To transmit or receive data, the modem must be in the on-line state. When placing a call, the modem is put on-line with the dial command. At the remote end, the modem goes on-line when it is instructed to answer a call automatically by setting the S-Register S0 equal to any number greater than 0 and less than 255. You can also perform this function with the ATA command.
After a connection has been established with a remote modem, you can return to command mode without breaking that connection by typing the escape sequence. The escape sequence consists of three characters (+++). It is not necessary to begin this sequence with the attention code, or by pressing [Enter] after typing the escape code.
The modem can be returned to the on-line state after the escape sequence has been issued. To return the modem to the on-line state, issue the ATO command. To return the modem on-line and then initiate an equalizer retrain sequence, use the ATO1 command.
Each command sent to the modem remains in the command buffer until the next command is sent, or until power to the modem is turned off. To repeat the last command issued, type the command A/. The A/ command is not preceded by the AT characters or followed by pressing [Enter]. See the command reference for more information.
Some command require a parameter to completely define them. If a parameter is omitted from a command that requires one, the command will use the default value.
A response is displayed on your screen by the modem, indicating the result after it processes, or tries to process a command. These result codes display the status of the modem, or the progress of a call sequence, and can take the form of either words or digits. The default consists of word responses that are defined by the ATV1 command. To receive digit responses rather than words, use the ATV0 command. To disable responses entirely, use the ATQ1 command.
All control commands to the modem are prefixed with either AT or at and terminated by a carriage return (Enter). Mixed case set ("At" or "aT") is not allowed. The "AT" sequence is called the "ATTENTION COMMAND". The Attention command precedes all other commands except RE-EXECUTE (A/) and the ESCAPE (+++)code.
Several commands which are preceded by AT can be entered in a single line followed by the carriage return character. Spaces can be inserted between commands to increase readability, but will not be stored in the command buffer, the size of which is 255 characters. The Backspace character can be used to erase mistakes but is not saved as part of the contents of the command buffer in terminal applications. Unsupported commands will be logged and an "OK" or "ERROR" will be returned.
Commands will only be accepted by the modem once the previous command has fully executed. A command line may be aborted at any time by entering "CTRL X". The AT sequence may be followed by any number of commands in sequence, except for commands such as Z, D, or A. Commands following Z, D, or A on the same command line will be ignored. The maximum number of characters on any command line is 56 (including A and T).