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DTE Command Lines
A command line is a string of characters sent from a DTE to the modem (DCE) while the modem is
in a command state. 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 008) 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.,
&
or
%
or
\
or
)
followed by a decimal parameter. Missing decimal parameters are evaluated as 0.
The modem supports the editing of command lines by recognising 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 < ctrl-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.
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.
Summary of Contents for TD-22 AC
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