Direct Modem-Modem connection

Posted on 2003-03-20
Medium Priority
Last Modified: 2013-12-27
I know it's possible to connect 2 modems directly (like a null modem) and to initiate a communication between them with some AT commands. Is there someone how can give me precise steps?
I read that I must do ATA on Modem1 end some commands like force "GO Online" on Modem2. But I never succeed!

Thanks for any help.
Bye bye

Question by:drouillon
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LVL 17

Accepted Solution

rayt333 earned 100 total points
ID: 8185040
The "null modem" connection does not use the modem, it uses the serial or parallel port to emulates the data transfer method between the computer and the Modem. That is exactly why it is called Null Modem, because it is an inexistent Modem. With the Null Modem the connection of two computers in relatively small distance betwwen them is achieved and the transfer of data is possible. Practically, this means that you can easily carry out the transfer of a file without having to interpose another storing mean whose capacity can limit the transfer of the file as for example a disk, something that finds application during the transfer of files from a table computer to a laptop or the other way round, or even the conduction of multiplayer games.
LVL 31

Expert Comment

ID: 8186523
For connecting two modems, I have had the following recommendation from a purportedly tech-savvy person:

If your modems are not explicitly said to support leased line or private line communication, you will have to fool them into believing they are on an ordinary phone line. The prerequisite is a "line voltage", which ordinary modems uses to understand that they are on-line. This line voltage can be simulated by connecting a 9-25 V voltage source (battery) in series with the modem - line - modem setup.

Next step will be to force handshake start - issuing the ATD  command (Dial!) on one end. The other end MAY need the ATA command (Answer!), unless it can be set to answer automatically on handshake tries. All such technical things should be in the modem manual.

I strongly suggest you go into this using proper hardware modems and a good portion of patience.

The battery mentioned should be inserted between one strand of the phone line (which is two-strand) and its modem terminal. Easiest way to connect is probably to snip off one strand in the cable and connect the battery into the "gap" thus produced.


Assisted Solution

RDavid earned 100 total points
ID: 8262282
The comment from /RID is a hardware hacker's answer.

If you are like me and do not want to cut phone lines and try to add batteries, the easiest way to get two modems to talk to each other is to use a "Central Office Emulator". That is a box which both modems plug into and it acts as if it were the phone office. One modem can dial the other (pick up the line, get a dial tone, and do the dial) and the other modem receives a ring. The Viking DLE-200B is an example of that and can be gotten from Greybar Electronics for about $175.

Once you have the electrical hookup ok, then the next step is to initialize the modems properly. On the modem that dials, the command "ATDT18005551212" is a valid initialization string and will dial the other modem. (Actually, with the device suggested, you only need a single digit.) On the other side, the modem needs to have been set into the answer mode. The command I use is: "ATE0M1Q0V1X4S0=1&C1&D1". Of this command, the "S0=1" is the one which means to answer on the first ring.

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