Connecting two computers using their modems, without a telephone line

Suppose you had two computers and two modems and wanted to transfer files between them, then this is what you could do.

One necessity for a reliable connection of this type is a "line simulator", a device to provide simulation of a telephone line, supplying the line current that modems are designed to work with. For our purposes, a very simple home-made circuit will do the job. This connects between two telephone modular jacks, and the two modems plug into the jacks.

Basic Circuit:

 

                             +  | | -
      	     GREEN----------+---||||---/\/\/--+-------------RED
                            |   | |     R     |           
                            |  9 to 24V       |            
                            |                 |           
                            +---||------------+          
                                C .47uF     
             RED--------------------------------------------GREEN

 

Note that the resistor value depends on the actual voltage used. For 24V about 1K Ohms max will give at most 24mA (12V @ 500 Ohms, 9V @ 380 Ohms); the resistance of the modem circuit will reduce this slightly. The telephone company guarantees about 20mA minimum in an actual phone line, and we want the new line to be about the same minimum.The capacitor is used incase you give the DC voltage using an adaptor from the mains, else for pure battery current, it is not necessary. This is used to reduce the "ripple" voltage (i.e., AC "noise"); be sure to match the polarity of an Electrolytic Capacitor to the polarity of the power supply.

Now, connect using a terminal software such as Hyperterminal (available with Windows9x or can be downloaded). Transfer files convinently or chat using the menus provided.

Abhijit Pai

Sources :
Jagworks Media Jag House (Jag's Used Mac Store) - John Meshkoff
http://www.hut.fi/Misc/Electronics/docs/old/telephone_intercom.html
http://www.helpdesk.umd.edu/faqs/comm/dial/atdirect.html
http://www.imatron.com/

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