User manual

2-4 AT COMMANDS: DATA MODEM
AT - Attention Code
The AT (Attention) Code, which introduces each command line, can be entered in
upper-case or lower-case characters.
Several commands (separated by spaces if desired) can be positioned one after
another, in one line. A command line must end with the ASCII character which is
stored in the S3 or S4 Register (or with both together). The standard value for S3 is
Carriage Return (<CR>=13 decimal) and Line Feed (<LF>=10 decimal) for S4.
A command line without <CR>,<LF> will remain in the command buffer until
<CR>,<LF> is entered, or until the action is broken off with <Cntrl-X>. Once
<CR>,<LF> has been received, the modem carries out the commands following
the AT and answers with an appropriate modem message.
The maximum length of a command line is 40 characters. If the capacity of the
command buffer is exceeded, the modem issues an Error message.
➮ The AT code enables the modem to recognize the speed, parity and character-
length of the communication programme
The ESC Sequence
If the modem has established a data connection, you can enter more commands at
any time, without breaking off the connection. This is achieved by sending three
ASCII characters (S2 Register) from the computer to the modem. The standard
setting is the “+” character. In order for this to be interpreted as the ESC sequence,
certain time limits must be observed in entering the plus-characters. Before the
first and after the last character, a Guard time is required (the standard is 1 second)
and the individual characters must not be separated from each other by longer than
this time period.
A - Answer Mode
When the A command is entered, the modem switches to the “Off-Hook” condi-
tion in the Answer mode and takes over control of the telephone line connected. In
this way, the modem transmits in the upper frequency band (upper channel)
If further commands are entered after the A command before a connection has
been made, the modem interrupts the establishment of a connection, switches to
command mode and issues a NO CARRIER message.
If no carrier is received from the remote station after the waiting time which is
set in the S7 Register, the modem responds with a NO CARRIER message and re-
turns to command mode. If the modem does receive the carrier signal, it issues a
CONNECT message and switches to Data mode.
➮ If the handset is not replaced after the data transmission has been completed,
the connection remains made and you will continue to pay for this !