Polarity
In the early years of the telephone industry when rotary dial instruments were in use, the correct polarity of tip and ring was important only for properly ringing a telephone, especially in party line service with selective ringing.
In the 1960 when Touch-Tone service was introduced, the DTMF tone generator also required correct polarity as it depended on the line power for operation. Later Touch-Tone telephones included a diode bridge that eliminated the polarity sensitivity, so that consumer telephone service is essentially immune to reversal today. However, some special circuits, such as some Direct Inward Dialing (DID) trunks, T-1 lines, and ground start lines connected to field side (terminal) equipment, e.g., a corporate private branch exchange (PBX) switch, operate correct only with proper tip and ring polarity.
Read more about this topic: Tip And Ring
Famous quotes containing the word polarity:
“The same polarity of the male and female principle exists in nature; not only, as is obvious in animals and plants, but in the polarity of the two fundamental functions, that of receiving and penetrating. It is the polarity of earth and rain, of the river and the ocean, of night and day, of darkness and light, of matter and spirit.”
—Erich Fromm (1900–1980)