USRobotics V.34 - Manual (Page 188)

COURIER HIGH SPEED MODEMS
G-6 Additional Operations
Software Commands

ATD, ATA
If your voice/data switch is set for a function other than
voice/data operations and you don't wish to change it, use AT
commands.
1. Call the other user to establish the bit rate, parity, word
length and number of Stop bits the other person's modem
accepts. Determine which modem will originate and which
will answer.
2. If you are to originate the connection, type the following
command:
ATD
NOTE: Be sure the modem is not set to X2, X4, X6 or X7, or
it will return the NO DIAL TONE result code and hang up.
3. The other party must then have the remote modem go off
hook in Answer mode. The following command is used to
do this:
ATA
4. If the remote modem also has handset exclusion, leave both
phones off hook in case you wish to switch back again to
voice after your data transfer. If the remote modem doesn't
have handset exclusion, switching back to voice may not be
successful. If you don't want to switch back to voice later,
hang up both phones as soon as the modems go off hook.