Have you ever connected two PCs together via
modems using a twisted pair cable and nothing happened? That’s because
the modems are expecting a phone line with all the signals and voltages
supplied by the local telephone exchange. This circuit simulates the DC
power and signal isolation but not the “dial tone” or the “ring signal”.
It suffices to connect two PCs together to communicate and exchange
files using HyperTerminal. The circuit is self-explanatory and needs
only one power supply for both modem lines. Although 50V DC is the usual
exchange line voltage, this circuit should operate down to 20V. A 600O
line transformer (eg. Jaycar cat. MM-1900) provides signal isolation,
while the resistors provide current limiting and keep the lines as
balanced as possible.
When using this set-up with HyperTerminal, you should not select a
Windows modem driver in the “Connect To” dialog. Instead, connect
directly to the relevant COM port. Next,
verify that the modems are working by sending information commands such
as “ATI1” or “ATI3”. If you don’t get a response using these commands,
try resetting the modem(s) using the “AT&Z” command. Assuming you do
get a response, set one in originate mode using the “ATD” command and the other in answer mode with the “ATA”
command. If all is well, you should now be able to type in one terminal
window and see the results echoed in the second PC’s terminal window.
To return to control mode, type “+++”. The advantage of using modems
instead of a serial cable between COM ports is
that the two PCs can be kilometres apart instead of a few metres. For
example, you could connect the house PC to the workshop PC on the other
side of the farm.
Author: Filippo Quartararo
Copyright: Silicon Chip Electronics