Common AT Commands

Jun 25,2009 by alperen

Command Function
&F The AT&F command resets most modems to their factory defaults.
&C This command configures the modem-for-modem control (C is for Carrier
Detect). C1 instructs the modem to use CD to reflect the actual connection status.
S0=1 There are a number of S series commands, of which S0 is the first. S0 controls
the number of rings before the modem answers; in this case the modem will
answer on the first ring. A setting of at least 2 is suggested for caller ID installations;
some secure installations use fairly high values—perhaps 10 rings or
more. This is because most “war dialers” (or automatic dialers) assume the
line is not terminated after eight or more rings.
&D The &D command relates to DTR. With a setting of D3, the modem will hang up
the line when the DTR drops. This is the normal configuration.
M0 This command turns off the audio output from the modem. This can provide a
great benefit when you are not troubleshooting; the screeching of the modem
connection sequence can be quite irritating.
L1 The L commands control the volume on the modem speaker. L3 would turn the
volume to maximum. Note that modems with external volume controls, such
as the U.S. Robotics Courier, will also require the physical knob to be turned.
&Q6 The &Q6 command is significant because it results in the DTE speed being
locked. This is discussed in greater detail in Chapter 25, “Using Microsoft
Windows 95/98/2000/XP,” but basically, this means that locking the DTE
speed can improve performance on lower-quality circuits.