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Using telephone wire and two modems to connect computers

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ryanl88

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I am doing a little science fair project and I need to connect two computers together using two dial-up modems, two phones (mic. to spkr. and spkr. to mic.), using no phone line, just plain old phone cable. I have powered the cables with 9V batteries and can use the phones as intercoms, but can I use them to establish a dial-up connection between two PCs? The reason I am doing this crazy set up is because I plan to hook the mic's and spkr's up to ultrasonic sound converting circuits and have the communication be done over air via ultrasonic sound.

Thnx for the help!
-Ryan
 

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Dialtone

New Member
The modems will talkover the circuit you have described, even one that is un-powered for a short distance, however, you are going to have some trouble getting them to connect together using the standard setup software normally provided. Consider the following 2 items you will need to address to get a successful connection.

1. The dialing computer modem will be looking for dial tone before it procedes to dial out.
2. The receiving computer modem will need power ringing in order to detect a connection attempt.

If the modems are compatible with the Hayes AT command set, you may be able to build init strings to place them in a direct connection mode.

Dialtone
 

ryanl88

New Member
Dial Tone

I have the sending modem set to not look for a dial tone and just dial. I haven't been able to simulate a ring to get the receiving modem to initiate a connection, this may require a seperate circuit. Dialtone- how would I do the direct connection setup?

Thanks for all the replies!
-Ryan :shock:
 

Phasor

Member
I haven't been able to simulate a ring to get the receiving modem to initiate a connection,

You will need to either manually command the modem to answer ("ATA") or write some sort of script which sends the ATA command to the receiving modem.
 

Dialtone

New Member
After re-reading your original post about using ultra-sonic communications, I have a suggestion that may simplify things considerably.
Dump the voice modems and switch your project over to using DSL modems. They already communicate in the ultra-sonic frequency range and can be directly connected back to back, over an un-powered line, and interfaced to the PC's via ethernet ports. They also would give you higher data thru-put than standard voice modems are capable of providing. I have not done this personaly, but know of some IT people who have used this technique to "extend" a high speed link between buildings via standard telephone cables already in place..
Just a thought
Dialtone
 

ryanl88

New Member
DSL 'Modems'?

Thanks for all of the help! I am confused Dialtone- DSL does use high frequency, but I thought that they used electricity, not sound? Maybe the electrical impulses can be converted to ultrasonic sound waves?

Thanks,
-Ryan
 

Dialtone

New Member
All modems use electricity for their means of transmission and the signal is the AC representation of the sound frequency. Standard voice modems
use transmit and receive frequencies generally below 4KHZ which is in the human audible range. This factor is what limits the maximun data transmission rates of voice modems. Telephone voice when converted from analog to digital, is sampled 8K times/sec, so it is impossible to modulate the voice signal faster than that and survive the conversion process intact. In reality anything faster than 1/2 the sample rate is prone to errors so the modulation rate is generally limited to 1/2 the conversion rate. To get higher rates than this, they employ other data compression techniques.
Since DSL uese high frequency tones for transmission, it can run at much higher rates, but only can be used on an analog cable pair. It's signal is usually bridged onto an existing telephone line with the normal voice signals and then seperated out at the end point by using a band pass filter to block out the voice components from causing interference whit the data tones. Since the tones are so high, the voice user does not hear them during normal voice conversations.
So, if you can build a suitable sound based transmitter/receiver, you should be able to feed the DSL signal(s) to it and get your over air link.
You may want to do a Google search on DSL and find out the frequencies the modems use.
No matter which way you go with your project, you are going to learn a lot about computers, telephones, and electronics in general. Sounds like a great project from an educational point.
Dialtone
 

ryanl88

New Member
Dsl frequency

Wow Dialtone, you know quite a bit! I have yet another question though. I have researched DSL frequency and I found that "data travels over the 100kHz - 2.2Mhz frequency spectrum and POTS uses 0 - 4 kHz". What I've done is purchased some SpeedStream 5250 DSL modems that can act as Point-to-poitn devices, from ebay. Now can i simpliy use this set up:

Computer--> DSL Modem --> RJ-11 Cable --> Telephone (MIC to SPKR and SPKR to MIC --> RJ-11 Cable --> DSL Modem --> Computer

Would this work, can phone speakers and mics use ultrasound?

Thanks,
Ryan
 

Dialtone

New Member
I could not say for sure, but my gut reaction would be that it probably would not work given the frequencies you stated. Keep in mind that the sound system (speaker/microphone) combination in a sound card is engineered to work in the human audible range (probably less than 20KHZ), but you can try it and see what happens. After all, a failure is just as important as success when viewed from a learning experience perspective.
Most likely, to go into the range you specified, you will need special transceivers designed to work in this frequency range. Check out some information on wall stud finders. I think they are basic ultrasonic devices. It may be possible to get a couple of cheap ones, dismantle them to get to the transceivers and use them in your project
Also keep in mind that higher frequency sound does not propogate as well as lower frequencies in normal air, so your transmit/receive range will most likely be severely limited and prone to errors. Also, there is a point , as the frequency increases, where the frequency will not propogate in air at all. When the wavelength of the frequency becomes shorter than the distance between the air molecules, it is not possible to vibrate them together to move the sound wave thru the air medium.
I do not intend to discourage you from trying , however. Like I said earlier, learning what does not work is just as important as learning what does work. The later just gives you a better feeling of accomplishment.
Good luck and keep us posted on your results.
Dialtone
 

jem

Member
If I may jump in... If your main goal in doing this project is to demonstrate digital communication using an ultrasonic link, then perhaps it would be better to simply use the serial port of each computer to modulate a pair of ultrasonic transducers on each end.

Concerning ultrasonic transducers here is $0.02 worth:

1. You can buy ultrasonic transceivers (Tx/Rx pairs) at Mouser (www.mouser.com) for about $6.00 ea. They operate at 40KHz.

2. Dialtone is correct about the propagation range of ultrasonic waves in air. I would say you can comfortably get about 15' range using the said transducers.

3. Communication will have to be half-duplex since both sides will be communication on a "carrier" frequency of 40KHz.

4. Bit rate will be quite low since the speed of sound in air is about 1127'/sec at normal rm. temp. Limiting the rate even more will be the fact that the Tx transducer will be vibrating for a bit longer then the applied pulse, and you need to space your pulses so that they do not jumble up the Rx.

5. The speed of sound in air varies quite a bit with temperature (being roughly 1087'/sec at 32F (0C). It also varies (but to a lesser extent with relative humidity). Since the altitude will be fixed, this can easily be compensated for.
 

ryanl88

New Member
COM

Thanks again for all the help, I would also like to try the com port setup. Would this set up be similar to an Infrared Com connection? Can you point me in the right direction to setting this up?

Thanks
-ryan
 

jem

Member
If you would like to use a COM port setup, first of all I would suggest that you try to communicate between the two PC's directly using a wired link (a NULL modem). At this point, you do not need to write any PC SW. Just use a terminal program running on both PC's. Check out this site for a very useable terminal program: https://bray.velenje.cx/avr/terminal.

Once you have established that you can actually communicate with wires, you can move to the next step of making an Ultrasonic link. As mentioned before, the transducers work on 40KHz. That means that you would need to modulate and demodulate that 40KHz carrier. For the modulator, you can use a simple 555-based 40KHz oscillator which is turned on or off directly by your COM port. To get max. range, run the 555 off 9 - 10 V. You will also need to translate the RS232 levels of the COM port to suit the 555. There are plenty of 555 based oscillator schematics around. so that should not present a problem.

For the receiver, you will need an amplifier to receive the weak Ultrasonic waves. Then, you will need to level shift the detected signals back to RS232 levels. Here is a simple discrete amplifier that I have designed. It has been tested on an similar project, and works pretty good.


 

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ryanl88

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Thanks for the help! I am new to building circuits, but I will give that a try! I haven't received the DSL modems yet, but I will keep you guys posted on that and on the outcome of the circuit for serial connections.

Thanks again!

-Ryan :wink:
 
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