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Modems and Analog Telephone Lines

Connecting to the telephone system without frying your modem.

Charles A. Plesums, Austin, Texas, USA

As most people know, connecting a PC modem (or fax machine or other analog device) to a digital phone line can damage (destroy) the modem. But if you aren't a technician with a set of test equipment, what can you do?

Every analog phone in the United States must carry the designation "...complies with Part 68 FCC rules..." and must have a Ringer Equivalence Number (REN). For the moment, don't worry about the value of the REN - just whether the phone has the label. If the phone currently connected to that telephone line has this information on it, it must be an analog phone, and you should be safe using that line to plug in your computer. If it does not, it must be one of the special digital phone lines used in many offices. If you try to use the modem on a digital line, it is unlikely to work, and will probably damage your computer.

Realistically, practically every home telephone is a "safe" analog line. Most hotel phones are analog, and certainly the "data" jack on a hotel phone is analog. But beware in large offices - many telephones are digital (more features with less wiring) rather than analog.

By federal regulation, the REN must be checked before you connect any telephone, answering machine, modem, or other device to the telephone network. Therefore, I'm sure everybody is expert on the subject, and has checked the REN at their home. If not, you can turn yourself in at the nearest federal penitentiary. One ringer equivalent is the amount of power it took from the phone line to operate the old electro-mechanical "ringer" (bell) that announced an incoming call. The phone company was expected to provide enough power to operate 4 ringers. The law says that if you plug your own phone into the telephone line, the total REN for all the devices plugged into that line must be no more than 4. If greater than 4 you will be using more telephone power than your share, and you will be personally responsible for the collapse of the world's telephone system.

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