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14-13910-A2-GN32-40 September 1998
AT Command Set and S-Registers
Overview
The AT command set provides an alternative method to the modem’s DCP for entering commands
that control the operation and configuration of the 391x Series modems. This chapter discusses
guidelines necessary to operate AT commands as well as listing the modifiers for all AT commands
supported by the 391x Series modems. AT commands are issued from asynchronous DTEs.
Operating Modes
The 391x Series modem has two operating modes, Command mode and Data mode. Before a
modem goes online (establishes a successful connection with a remote modem), it is considered to
be in Command mode; an idle state where you can modify its operating parameters or issue
modem commands. (Any command issued is acknowledged with a response in either words or
digits known as Result Codes. Refer to Table 14-1 for a listing of result codes.)
Once the modems are online, either by answering or originating a call, they automatically switch to
Data mode. Data mode is a state where any entries made from the DTE are considered data and are
transmitted and received between modems. The modems remain in Data mode until the connection
is broken or until they are forced into online Command mode using the escape sequence.
Switching Between Data Mode and Online Command Mode
Sometimes it is necessary to change operating parameters while the modems are online. The
Escape Sequence allows you to toggle the modem between Data mode and online Command mode
while maintaining a connection with the remote modem. This is accomplished using the escape
sequence ( + + + ) to exit Data mode and the O command to return to Data mode.
Escape Sequence and Escape Guard Time
The escape sequence is only issued when the modem is online and in Data mode. The 391x Series
modem uses three consecutive plus ( + ) characters as the escape sequence. (To change this value,
refer to S-Register S2 discussed in Table 14-3 in the S-Register Format section.)
To prevent the modem from interpreting an embedded + + + in data as an escape sequence, the
Escape Guard Time value determines the length of the pause before and after the escape sequence
is issued. The 391x Series modem uses a one-second pause as the Escape Guard Time. (To change
this value, refer to S-Register S12 discussed in Table 14-3 in the S-Register Format section.)
To enter online Command mode while in Data mode, enter the following sequence:
TYPE: + + +
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