AT Entry

Most commands are sent to the modem in the format: ATxxxx&ltCR&gt, where &ltCR>is the character defined to represent a carriage return and xxxx represents an AT command. For example, ATE1 enables offline character echo. The AT can be in upper or lower case but not mixed case. The modem determines the DTE speed (AT Speed) from the A (or a). The DTE speed is noted because it is an important modifier for some commands.

Notes:

1: If the modem determines that the first characters of a new command line are not AT or A/ several times in a row, then the modem may ignore characters from the DTE for up to 2 ms. This is to avoid processing what is likely to be a string of nonsensical characters.

2: Echoed characters and result codes may be sent to the DTE at 500 or 1000 Hz instead of at the full DTE speed.

3: The modem may be able to accept the beginning of the next command line immediately after sending a result code.

In the following chart the rows represent the cumulative elapsed time required to perform the command.

Entry/Edit from  Modem      Execute Send     Modem 
DTE [1]          Pause [2]  Commands [3]     Result Code Delay [4] 

AT commands     <CR>125 ms   execute         result 500 µs 

AT commands     ^X 125 ms    no action       OK 500 µs 

A/ (no commands) 125 ms      execute         result 500 µs

Notes:

1: Only the commands can be edited.

2: The 125 ms pause permits additional characters from the DTE (e.g., &ltLF&gt) that are ignored.

3: The execution of each command includes the modem sending info-text to the DTE before the next command is executed.

4: These 500 microseconds are the maximum delay permitted for the modem firmware to get ready to accept an AT entry from the DTE.

The binary representation of ASCII characters a and A are 1000011P and 10000. The leading A and T of a command line are checked to determine the DTE speed (AT speed) and parity. If the first character of a command line is not an A (or a), it is ignored. If an A is not followed by a T or a/, then the A is ignored.

The leading A and T of a command line are checked to determine the DTE speed (AT speed) and parity. If the first character of a command line is not an A (or a), it is ignored. If an A is not followed by a T or a /, then the A is ignored.

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