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AT Command Reference Manual
2. COMMAND SYNTAX
2.1 DTE/DCE INTERCHANGE CIRCUITS
Communication between the DTE and modem is half duplex (i.e., only one entity 'talks' at a time).
2.2 COMMAND SYNTAX AND GUIDELINES
2.2.1 DTE Commands
The ISO 646 character set (CCITT T.50 International Alphabet 5, American Standard Code for Information Interchange) is
used for the issuance of commands and responses. Only the low-order 7 bits of each character are used for commands or
parameters; the high-order bit is ignored. Upper case characters are equivalent to lower case characters.
2.2.2 DTE Command Lines
A command line is a string of characters sent from a DTE to the DCE while the DCE is in a command state. Command lines
have a prefix, a body, and a terminator. The prefix consists of the ASCII characters "AT" (065, 084) or "at" (097, 116). The
body is a string of commands restricted to printable ASCII characters (032 - 126). Space characters (ASCII 032) and control
characters other than carriage return <CR> (default value = ASCII 013 = 0Dh, see register S3), backspace <BS> (default
value = ASCII 008 = 08h, see register S5), and cancel <cntrl-x> (ASCII 024 = 18h) in the command string are ignored. The
default terminator is the <CR> character. Characters that precede the AT prefix are ignored.
2.3 AT COMMAND GUIDELINES
Modem operation is controlled by generic AT commands. These AT commands may be basic AT (i.e., commands preceded
by AT, AT&, AT%, AT*, AT\, AT), AT-, or AT#), S-Register (e.g., S6=n), Fax class 1 (e.g., +FTM), Fax class 2 (e.g.,
+FDCS:), or voice (e.g., #VBS) commands. The command syntax and operation guidelines governing each of these
command categories are described in subsequent sections.
2.3.1 Basic Command Syntax
Characters within the command line are parsed as commands with associated parameter values. The basic commands
consist of single ASCII characters, or single characters preceded by a prefix character, followed by a decimal parameter
(e.g., "&D1"). Missing decimal parameters are evaluated as 0.
2.3.2 Extended Command Syntax
The facsimile commands use extended syntax. They are preceded by the "+F" characters, and they are terminated by the
semicolon ":" character (ASCII 059) or by the <CR> that terminates the command line.
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