A note about polarity and phone connections:

The two wires of an analog telephone connection are called Tip and Ring.
Tip is normally at ground potential. Ring is normally at minus 48 volts with res
pect to ground, not + 48 volts as the diagram shows.   Negative battery supplies
 (with respect to ground) have always been used with phone systems, so that leak
age currents to ground caused by moisture do not electroplate away the copper in
 the wires.  Some modems, e.g. certain US Robotics models, do not work correctly
 if the polarity is reversed (they will not detect ring), but most modern modems
 use diode bridges so that polarity doesn't matter.   The 2KOhm resistor shown s
hould work fine, but it only provides about 1/2 the normal current that each mod
em normally expects to see-- (each phone wants to 'see' about 20 ma) -- this sho
uld work fine, tho 1K to 1.5K would be more accurate.

Wagner's ring circuit is very clever, do you see that when the relay contact swi
tches, the phone 'sees' a change of 96 volts.  But be sure you get the capacitor
 the right way, or you will have quite an explosion, and with tantalum caps, you
 will have flames.

The transformers shown are 'inside' the modem -- but the vast majority of modem
circuits do not pass the loop current thru the transformer, because the transfor
mer core size required to avoid saturation means you need a bigger more expensiv
e transformer.  They couple to the transformer thru a cap, and send the loop cur
rent thru a resistor or resistive bridge.

Wagner, you have a great way with ASCII-based circuit diagrams!
   Regards,
     Ron Fial
--------------------------------------------------------------

>>Wagner Lipnharski wrote
>If you want to simulate the phone line voltages and currents:
>
>                    + 48Vdc
>                    |
>                    R 2 kOhms
>         Current    R
>Modem #1   <---     |     --->      Modem #2
>                    |
>---.  .----RRR------o------RRR----.  .---
>   3|(   150 Ohms        150 Ohms  3|(
>   3|(                             3|(
>   3|(          Phone Line         3|(
>---'  '-------------o--------------'  '---
> 600 Ohms           |            600 Ohms
>                  Ground
>