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Chapter 4—AT Commands, S-Registers, Result Codes
Dual Tone Multi-frequency (DTMF) was a technique proposed by the
phone company to replace pulse dialing to make dialing faster and
more reliable. In anticipation to adding other customer services, 6
additional digits were included, totaling 16 digits. Most commercial
phones only include 12 digits. Many specialized phones and telephone
equipment (such as ADSI phones, PBXes, etc.) utilize all 16 digits.
DTMF was first introduced in the 1960s to the general public as touch
tone dialing. The conversion from pulse dialing to touch tone
dialing took over 20 years in the US. To this date (and in the
foreseeable future) all phone companies support pulse dialing, and
most phones you buy still allow you to switch to pulse. In the 80s,
DTMF found a complete new use as a key pad for voice mail and
interactive voice response systems. Since these systems have become
a way of life now, DTMF has become the most common man machine
interface.
S-Registers
Certain modem values, or parameters, are stored in memory locations
called S-registers. Use the S command to read or to alter the contents
of S-registers (see previous section).
REGISTER UNIT RANGE DEFAULT DESCRIPTION
S0 1 ring 0, 1255 1 Sets number of rings until the modem
answers. ATS0=0 disables autoanswer
completely.
S1 1 ring 0255 0 Counts the rings that have occurred.
S2 decimal 0127 43 (+) Sets ASCII code for the escape
128255 sequence character. Values greater than
127 disable escape.
S3 decimal 0127 13 (^M) Sets ASCII code for the carriage return
character.
S4 decimal 0127 10 (^J) Sets ASCII code for the line feed
character.