Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site brl-tgr.ARPA Path: utzoo!linus!philabs!cmcl2!seismo!brl-tgr!ron From: ron@brl-tgr.ARPA (Ron Natalie ) Newsgroups: net.dcom,net.periphs Subject: Re: building a modem - How do I use the telephone lines? Message-ID: <10625@brl-tgr.ARPA> Date: Mon, 13-May-85 15:20:08 EDT Article-I.D.: brl-tgr.10625 Posted: Mon May 13 15:20:08 1985 Date-Received: Thu, 16-May-85 02:49:47 EDT References: <2371@wateng.UUCP> Organization: Ballistic Research Lab Lines: 20 Xref: linus net.dcom:793 net.periphs:609 > Questions - the jack that plugs into the wall has four coloured wires > coming out of it -- what do the four colours mean? What happens when > the phone rings? Does the voltage on one of the wires go high or something? > How can you tell if the phone is off the hook? Any ideas on how to > actually interface the TMS99532 to the phone lines? I'm told I need > to use an isolation transformer... > Well, it would be safer to use a transformer or a capaciter. Only two lines get used essentially (the center two, if I remember properly). There is about 45 volts across them. The ringing is done by putting 90 volts of low frequency AC on the line (you can make a really nice visual ring indicater with a neon bulb since the thing is designed for that volatage and as very high resistance when it is off). Of course the real problem is that you can not legally just hook up to these wires. The interface has to be approved. This means either getting some kind of DAA or using an approved interface chip. I don't know enough about the chip your using or chips in general to recommend anything. -Ron