B Dial Pulses and Tones
CommPlete Communications Server 69
Appendix B 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 are the same as the digit they encode; thus, the digit

1

is
represented by one pulse, the digit

2

by two pulses, etc. In Figure B-1, 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 r ate of 10 pulses per second. The
interdigital pause time is 800 ms. The pulse ratios are controll ed by the &P command.
Closed
Open
Digit 2 Digit 1
A
B

Figure B-1. Dial pulses

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-freque ncy tones (697, 770, 852, 941 Hz),
while the three vertical columns use three high-frequency ton es (1209, 1336, 1477 Hz). The tone frequency
tolerance is ± 0.02%.
For example, the digit 4 is dialed by combining two tone frequencie s: 770 Hz from the second row, and
1209 Hz from the first column. In another example, the digi t 9 is dialed with tone frequencies 852 Hz and
1477 Hz.
123
456
770
697
789
* 0 #
941
852
147713361209
Hz
DigitsHz