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"POTS" stands for "Plain Old Telephone Service," which defines the telephone industry requirements.
When the first telephones were installed, a method of informing a person that a call was waiting had to be designed. This was accomplished by having a bell ring. In the 20s and 30s you actually could see the bell structure with one or two coils of wire, a bell housing, and a clapper, which would ring the bell when an AC voltage was applied to the coil. The power to ring this bell came from the phone line. A ringer supply would develop the ring voltage.
Today most phones have their own little amplifier/speaker combination that produces the ringing sound. They also have sense circuits that detect the 20 Hz ring voltage and then activate their ringing sounder. The load defined as a REN represents the load of one telephone or the RingerEquivalent Number. This load presented by one phone can be simulated as an RC circuit consisting of about 6 k and 8 µF.
Principles POTS/PSTN
- The signals on POTS port are RING and TIP which are as same as PSTN line.
- The voltage is 48V when on-hook; 8V when off-hook.
- The frequency can be set by S-register S121. Default ring frequency (cc=255) is 40Hz
