Command Line
A command line is a string of characters sent from a DTE to the modem, while
the modem is in a command state (idle, online command or analog loopback
self-test state) that starts with AT and ends with <CR>.
Spaces in a command line, after AT, are echoed if command echo is enabled
but are ignored by the modem, unless they are part of a string variable
input. Spaces are retained in the command line buffer. Spaces are sent to
the modem by 01P, respectively, with the left most bit sent first and where
P represents the parity bit. The modem estimates the duration of the start
bit that precedes the leading 1 bit of a or A to determine the DTE speed.
The bits of the A or a and T or t are checked to determine the parity used
by the DTE, if any.
Characters that may precede the AT are ignored. That is, a string such as
ZXDBNMJAAT is interpreted as AT. All characters that follow the AT are treated
as commands.
The maximum number of characters in a command line, excluding the <CR>and
the AT, is 255. If more than 255 characters are entered before the <CR>,
then the modem returns ERROR after the <CR>is entered and does
not execute any of the command line. The command buffer is cleared and the
modem then waits for a new AT command.
The characters of a command line that follow the AT may be corrected by
entering backspace characters to erase them. The backspace character is
defined by S5, and its default value is the ASCII backspace.
The modem determines if a received character is <CR>then <BS>before
any further decoding. This helps to assure <CR>and <BS>are recognized
even if they are defined to be characters that the DTE uses for other purposes.
The Ctrl-X character (ASCII CAN) tells the modem to clear its command line
buffer and ignore the command line. The Ctrl-X is not followed by a <CR>
it is executed immediately, terminating the command process, and produces
the OK result code.
The response of the modem to a change in the DTE Speed during the entry
of a command line is not defined.
Except for character echo during command entry, simultaneous communications
between the DTE and modem may result in undefined behavior. Therefore, entering
commands to the modem during info-text or result codes is not recommended.
The results of command entry which interrupts or which is interrupted by
a RING or NO CARRIER result code is unspecified.
Generally, when the DTE sends a <BS>, the modem responds by sending
<BS><space><BS>back to the DTE. If E0 is in effect, <BS>does
not result in any response screen display in any case. This sequence is
intended to write over the last-typed character on the terminal screen and
leave the cursor positioned there. However, if the DTE sends a string of
<BS>characters, the modem may not be able to respond to all of them
because it attempts to send three characters to the DTE for each received
<BS>. This causes undefined behavior due to simultaneous communication
between the DTE and the modem.
When the DTE sends a <BS>to the modem as if to backspace over the
T of the AT, then the modem responds by sending <BS>T. On a terminal
screen, the AT would not appear to be removed. Also, if the DTE had sent
at, then the screen would depict aT after responding to a <BS>character.
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