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.

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