
      switch-hook, i.e., goes off hook 0.5
      seconds, on hook for 0.5 seconds, and
      off hook again to dial the specified
      extension.  The following example
      includes instructions to return to
      Command mode (;) and to hang up (H).

             ATDT !1234;H <Enter>


Wait for an Answer (@)   

      Some online services answer the phone
      and return a tape-recorded request for
      information before processing
      transactions.  In such instances, the @
      command can be used in the Dial string
      to tell the modem to detect at least one
      ring, wait for five seconds of silence
      at the other end of the call, and then
      continue to execute the Dial string.

      To use the @ command, set the modem to
      X3, X4 or X7.  If the modem is set to X2
      or lower, the modem returns an ERROR
      message when encountering the @
      character in a command string.  If set
      to X5 or X6, the modem hangs up when it
      detects a voice answer and sends the
      VOICE result code.

      In the next example, the modem is set to
      the X4 result code option and dials a
      banking service.  Each occurrence of @
      in the example indicates a five-second
      wait for silence, that is, for taped
      requests from the bank for a password
      (12345), an account number (6789) and a
      transaction code (2).  The transaction
      code might indicate, for example, a
      request for an account balance.

             ATX4 DT5551234 @ 12345 @ 6789 @ 2
             <Enter>

      If the necessary conditions do not
      occurno rings, or no following five
      seconds of silencethe modem times out
      as it normally
      would (after 60 seconds).  It then sends
      the message NO ANSWER to the screen and
      aborts the command.


Reversing Originate/Answer Frequencies (R)

      This command allows calls to an
      originate-only modem.  It reverses the
      modem's originate/answer frequencies,
      forcing the Courier to dial out at the
      answer frequency.  The command follows
      the Dial command, before or after the
      phone number:

             AT D1234567R <Enter>
             AT DR1234567 <Enter>


Dialing a Stored Number (DSn)

      Chapter 5 includes instructions for
      storing up to four telephone numbers in
      nonvolatile random access memory
      (NVRAM).  To have the modem dial a
      stored number use the DSn command, where
      n is the number's position, 0-3, in
      NVRAM.  In the first of the next two
      examples, the phone number is stored at
      position 0, assumed by the modem if
      there is no numeric parameter:

             ATDS <Enter>
             ATDS3 <Enter>

      You can store a partial dial sequence,
      for example, the phone number of a
      system, to which you might want to add
      different codes for different
      extensions.  Store the phone number so
      that you don't have to type it each
      time.  Use the DSn command to dial the
      number; then have the modem dial
      whichever code you want, as in the
      following example.  The phone number is
      stored at position 1.  Spaces in the
      command string are included here only
      for readability.

             AT DS1 D5678 <Enter>


REDIALING

      The most frequent reason for redialing
      is receipt of a busy signal.  The
      Courier provides three ways to redial,
      as follows.
Dial the Last Dialed Number (DL)

      When sent this command, the modem dials
      the last dialed number, which it has
      stored in a special buffer:

             ATDL

      To display the number stored in the
      last-dialed buffer, use the following
      command:

             ATDL?

      To write the last number dialed to
      NVRAM, use &Zn = L where n is the
      position in NVRAM.  The following
      example stores the last dialed number at
      position 3:

             &Z3 = L

      If a number is already stored at
      position 3, that number is overwritten
      with the last-dialed number.


Re-execute the Last Command (A/)

      The A/ command, which does not take the
      AT prefix or a Carriage Return, redials
      one time:

             A/

      When the modem receives a command, it
      stores the instruction in its command
      buffer until it receives the next AT
      command.  Note that if you've sent the
      modem an additional command since the
      Dial command, A/ re-executes that
      command instead of redialing.

Automated Redialing (>, A>)

      These two commands, while they can be
      used to continuously repeat any command,
      are designed for automated redialing.
      The first (>) is included in the Dial
      command.  The second (A>) is used alone
      to continuously redial the command
      string in the buffer.
Continuous Repeat (>)

      If you know the modem you are calling is
      frequently busy, include the Repeat
      command in the Dial string, as follows:

             AT > DT 1234567 <Enter>
             AT DT 1234567 > <Enter>

      The modem enters Repeat mode, dials the
      number, waits the default 60 seconds for
      a carrier, and hangs up.  Then, after a
      two-second pause, it redials.

      The cycle continues until the modems
      connect or the modem reaches a maximum
      of 10 attempts.  The 10-try limit is
      mandated by the Canadian Department of
      Communications (DOC) to prevent tying up
      local telephone company exchanges with
      unconnected calls.


Continuous Re-execute (A>)

      This command combines the features of
      both the A/ and > commands.  The modem
      enters Repeat mode as described above,
      and redials the Dial string in the
      command buffer.  Like the A/ command, A>
      does not take the AT prefix or a
      Carriage Return:

             A>


Exiting Repeat Mode

      Should you use > or A> with a command
      other than a Dial string, abort the
      cycle by pressing <any key>.
      To abort automated redialing, be sure to
      press <any key> when the result code
      appears, during the pause before the
      modem begins dialing again.  If you
      press <any key> while the modem is
      dialing, that dial attempt is canceled
      but the cycle continues.


ESCAPE CODE OPERATIONS (+++)  

      Once the modem is online to another
      system, the only command it recognizes
      is an escape code of three pluses, which
      forces the
      modem back to Command mode.  Do the
      following when issuing the command:

          Wait one second after sending the
           last item of data
          Type: +++
          Wait one second before typing any
           data

      Do not type the AT prefix or a Carriage
      Return.  The guard time of one second
      before and after the code prevents the
      modem from misinterpreting the
      occurrence of +++ in the transmitted
      data stream.

      If necessary, the character used in the
      escape code or the duration of the guard
      time can be changed by resetting
      Register S2 or S12.  See the S-Register
      Summary in Appendix B.


Modem Response to +++

      The modem returns to Command mode when
      it detects the escape code.  However, it
      keeps the line open or hangs up,
      depending on the setting of DIP switch
      9:

      DIP Switch 9  Response to +++
      
      UP        Modem goes on hook (hangs up),
                sends NO CARRIER result code
                (factory setting)
      DOWN      Modem maintains connection
                (Online-Command mode), sends
                OK result code

      The factory setting (UP) forces an
      automatic disconnect when you issue +++.
      An advantage of this is that you are not
      likely to inadvertently run up an all-
      night phone bill.

      Set DIP switch 9 DOWN if you want the
      modem to respond to +++ by entering
      Online-Command mode, enabling it to
      execute commands and return online.
      (See the O command, below.)

      WARNING:  For unattended modem
      operations:  in rare instances, the
      modem may fail to recognize the +++
      escape code sequence.  If you are
      running the modem under software control
      for unattended operations, we suggest
      you use the surefire method of dropping
      the DTR signal from the DTE for at least
      50 milliseconds, to ensure against
      costly phone charges.  Methods of turn-
      ing the DTR signal offfor example,
      closing the communications portdiffer
      from one computer to another.
RETURNING ONLINE (O)

      If DIP switch 9 is DOWN (on detection of
      the escape code the modem maintains the
      connection), you can issue commands and
      then toggle the modem back online with
      the O command, as in this example:

             AT Q1 O <Enter>


HANGING UP (Hn)

      If DIP switch 9 is DOWN (factory
      setting), the escape code forces the
      modem back to Command mode but leaves
      the line open.  If you want the modem to
      hang up, issue the following command
      once the modem sends the OK result code.

             ATH <Enter>

      If DIP switch 9 is UP, the modem
      automatically hangs up on receipt of the
      escape code.


AUTOMATIC ANSWERING

      The Courier is shipped with DIP switch 5
      DOWN, Auto Answer suppressed.  To set
      the modem to automatically answer incom-
      ing calls, do one of the following:

      1. Before powering on your system, set
         DIP switch 5 UP.  When you turn the
         computer on, the modem answers
         incoming calls on the first ring.

      2. When the modem is on, use software
         control.  The following command
         instructs the modem to answer on the
         first ring.  (You can substitute a
         higher value.  See the S-Register
         summary in Appendix B-5.)

             ATS0=1 <Enter>

      When the modem senses a call coming in,
      it sends the result code RING to your
      screen, goes off hook, and sends the
      remote modem a high-pitched answer tone.
      If there is no Carrier Detect within 60
      seconds, the modem hangs up. If the
      connection is made, the modem sends a
      CONNECT result code.  When the call is
      disconnected by you or the remote user,
      the modem hangs up and returns the NO
      CARRIER code.
Suppressing Auto Answer

      To disable Auto Answer, reverse Steps 1
      or 2 above.  Set DIP switch 5 DOWN
      before powering on the modem or set the
      modem to answer on zero rings:

             ATS0=0 <Enter>


Points to Remember

      1. If the modem is attached to a
         computer, you can set the modem to
         receive calls when you're not at your
         computer.  Load your communications
         software as you normally do, and set
         the modem to Auto Answer.  Also set
         your software's file-save function to
         save incoming messages and/or files.

      2. If you've attached your phone so it
         can be used for conventional calls,
         disable Auto Answer when you are not
         expecting incoming data calls.
         Otherwise, your modem may answer the
         phone before you do, greeting a voice
         caller with a high-pitched,
         irritating answer tone.


INTERNATIONAL CALLS

      There are three commands that apply to
      international calls at 1200 bps and
      above.

      NOTE:  To call or answer overseas modems
      at 300 bps, set the modem to CCITT V.21
      mode.  See Register S27 in Appendix B.


Handshaking Options (Bn)
      This command is used primarily to select
      HST or V.32 bis operation.  However, to
      answer international calls, use the B0
      setting, as described below.

      B0  This setting is required for
          V.32/V.32 bis operation.  It also
          selects the CCITT V.25 answer tone
          used outside of North America.  This
          answer tone is incorporated into the
          V.32/V.32 bis recommendations.

          This setting is the default for the
          Courier V.32 bis and Dual Standard
          modems.  All you need to do is make
          sure that the modem is set for Auto
          Answer (ATS0=1).
          Set Courier HST modems to B0 to an-
          swer HST-mode international calls.
          The B0 setting does not affect the
          modem's ability to call or answer
          domestic modems.  It takes slightly
          longer, however, to connect.

      B1  This setting is required for HST
          operation.  It also selects the Bell
          2225 Hz answer tone used in the U.S.
          and Canada.  This is the default
          setting for the Courier HST.


Guard Tone (&Gn)

      This setting applies only to overseas
      calls at 2400 or 1200 bps.  British
      phone switching systems require the
      modem to send an 1800 Hz guard tone
      after it sends an answer tone.  Some
      other European phone networks require a
      550 Hz guard tone.  Guard tones are not
      used in the United States or Canada.

      &G0 No guard tone, U.S./Canada.
          Default.

      &G1 550 Hz guard tone follows answer
          tone.

      &G2 1800 Hz guard tone follows answer
          tone, United Kingdom and some
          Commonwealth countries.  Requires B0
          setting.


Pulse Dial Make/Break Ratio (&Pn)

      This command sets the ratio of the off-
      hook/on-hook (make/
      break) interval for pulse dialing.  The
      default sets the modem for use in North
      America.  The ratio must be changed if
      the modem is used in the United Kingdom
      and some Commonwealth countries.

      &P0 Make/break ratio, U.S./Canada:
          39%/61%.  Default.

      &P1 Make/break ratio, United Kingdom,
          some Commonwealth countries:
          33%/67%.
