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Chapter 3 Modem Operation and Response Codes
This chapter describes modem states, commands and response
codes and explains how to issue commands to the modem.
3.1 Command State and Data State
During operation, your modem is either in command state or data
state. The figure below shows the relationships between the two
states and how to enter and abort from each of them.
In command state, the modem interprets all the characters it
receives as commands. The command state may either be on-line
or off-line. When the modem is holding a connection with a rem ote
modem or when it is in an analog loopback test, it is on-line.
Otherwise, it is off-line.
In data state, the modem is a transparent receiving and sending
device. It assumes everything it receives (except the escape
sequence) as data and conveys them to the local computer or
remote system.
3.2 The Escape Sequence (+++)
The escape sequence instructs the modem to leave (escape) the
data state and enter the command state, without breaking the
connection. It is the only data string that is interpreted as a
command when the modem is in data state.
After switching to command state, the user can issue further
commands, such as changing parameter settings, etc. To resume
data state, simply issue ATO.
The escape command in default is three consecutive “+”
characters. For the modem to distinguish the escape command
from the data, a certain duration of guard time is defined to exist
both prior to and after the three consecutive escape characters.
The escape sequence includes a guard time, three consecutive
escape characters, and a guard time, in that order.
The default guard time is 1 second. It can be changed by
assigning a different value to register S12.
In the same manner, you may change the escape character to a