Ron and Javier,
The polarity is really -48V and Ground, even that in a bench simulator
it doesn't matter  :)
You are right about the phone line transformer, in real, actualy they
are smaller in size and most of them requires a 30-100 Ohms resistor in
series to reduce a bit power over the coils.

There are some solid state phone line transformers, using proportional
LEDs and PhotoTransistors. I believe I saw something at CPClare.

I already used two resistors and two capacitors instead the transformer,
it works pretty nice, but there are some problems related to voltage
saturation.


--------------.     0.47uF 250V    Answer Relay Sw
              |                        /
              |---o------||-----o-----o/  ^----> To Phone Line
              |   |             |
              |   R             R
MODEM Circuit |   R   600 Ohms  R
              |   |             |
              |---o------||-----o--------------> To Phone
Line               |
--------------'
                    0.47uF 250V


Javier wrote:
> I want to connect the handheld PC(with internal modem) and the desktop
computer, is the resistor + 9v batt method reliable, what baud rate
could I
use ?? I want to connect them via modem cause I don4t have the serial
cable
:-(

Your serial port UART can easily goes up to 115kbps in direct attachment
using a null modem cable (just cross wires between two computers), while
your portable PC internal modem (can goes from 2400 to 28800, if it is
old, I guess it is 2400 max 14400bps).

To hookup your handheld to talk to your PC it will give you some
problems, first because you will not have the ring signal at the fake
phone line, you will need to know how to make both modems handshake and
start the connection.

I would go after a null modem cable, if you have a free serial port in
each computer.
If you have two female DB9 connectors (or DB25 if your computer uses it
as a serial port), it is also easy to build this cable, in real it is
just few wires, nothing special.

Wagner