What's the limiting factor in USB cable length?

Discussion in 'General Electronics Chat' started by moeburn, Sep 7, 2013.

  1. moeburn

    Thread Starter Member

    Aug 16, 2013
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    So I want to be able to use my USB webcams as security cameras, without having to have a small laptop or RaspPi within 3-5 metres of each webcam.

    What exactly makes the USB cable length limit so short? Is it the quality of the cable they use? Or the type of signals they transmit? Or some combination of both?

    I have several 5m USB extension cords. Individually, they all work fine with my webcam (which itself also has a 5m USB cable). But if I try and patch two or more of them together, the webcam is unrecognized by the operating system. But the other day I noticed that if I put a powered USB hub in between one of the cables, it works just fine! And the usb hub's cable itself adds another foot, bringing the total to 10.3m! So clearly the USB hub is doing some amplifying of the signal.

    So why is it that USB can only transmit up to 5m, but ethernet Cat5 can transmit up to 100m? Is it only the difference in quality of cable, or also the difference in type of signal?

    I ask because I'm wondering if I can just make a super-long USB extension cord out of a usb socket, a usb plug, and a whole bunch of high-quality Cat5e ethernet cable. I'm not assuming I'll get a whole 100m out of it, but something like 20 or 30m would be nice. For starters, I've noticed that every single USB cable I've ripped apart has used 26 AWG wire, while every single ethernet cable I've seen has used 24 AWG wire, some even going as big as 23 or 22 AWG. So that's promising.

    Ultimately, I'm going to try it anyway, but I figured I'd let someone more scientifically informed than me about this kinda thing chime in, while I try to figure out how the hell to desolder a USB socket off an old device (it's really difficult!).
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
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    5 meters is built into the USB standard. Exceed it and you get timing issues no matter what kind of wire the cable uses.

    Hubs have active elements that receive, decode, process, and retransmit the signals so they start as a new cable so you can string together several hubs to get somewhat further. AFAIK there is a limit (five?) on how many series hubs will work too.

    There are translators out there to convert USB to a signal type designed to go long distances to make a long haul, or even go wireless. (One example of both a unit and vendor I have no experience with.)

    Bottom line is using your inexpensive web cams may be the most expensive way to go.
     
    PackratKing likes this.
  3. Papabravo

    Expert

    Feb 24, 2006
    10,137
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    The short answer is that any cable or transmission line looks like a distributed series inductance and a distributed shunt capacitance. They are measured in Henries per meter and Farads per meter. Distributed components behave differently than lumped components and their properties are frequency dependent. The primary frequency of interest is not the frequency of the data, which for USB 2.0 is either 1.5 Mb/sec, 12 Mb/sec, or 480 Mb/sec, but the rise and fall time of those signals. Reflections from unterminated transmission lines will make hash of any sufficiently fast digital signal.
     
    Metalmann likes this.
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    you can get some cheap ($10-15) USB to ethernet adapters
     
  5. BReeves

    Member

    Nov 24, 2012
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    Ernie's link is to CAT5 converters for $10.70 ea. if you buy them two at a time. Times have really changed, several years ago we were putting point and shoot cameras on a rock climbing wall and the only thing available at the time was over $100.00 a pair.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    You can also get usb boosters to extend range. They are often called active usb extension cables, since they need power.
     
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