Is it possible to conect two modems directly?

[danielshoes]

Lets suppose that you want to make a file transfer between two PCs that don't have an ethernet adapter or a serial-to-serial cable, only 56K modems built in. I would like to know if it is possible to connect directly the two modems by using a conventional RJ-11 cable.

I am wondering if there is an AT command (to be issued through Hyper Terminal) that allow you to stablish this direct connection. The main difference is that when you use the POTS to reach the modem in the other end, you have to dial a number (no matter if you use the Hyper Terminal or the Windows dial-up, you have to dial a number), pass through a switched telephone network, phone rings in the other end, the modem picks up the call, the handsaking is performed and then you have an end-to-end link.

In this case, no dial-up should be necessary, because the physical connection is already stablished through a direct copper pair. There must exist just a single "connect" command... this is my guess. Any ideas?

 

[JackMDS]

The Modem's Electronic circuitry is build to Interact with the Telco. circuitry.

How I connect two computers to exchange files (No Network or Internet Sharing)?

 

[ddiccico]

http://www.jagshouse.com/modem.html

 

[danielshoes]

hummmm... very interesting. A circuit simulating the electricity provided by the phone lines. It makes sense. But I am not absolutely sure if this is necessary, I mean, if the modems really need this external electricity to operate.

 

[prosaic]

Yes, they do. JackMDS told you why. No electrons, no worky. The modems themselves don't have the oomph to push data across the wire. They can only go off hook to signify readiness. The phone company has to supply the power.

- prosaic

 

[JackMDS]

15 years ago I probably would resort to the Line simulator solution.

I guess it depends on geographical location and availability.

As far as I am concern it down to what is easier and cost effective to achieve the goal. I.e. transferring files.

I notice that you are posting from Brazil. In the US this month one store sells Network Cards for 99 cents.

Two cards and a cross over cable probably will cost few dollars more then the components needed to build a line simulator.

However Thanks, I saved the page. Good project for my Grand son science class.

The links that you provided lead to another good link that can be very useful to Audiophiles.

http://www.epanorama.net/index2.php?section=documents&index=audio

 

[ScottMac]

Some (external modems anyway) may be able to operate on "cold" copper (like a line driver).

They way we did it with the old Hayes Modems (1200-2400 baud) was to do an ATA on one side, and an ATO on the other (ATA - force answer tones , ATO - force originate tones).

I haven't tried this in better than a decade, things might have changed somewhat in that time span .

and (This is always a popular answer) Check the book/documantation that came with your modem. The Hayes book had printed examples of how to do a cold copper / line driver setup (as well as Sync, async, etc).

Good Luck

Scott

 

[prosaic]

If any type of modem could do it it would be external ones. It would be cool if this would work for him. I just tried the trick you suggested (I remember it, but I don't remember it ever working without a line simulator.), but it was between an internal WinModem and an external modem. Didn't work in that case. I'll cross my fingers.

- prosaic

 

[danielshoes]

ScottMac, if you say you did it, so I still believe that it is not impossible. I will try that AT commands. I don't see reason for changes (the basic ones are the same, no matter the modem model or age).

JackMDS, ethernet adapters also can be found here in Brazil at very low cost everywhere. Certainly if I would search for an efficient and cost effective solution, there is no better choice than use the ethernet cards, you are right. But it is always good to know some tricks. Maybe someday I face a situation where no ethernet ports are available and I will have the opportunity to show why I am an Anandtech's forum member...

Thank you guys for your inputs!!!

 

[glugglug]

You can connect two modems together just fine. The thing is by default it looks for a dialtone and will return a "NO DIALTONE" message when it doesn't find one. On mostmodem chipsets ATX3 will disable the dialtone detection. You can issue it as a separate command or put it in your dial string, i.e. use ATX3D to "dial"

On the "answering" side use the ATA command to pick up.

With most modems, this will result in the highest possible connection speed, up to 33.6 every time. (56K not supported with neither side digital) USR chipset based modems below 56K, however, will often end up in an endless spiral of retraining to lower speeds. My best guess is that this is because the USRs actually transmit too loudly on the line, and actually rely on the phoneco to attenuate it (or in many cases just clip the tops/bottoms off all the waveforms resulting in a shitty connection). If you use an AT%L command while connected to check line level or set S194=170 on a supra with the 2 char display for lots of extra info to be displayed, you will see that those particular modems transmit 6dB louder than everyone else.

 

[danielshoes]

AHAAAA!!! Ok... it seems we have an official confirmation. Thanks glugglug!!!

 

[danielshoes]

glugglug, bad news: I tried you suggestion, but it didn't work anyway. The two modems I am using are an external Multi-Tech 5634ZBA and and the internal of my Toshiba laptop. Tried to dial frist (ATX3D) then pickup the call (ATA) with the other modem, without results. Changed modem direction, changed command sequence (first placing the ATA in one modem and then the ATX3D in the other). In all times both modems start to make some weird noise, but not the typical handshaking all we know. I sincerely don't think that it has something to do with the fact that one modem is internal and the other one is external.

glugglug, have you been able to make this kind of direct connection or was that only a guess?

 

[prosaic]

I have hope because of ScottMac's information, and I'll keep my fingers cross. But I remember trying this with several other pretty talented communications engineering types a long time ago with standard Hayes external modems (300 and 1200 baud). We never got it to work without simulating the external circuitry provided by the phone company. If the sound you're talking about hearing from the modems is a sort of decreasing frequency howl (like old AM radio tuners made when you were adjusting them) then that would be typical of what I remember from the modems we experimented with back then, and the sound I heard last night when I tried a connection (just for grins) between an internal and an external modem on two systems I have at home. I think that may be the sound that glugglug refers to as an endless spiral of retraining to lower speeds.

- prosaic

 

[danielshoes]

prosaic, the sounds that I heard are typical when you type the commands ATA and ATX3D. Even with the phore wire disconnected from the jacks, those sounds occur. I am sure that these sounds are not the called "endless spiral of retraining to lower speeds", because no negociation started between the two modems indeed.

Folks, if such kind of direct connection is *really* possible, I would like to hear a clear and direct "yes, it is. I did it once and it worked".

 

[JackMDS]

There are certain things that with the advance of Technology become less flexible because some of the ancient functions are not needed any more.

As I indicated you need to simulate the line. Scott remembers old modems that had line simulation in.

With the developing of Network there was no reason any more to do this, two modems will move files at a speed of 5KB/sec. When the only mode of transfer were floppies it made sense.

The Internal Win-modems that are prevalent in the last 5 years have much less circuitry and components then the old modems. They would probably work only with what they meant to work with, Telephones.

I do not have a schematic of a current Modem, but I will not be surprise that there is Diodes (or some form of Gates) at the circuit entry that does not let the modulated signal to function unless it sit on a carrier of few volts (which is what the similutator provides).

 

[prosaic]

I can only vouch for the fact that I've tried, along with lots of other people I know, and failed. I know. Not the kind of confirmation you wanted. Back when I tried it I was trying to set up a "LAN" this way in my home between an Apple //e (son's computer) and an Atari 800XL (daughter's computer). This was when those computers were new, so... it was a while ago.

- prosaic

 

[Oaf357]

You're forgetting two things.

1) Dial-up modems need dial tone.

2) The RJ-11 would have to be rolled.

 

[glugglug]

It definately works I've used the technique quite a bit testing stuff at an ISP before.

Also, I checked the multitech manual online here and it definately supports ATX3 for no dialtone check, but it doesn't say whether it allows it IN the dial string.

So you might have to do ATX3 (return), then ATD (return) on the multitech side, then after it picks up (probably just a click sound) use ATA on the toshiba end.

 

[danielshoes]

glugglug, the X3 command (blind dialing) is universal and I am sure that all modems do support it. Regarding placing it in the dialing string, I am also sure that there is no problem with it, considering that you place the commands in the correct sequence, because they are executed in sequence, from left to right. So, if you issue the string ATX3D, the X3 command will be processed before the D command. This is not the problem. I will try another modems combinations.

Oaf357, the dial tone is not necessary and the rolled cable does not apply to POTS lines.