On Wed, 15 Jul 1998, Max Eskin wrote: Well, it wouldn't be on topic. Telephony is a whole 'nother subject. Try calling your local telco and say you're a student interested in learning more about the phone network. If they ask if you're a phone phreak, say you've never even heard of that term. Or, a good book to start on would be _Understanding Telephone Electronics_. Its a Sam's book but you can also find a Radio Shack branded version at Radio Shack (they all carry it AFAIK). I recommend it. Just a quickie, the phone network design has many parallels to the Internet, or a basic datacom network topology. You've got your end offices (sort of like level 3 routers), tandem switches (sort of like level 1 baclkbone routers). Ask me more questions in e-mail. When a phone is on-hook, the ringer circuit is connected in parallel with the line. The tip and ring (telephone-speak for positive wire and negative wire, or commonly color-coded as green and red respectively, except when dealing with a multiple pair cable) each have a capacitor in series with the ringer. Like so: + (tip) - (ring) | | |--||--[BELL]--||--| | | | \ | |______o \o______| Hookswitch The capacitors allow current to flow during the AC ring cycle (about 90 to 120v) which allows the bell to ring. Once the phone goes "off-hook" (someone picks up the hookswitch and closes the loop) the telco senses loop current and ceases the ring current, and puts 12v DC at ~20mA on the line. When the phone is on-hook, there is a 48v potential on the line. Nope, the caller ID data is a 1200bps data burst. Not enough current to activate the ringer. Bzzt. But thanks for playing :) The caller ID box camps on the line with a high impedance monitor circuit (basically has capacitors in parallel with the line like the ringer circuit, with over-voltage protection to keep the nasty ring cycle away from it. This allows the AC signal to pass but prevents a DC loop which would look like an off-hook to the phone company). If you ever see a telephone technician, ask him to demonstrate his/her lineman set for you. It allows you to camp on a line and listen to the conversation without actually taking the line off-hook. Sorry for the off-topic. Please send all further queries to e-mail. Sam Alternate e-mail: dastar(a)siconic.com ------------------------------------------------------------------------------- Ever onward. September 26 & 27...Vintage Computer Festival 2 See http://www.siconic.com/vcf for details! [Last web page update: 07/05/98]