w.r.t. telephony, a wet interface is one which includes the loop (DC) current. A dry interface is typically on the other side of a blocking capacitor of some sort. In your case, your little transformer probably doesn't like the continuous DC. Bob Ammerman RAm Systems ----- Original Message ----- From: "Mauricio Jancic" To: Sent: Tuesday, September 16, 2003 3:57 PM Subject: Re: [EE]: Using CMX868... > Can you please explain to me what is the difference between a "wet" and > a "dry" interface? > > Thanks > > Mauricio > > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of David VanHorn > Sent: Martes, 16 de Septiembre de 2003 14:24 > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE]: Using CMX868... > > > At 01:45 PM 9/16/2003 -0300, Mauricio Jancic wrote: > > >Hi, > > I'm currently trying to develop a application which involve > >some telephone communications. I've decided to use CML's CMX868 > >(formerly from MX-COM). > > Now, my problem is that someone (Hi there Dennis) told me to > >make a power supply to emulate the phone line in order to be able to > >establish communication between the two modems. I've succeeded on this > >issue and successfully connected two computers via modems. > > The circuit is a simple LM317 in current regulator > >configuration and another one in series in volt reg configuration, this > > >will provide 9V with 13mA max. > > > > On the modem circuit, I've used CML recommended circuit for USA > > >and EUROPE with caller ID feature. The transformer is a 60ohm > >(multimeter says so:). > > > > When I connect this circuit to the phone line or to the phone > >line simulator it draws a lot of current, so it brings the PSU down and > > >I guess that's why it doesn't work. > > > > Any tips? > > do yourself a huge favour, and grab a copy of 'understanding telephone > electronics' off amazon. this used to be a five dollar book at radio > shack, and now i think it's something like 30, but still worth every > penny.. > > basicaly, what you're trying to do with that circuit, as i understand > it, is a little bogus. the problems are current draw, and impedance. > your 60 ohm transformer is designed for a 'dry' interface, and it sounds > like you're using it in a 'wet' interface. in a 'wet' interface, the > line current is pulled though the transformer. this transformer dosen't > have enough dc resistance for that. > > also, minimum current on a us phone line is 20ma. > impedance is 600 ohms. > your simulator circuit needs to provide at least 20ma, a bit more would > be nice, and you need probably at least 12v on the power supply.. phone > stuff doesn't run by voltage, it's all current. > > /nobody start a war on this, i know you can't have current without > voltage, except in a superconductor, but all the measurements and specs > on phone systems are current. voltage is incidental/ > > you need to emulate a central office. > there's a box called a tls-2, made by teltone, that will do this job for > you, including simulating dial tone, ringback, and ringing voltage on > the called line. they have two ports, '29' and '40' 40 picks up, and > dials 29, 29 rings, and off you go. > > you find them on ebay from time to time. > they are a bit pricey off the shelf. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.