RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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7-1
7. DTMF DIALING WITH AUTO DIALER
The modem includes tunable oscillators that can be used to perform dual-tone multi-frequency (DTMF) dialing. The
frequency and amplitude of each oscillator output is under host control. A programmable tone detector can also be used in
call establishment to recognize an answer tone.
This section describes the method of oscillator and filter tuning by the host processor and provides an example of an
autodialer routine that may be programmed into the host.
7.1 DTMF REQUIREMENTS
EIA Standard RS-496, paragraph 4.3.2, specifies requirements that ensure proper DTMF signaling through the public
switched telephone network (PSTN). These tones consist of two sinusoidal signals, one from a high group of frequencies
and one from a low group of frequencies, that represent each of the pushbutton telephone characters shown in Table 7-1.
Table 7-1. DTMF Signals
Low
High Frequency
Frequency
1209 Hz
1336 Hz
1477 Hz
1633 Hz
697 Hz
1
2
3
A
770 Hz
4
5
6
B
852 Hz
7
8
9
C
941 Hz
*
0
#
D
Signal power is defined for the combined tones as well as for the individual tones. Both maximum and minimum power
requirements are functions of loop current. By combining the various requirements of RS-496, compromise power levels can
be determined that meet the power specification for all U.S. lines (when driving the PSTN from a 600 ohm resistive source).
The high frequency tone should be at a higher power level than the low frequency tone by approximately 2 dB. The
maximum combined power, averaged over the pulse duration, should not 1 dBm. The minimum steady state power
of the high frequency tone should not be less than -8 dBm.
When connecting the modem circuit to the PSTN by means of a data access arrangement (DAA) set for permissive mode,
the DAA gain is -9 dB. The modem circuit must, therefore, drive the DAA input with +1 dBm of steady state high frequency
power and -1 dBm of steady state low frequency power in order to meet all of the listed conditions. Since +0.5 dBm is the
maximum undistorted power level for individual tones generated by the modem, the user may need to add gain in front of the
DAA during DTMF dialing.
The required duration of the DTMF pulse is 50 ms minimum. By experience, a pulse duration of approximately 95 ms is more
reliable. The required interval between DTMF pulses is 45 ms minimum and 3 seconds maximum. Again, by experience, an
interdigit delay of approximately 70 ms is preferred.
The remaining requirements of RS-496, relative to DTMF dialing, are not influenced by the host processor. These
requirements are all met by the modem's oscillators.
7.2 SETTING OSCILLATOR PARAMETERS
The oscillator frequency and output power are set by the host computer in DSP RAM using the microprocessor bus and
diagnostic data routine. For a description of the microprocessor bus and other interface considerations, refer to sections 2, 3
and 4.
To generate a DTMF tone, place the modem into TONE configuration (CONF = 80h). The user must program the
frequencies and the levels of each tone. To set the frequency of tone 1, write a 16-bit hexadecimal number into RAM using
RAM access code 21h with AREXx = 0, BRx = 0, DRx = 0, and CRx = 1. When setting the frequency of tone 2, use the
same procedure with the RAM access code 22h with AREXx = 0, BRx = 0, DRx = 0, and CRx = 1. Set the power levels of
tone 1 by writing a 16-bit hexadecimal number into RAM using RAM access code 22h with AREXx = 0, BRx = 0, DRx = 0,
and CRx = 0. The RAM access code for the power level of tone 2 is 23h with AREXx = 0, BRx = 0, DRx = 0, and CRx = 0.
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