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Modem/Modem connection (AGAIN)

Posted on 1998-08-25
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Last Modified: 2013-12-27
I was just browsing through some modem problems and I was surprised by some people's statement that you can connect modems without a line/line simulator.
I know how to deal with serial connections, but I have a similar problem that I have to connect two modems. Usually there will be a real phone line in between, but there are cases when I don't have that line. Until now I thought I'd have to buy one of those (expensive) line simulators, but now it seems I should be able to handle it with s/w only.
So here is my question: Is it really possible to communicate modem to modem without telephone hardware in between? What cable do I use (crossed over phone cable?)? What do I have to do on the s/w side (I saw a AT command there ...)?

Please do not start the discussion again IF it's possible, only comments HOW to do it! Trust me, I never thought it would be possible.
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Question by:griessh
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joe_massimino earned 100 total points
ID: 1144668
Phone lines have a DC voltage riding on them, but the modem has enough drive to handle connecting two of them together with a standard phone cord, just like th one you use to plug your phone into the wall.  Now instead of dialing you would just issue the command to answer. AT A is what I remember using.  if you lok into the modem commands that came with the book for your modem you will see that you can force your modem to answer and go online. You should be able to do this on both modems at the same time and get them to connect.  Keep in mind that you will not be able to do any of the digitally enhanced speeds, ie. 56k, but 33.6 should be ok.  Try it.  I use to do this to transfer files between PC's that were in the same room when I didn't have LapLink or some other software product. I would use  Procom and Zmodem to copy files.
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by:joe_massimino
ID: 1144669
Also, If you wanted to run a phone line a very long distance, you could also supply your own talk battery if you wanted to use telephones.  Modems supply a carrier where phones have no way to supply power on their own in any amount.
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by:joopv
ID: 1144670
For Joe : is this going to work ?  i believe one modem should be in answer mode, and the other one in originate mode.

For a modem to start in originate mode, you will have to issue AT command to stop the dialtone detection.

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Expert Comment

by:colinmiller
ID: 1144671
As far as I recall we havn't had to worry about about originate/answer since the days of 300 baud - remember that?

Why would you use a modem to modem link to connect PC in the same room. A null modem cable would do just fine and run at 115,000 baud. Procomm can handle a direct connect type link and so can most other things.

Modem to modem is really only required if you are driving over a substantial distance. A made up or $5 null modem plug will get you within most poffices or factory complexes.
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by:griessh
ID: 1144672
Thanks a lot Joe, that's what I needed! The first time somebody made a clear statement in this discussion. I haven't had a chance to try it yet, but this weekend I'll give it a shot.

A comment for colinmiller:
Imagine you are designing a hardware that has the modem chipset on board, is supposed to work over modem(with phone line), but should also work in a closed enviroment like a building (where you don't have a telephone system). If you can do the direct connection, you can deal with the whole thing from the s/w side instead of having different h/w designs.
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