Hayes commands start with "AT", because it enables baud, parity, and wordlength detection. Has nothing to do with "ATtention". For it to work properly, there has to be a transition at the trailing edge of the start bit. So the low order bit of the first character sent (the one used to measure the start bit width) has to be a 1.

 A	 T
0100001	01010100

Notice that the A has only two one bits and they are 7 bits apart (not all serial system support 8 bit transfers) and the T has an alternateing bit pattern with the last bit set off.

Conveniently, the US ASCII character carriage return also has the low order bit set.

00001101

See also:

• http://en.wikipedia.org/wiki/Hayes_command_set The Hayes Command set+
• http://michaelgellis.tripod.com/modem.html A complete summary and explanation of the AT command set for Hayes compatible dial-up modems+