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AT Commands
5-2 Modem 3600
Offline Command Mode
In offline command mode (generally referred to as command mode), the
modem communicates with the computer or terminal. Commands can
be entered separately or in strings. There is no data communication link
established in this mode.
Online Command Mode
This mode is entered from the data mode after the escape command has
been entered. The escape command is performed by entering the escape
character (+ is the default) three times. The data communication link
remains established but data transmission is suspended. The modem
then accepts commands as it does in offline command mode.
Data Mode
The modem goes to data mode (online) after it acknowledges the proper
signal and successfully connects with a compatible modem. In data
mode, the modem sends and receives data, but does not accept or
execute command instructions.
Example: The modem is in the command state. The D command and
phone number are used to dial a remote modem. The local modem waits
to receive an answer back tone from the remote modem. When the local
modem receives the carrier, it leaves the command state and goes online
in the data mode. At this time, both modems are using the telephone line
and a communication link is established.
Sending Commands to the Modem
When the computer, modem, and monitor are on, an instruction can be
sent to the modem telling it what function or activity to perform. The
instruction, called a command statement, command string, or command,
is typed using the computer/terminal keyboard. The command statement
temporarily resides in a section of memory called the command buffer.
Each command statement is made up of characters, numbers, and
keyboard symbols such as the & and % signs. Commands must be
written in a specific form so the modem recognizes and follows the
instruction.