Appendix

Appendix H: Dial Pulses and Tones

Dial Pulses

When you pulse dial, as when you make a call with a rotary dial telephone, your telephone or modem generates codes in the form of pulses that simulate the opening and closing of old-fashioned electric relays, or switches. The number of pulses in a code is the same as the digit they encode; thus, the digit 1 is represented by one pulse, the digit 2 by two pulses, etc. In the figure below, the digit 2 is pulse dialed, followed by the digit 1. Each pulse consists of an A ms open (break) and a B ms closed (make), where A is either 60 or 67 ms, and B is either 40 or 33 ms, for a total of 100 ms per cycle, or a rate of 10 pulses per second. The interdigital pause time is 800 ms. The pulse ratios are controlled by the &P command.

Closed

Open

B

A

Digit 2

Digit 1

Tone Dial Frequencies

The tone dialing method combines two frequencies for each of the twelve digits found on a touch-tone dial pad. This method is referred to as dual-tone multi-frequency (DTMF) dialing.

The four horizontal rows on a touch-tone keypad use four low-frequency tones (697, 770, 852, 941 Hz), while the three vertical columns use three high-frequency tones (1209, 1336, 1477 Hz). The tone frequency tolerance is ± 0.02%.

For example, the digit 4 is dialed by combining two tone frequencies: 770 Hz from the second row, and 1209 Hz from the first column. In another example, the digit 9 is dialed with tone frequencies 852 Hz and 1477 Hz.

HzDigits

697

770

852

941

Hz

1

4

7

*

1209

2

5

8

0

1336

3

6

9

#

1477

150

MultiModemISI Hybrid Series, ISIHI-2S

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Multi Tech Equipment ISIHI-2S manual Appendix H Dial Pulses and Tones, Tone Dial Frequencies
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ISIHI-2S specifications

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