long distance serial communication

Discussion in 'Wireless & RF Design' started by pal_87, Aug 21, 2008.

  1. pal_87

    Thread Starter New Member

    Aug 21, 2008
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    hii...
    i am working on a project where i need to transmit serial data over a long distance approx 2 miles...on twisted copper lines
    can you offer suggestions on how it is possible...
    i googled it and found out that there are many drivers and modems for this but with insanely high prices like a 1000$

    please help me out i am really stuck against a wall here.... :(
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It's easy to do at low data rates. They used to transmit Morse code over longer distances. How fast do you need to transmit? Uni- or bidirectional?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Why not telephone based circuits, using modems?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Ask has already been mentioned, we will need more details about your specific application before we can assist you?

    hgmjr
     
  5. kammenos

    Active Member

    Aug 3, 2008
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    Have you consider using optic fibers? Because 2 miles on copper for fast baud rates would be difficult. Not impossible but difficult.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    EIA-485 (formerly RS-485) is good for 4,000 feet over differential pair. You might be able to split your 2-mile wire run into three pieces, or about 3,600 feet (1,200 yards) each, and use a couple of tranceivers as repeaters at each break.

    AWG 20 copper wire is roughly 10.4 Ohms per 1,000 feet, or 333.3 yards. If you powered one repeater from each end, you would need at least 4 conductors from the two far ends towards the middle, and two conductors between the two repeaters. The power supply line loop would then have roughly 75 Ohms of resistance; half via the supply, half via the ground.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    I did a quick check of the cost of twisted pair 2-conductor 20AWG wire and found the cost to be around $200 per 1000 feet. That means you may need a cool $1000 for the two mile stretch.

    hgmjr
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    That will also mean you will have a problem with theft of the wiring. :eek:

    During the Korean War, Marines and Soldiers were constantly laying commo wire for field telephones. However, the Koreans were so poor and so desperate for such supplies that they would chop out large sections of the wire for use on their farms; it was like a windfall to them.

    With the price of copper going through the roof, it's very plausible that theft would be a huge problem for a hard-wired network, even in the USA.

    However, fiber optic cable is relatively dirt cheap, and would have virtually no salvage value.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    I would expect the wire to disappear within days of its installation.

    I would go fiber-optics for both its lower cost and its low salvage value.

    hgmjr
     
  10. gerty

    AAC Fanatic!

    Aug 30, 2007
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    I agree,but, the thieves won't know it's fiber untill after they cut it.
    I ran into this problem at a factory that I was putting a fiber network into. One of the employees' saw the fiber on a spool laying on the floor. He decided he needed some 'speaker wire' so he cut off about 50'. He was nice enough to warn me when I came back not to bother pulling that 'wire' ,said they screwed me outta the copper. The bright orange jacket and the words 'optical cable' running down the side of it meant nothing to him.
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    CAT3 gel-filled runs about half that much, around $80 to $110, depending on how much one buys. Still, five hundred bucks is more than my wife would let me spend on one project.
     
  12. pal_87

    Thread Starter New Member

    Aug 21, 2008
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    0
    heyyy thanx a lot for the replies guys...
    well here is the project in detail..

    i am working on a project where i need to transmit serial data over a long distance approx 2 miles...on twisted copper lines(2 wires)
    can you offer suggestions on how it is possible...
    i googled it and found out that there are many drivers and modems for this but with insanely high prices like a 1000$

    Its a Data acquisition system
    i am using avr to measure temp humidity and power and tx it via uart to a PC
    there will be many avrs joining 2 the pc... i am using only 2 wires tx and gnd
    no feedback needed, the problem is...
    the distance between avr and pc is approx 6 km(max)


    well actually i am doing this project for a company and they have the telephone leased wires securely placed...

    they have many units some 2km,some lesser than 2 km, some as far as 6 km...

    they can provide me only pairs of copper wires for the communication...
    so fiber is out of the question...

    also these wires are already placed underground so using repeaters at regular intervals wont be possible..

    i dont need a fast transmission scheme even 1200baud rate or lower will do.

    i just need 2 get the thing running.
    can chips like max485 make it.

    i did consider telephone modem chips but i dont think they are available retail and some are too complicated with mannny features which i dont need.
    if u no any ics for this please do mention it.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    No, the MAX485 would require repeater stations. See figure 23 in the datasheet.

    26 gauge wire has a resistance of roughly 41 Ohms per 1000 feet, thus one strand of AWG 26 wire would have a resistance of about 433 Ohms over a distance of two statute miles (5,280 feet x 2).

    One of the limiting factors will be the capacitance of the wire. It takes time and current to overcome the capacitance.

    You might actually be able to use serial communication over these distances if you keep the baud rate down to around 300 BPS.
    Slow rise/fall times would help limit reflections.
     
    Last edited: Sep 1, 2008
  14. AlexR

    Well-Known Member

    Jan 16, 2008
    735
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    For a run of a couple of miles you are going to need some form of modem. While I am sure there are chips out there to roll your own you are much better off buying a pair of "off the shelf " short haul modems. I'm sure if you look around you will find units that are well under $1000. A quick google threw up this link, I don't know how good it is but for the price you really can't go too far wrong.
     
  15. kammenos

    Active Member

    Aug 3, 2008
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    RS232 would not be enough without an amplifier, even at 300baud. All computers do not really have same output voltage so that would not be reliable.

    A simple amplifier to keep some 15-20 volts may be a good idea. And low baud rate. There will be no problem.
     
  16. hgmjr

    Moderator

    Jan 28, 2005
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    The short-haul modem approach recommended by AlexR looks very attractive. I visited the www.blackbox.com website and there are several models from which you may choose. They are priced at around $100 which seems reasonable to me for the features they provide.

    Since the wire has already been installed by the company, you don't have to worry about that headache.

    hgmjr
     
  17. pal_87

    Thread Starter New Member

    Aug 21, 2008
    3
    0
    cant i use max485 ic with an amplifier.
    I mean the differential voltage is around 3 V in Max485
    but if i take the two pins individually .
    use individual amplifiers to increase the differential voltage and make the difference more like 7-8V and then transmit and scale down similarly at the transmitter. Then is it possible to transmit over such long distances.

    can opamps be used for such purposes...
    can that be done....?
     
  18. AlexR

    Well-Known Member

    Jan 16, 2008
    735
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    I'm sure you could design and build your own modems but is this really what you are being payed to do? Its not just a matter of hooking a couple of ICs together.
    To drive a line that's a couple of miles long you will have to design an amplifier with a frequency/phase response that matches the line characteristics. Get it wrong and you will have high error rates due to ringing and echos on the line.
    Modem designers have already done the hard work for you so why not use the fruits of their labour.
    You complain about the prospect of spending $1000 dollars on a modem but it would cost many thousands of dollars worth of your time to design and build a reliable modem.
     
  19. SgtWookie

    Expert

    Jul 17, 2007
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    The MAX485 would be the wrong IC to even start on long distance communication; it has much too high of a slew rate. Attempting to amplify it's output voltage would make the situation far worse.

    Those series of ICs (MAX481-MAX491) have no built-in method of error correction/detection. Modems at least have the capability of parity check, if such checking is enabled. Even that is of limited usefulness if more than one bit is incorrect.

    Commercial hardware has been designed to take care of that kind of thing for you. Trying to build such a thing yourself will certainly be a learning experience, but it will wind up costing your employer a great deal more than if you had simply purchased suitable equipment to begin with.
     
  20. hgmjr

    Moderator

    Jan 28, 2005
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    I agree with those who counsel against attempting to shoehorn into your design, devices that are not intended for use in driving a signal down a 2 mile length of cable.

    hgmjr
     
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