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Chapter 3 - AT Commands
3.7 Online Connection Commands
The following commands control the conditions of the online connection.
#An Auto Speed Detection
n = 0–3
Default: #A0
An MT2834MR6 modem can operate as a 33,600 bps, 28,800 bps, 19,200 bps, 14,400 bps,
9600 bps, 4800 bps, 2400 bps, or 300 bps modem. Use the #A command to select the
speed that the modem uses for initial handshaking and subsequent speed selection. The
#A command does not control the originating bps rate of the modem (that is done by the
modem baud rate command $MB), but only the answer mode starting speed.
#A0 causes the modem to start at maximum speed, with fallback to 31,200 to 28,800 to
26,400 to 24,000 to 21,600 to 19,200 to 16,800 to 14,400 to 12,000 to 9600 to 4800 to 2400
to 1200 to 300 bps.
#A1 causes the modem to operate at maximum speed only.
#A2 causes the modem to start at maximum speed, with decremental fallback to 4800 bps.
#A3 causes the modem to start at 2400 bps in V.22bis mode, with fallback to 1200 to 300
bps.
Bn Answer Tone
n = 0 or 1
Default: B0
The B command selects the frequency the modem uses for its answer tone. (The answer
tone is the tone a modem receiving a call transmits to the modem that called it, thus
initiating the handshake between the two modems.) At higher speeds (2400 to 33,600 bps)
there is no conflict, because all protocols use the Bell frequency of 2225 Hz. However, the
ITU-T specification for V.22 has an answer tone frequency of 2100 Hz. This command is
available only in International models.
B0 enables ITU-T frequencies including V.21 (300 bps) and V.23 (1200/75 bps).
B1 enables Bell frequencies including Bell 103 (300 bps).
%DCn AT Command Control
n = 0 or 1
Default: %DC0
The %DC command allows you to disable the modem’s ability to respond to AT commands.
It can be used with autoanswer applications in which no call origination is required, with
some UNIX applications, and with other applications that require you to disable the
modem’s command mode.
%DC0 enables command mode.
%DC1 disables command mode.

Note: If you save %DC1 as part of your default configuration, you can re-enable command mode by typing

AT%DC0 and pressing ENTER in the first ten seconds after you power up the modem.