Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!world!jon_sree From: jon_sree@world.std.com (Jon Sreekanth) Newsgroups: sci.electronics Subject: Re: phone line specs Message-ID: Date: 20 Dec 90 16:39:10 GMT References: <5510@mit-caf.MIT.EDU> Sender: jon_sree@world.std.com (Jon Sreekanth) Organization: The World Lines: 35 In-Reply-To: grnberg@mit-caf.MIT.EDU's message of 19 Dec 90 16:16:08 GMT In article <5510@mit-caf.MIT.EDU> grnberg@mit-caf.MIT.EDU (David R. Greenberg) writes: I realize that this may be a very common question, but I haven't read this bulletin board for some time now. Could somebody please post the specifications for a standard phone line jack i.e. what the various voltages are on the four wires during various phases of phone operation? When the phone is on hook (open circuit), the voltage is -48V DC. The connection upto your house is usually modelled as a 48V battery in series with 0.4 - 2 K ohm of DC resistance. When you pick up the phone (make it go off hook), your phone has 200 to 600 ohm of DC resistance. In terms of AC impedences, the phone wire is usually considered to be a 600 ohm load. The signal power on the line is usually referenced to 1 mW through a 600 ohm load ( 0 dBm is 0.775 V of signal). DTMF signalling is around - 6 dBm, voice is a little below this. Ringing is nominally 20 Hz, 90 VAC superimposed on the 48V battery. Ring voltage stops when the phone goes off hook. All these numbers are from memory, and they're all nominal (I've found various books disagreeing on these 'nominal' values). The good part is that the system works well with a wide range of values. Most common telephones can be rung with as little as 40 VAC of ring voltage, and don't need 48V DC bias. The 'official' specs are maintained by Bellcore. You'll want section 6 (and possibly others) of the LSSGR / Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com