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MT3334HD8 User Guide
12 CommPlete Communications Server
Working with AT Commands
The MT3334HD8’s modems are controlled by instructions called
AT commands
, so called because the
attention characters
AT
precede each command or command string (sequence of commands). You can
send commands to the modem from your keyboard while in terminal mode, or you can use communi-
cations software to issue these commands automatically.
A MT3334HD8 modem is in command mode when it is not dialing or online. When it is in command
mode, you have access to a complete communications system that allows you to use several features,
including the basic AT command set described in this chapter. Using the basic AT command set, you can
enter phone numbers for automatic dialing, configure modem options, and monitor telephone activity. In
addition, you can command your modem to perform advanced features such as error correction, data
compression, speed conversion, and more.
This chapter describes the MT3334HD8’s operational modes, and shows you how to use each of the
MT3334HD8’s AT commands. MT3334HD8 commands and responses are compatible with all systems and
with all data communications software using the AT command set.
For easy reference, the MT3334HD8 has a help command that provides you with short, on-screen
descriptions of the modem commands. The help command is explained later in this chapter.
Modes of Operation
The MT3334HD8 operates in two basic functional modes:
command mode
and
online mode
. (There is also
an in-between state,
wait-for-carrier
, in which the modem is out of command mode but not yet online.)
When you power up the modem, it is in command mode, and is ready to accept and respond to commands
from your keyboard or software.
An MT3334HD8 modem enters online mode after it dials, connects with another modem, and detects a
valid carrier signal. If it does not detect a carrier signal within the time frame controlled by register S7, the
modem abandons the call and re-enters command mode.
You can make the modem enter online mode without dialing by typing either the D command or the A
command.
The modem exits online mode if the carrier signal is lost or intentionally dropped. When this happens, the
modem hangs up and re-enters command mode.
By sending certain “escape” characters to the modem while online, you can make it enter command mode
without losing the carrier signal.
For auto answer-only applications, you can disable a modem’s ability to recognize AT commands by using
the %DC1 command. When you do this, the MT3334HD8 ignores all commands, and functions as a non-
intelligent modem. The %DC1 command and other MT3334HD8 commands are explained later in this
chapter.
Command Structure
You can control a wide variety of modem operations and options when the modem is in command mode.
AT commands tell the modem to dial a number, to answer a call, to operate at a certain speed, to use a
certain compression technique, and many other functions. AT commands consist of one or two letters,
which may be preceded by an ampersand (
&
), a dollar character (
$
), a pound character (
#
), a percent
character (
%
), or a greater-than character (
>
). The Q command, for example, determines whether the
modem returns result codes, while the &Q command selects which AT command set the modem uses.