3-1
3. AT COMMAND SET
3.1.
AT COMMAND GUIDELINES
The basic AT commands used to control modem operation are defined in this section. These commands are summarized in
section 2. The default values are typical of a fully configured modem supporting all data rates and options.
3.1.1. AT Commands, DTE Adaption
Under AT operation, the serial interfaced modem performs an autobaud/autoparity/autolength function on each AT header entered. The
autolength/autoparity facility can detect 7- or 8-bit characters of even, odd, or no parity with one stop bit.
3.1.2. AT Command Format
A command line is a string of characters sent from a DTE to the modem (DCE) while the modem is in a command mode. A command line
has a prefix, a body, and a terminator. Each command line (with the exception of the A/ command) must begin with the character sequence
AT and must be terminated by a carriage return. Commands entered in upper case or lower case are accepted, but both the A and T must be
of the same case, i.e.,
“
AT
”
= ASCII 065, 084 or
“
at
”
= ASCII 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 CR (ASCII 013) and BS (ASCII 010) in the command
string are ignored. The default terminator is the ASCII <CR> character. Characters that precede the AT prefix are ignored. The command line
interpretation begins upon receipt of the carriage return character.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
(e.g.,
“
&
”
), followed by a decimal parameter. Missing decimal parameters are evaluated as 0. The modem supports the editing of command
lines by recognizing a backspace character. When modem echo is enabled, the modem responds to receipt of a backspace or delete by
echoing a backspace character, a space character, and another backspace. The hex value to be used for the backspace character is
programmable through register S5. Values equal to 0 or greater than 127, or the value which corresponds to the carriage return character,
cannot be used for the backspace character. This editing is not applicable to the AT header of a command. A command line may be aborted
at any time by entering < cntrl-x > (18h).The AT sequence may be followed by any number of commands in sequence, except for commands
such as Z, D, or A. Commands following commands Z, D, or A on the same command line will be ignored. The maximum number of
characters on any command line is 39 (including
“
A
”
and
“
T
”
). If a syntax error is found anywhere in a command line command, the remainder
of the line will be ignored and the ERROR result code will be returned.Most commands entered with parameters out of range will not be
accepted and the ERROR response will be returned to the DTE.Commands will only be accepted by the modem once the previous command
has been fully executed, which is normally indicated by the return of an appropriate result code. Execution of commands D and A, either as a
result of a direct command or a re-execute command, will be aborted if another character is entered before completion of the handshake.
3.1.3. Escape Code Sequence
When the modem has established a connection and has entered on-line data mode, it is possible to break into the data transmission in order
to issue further commands to the modem in an on-line command mode. This is achieved by the DTE sending to the modem a sequence of
three ASCII characters specified by register S2. The default character is
‘
+
’
. The timing of the three characters must comply with specific time
constraints. There is a guard time before the first character (the pre-sequence time), a guard time following the third character (the post-
sequence time), and a guard time-out between the first and second characters and between the second and third characters (the inter-
character time). These times are controlled by the value recorded in register S12.
