USB cable length?

[Tahnok]

I read somewhere that USB cable has a 6' maximum before it becomes unreliable. But I just realized that my web cam has a 12' USB cable! I was wondering if it would be possible to extend a gamepad hack to 12'.

My cab is across the room from my computer, right now I send a 15' length of wire from the PS/2 port without any problems. 12' would be enough, though it would be nice to have it a bit longer.

I was also wondering if USB cables must be manufactured in a certain way. I know certain network cables can not be spliced without messing them up, is USB similar? If so, does anybody know where to get the cabling and a good guide to working with it?

Thanks,
Tahnok100

[SideShow Rob]

From the USB.org FAQ:

" Q1: How long of a cable can I use to connect my device?
A1: In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).

Q2: Why can't I use a cable longer than 3 or 5m?
A2: USB's electrical design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment, the range limitations were deemed acceptable. If you're familiar with transmission line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.

Q3: How far away from a PC can I put a USB device?
A3: With the maximum of 5 hubs connected with 5m cables and a 5m cable going to your full speed device, this will give you 30m of cable (see section 7.1.19 for details). With a low speed device, you will be able to get a range up to 27m, depending on how long the device's cable is. With a straightforward cable route, you will probably be able to reach out 25m or so from the PC. "

Hope this helps..

[daywane]

I watched Patrick and Leo on screen saver extend over 100feet.
I wish i could remember how they did it but they had web cams running on both ends of the building
try

[IntruderAlert]

I saw that too.
They just kept hooking USB extension cables together and it still worked fine.

[IntruderAlert]

USB extension limits
Tom in Arlington, Virginia, took time out from his duties as a TechTV Chat moderator to give us a call. All hell broke loose in the chat room, but Tom really wanted to ask about the cable limits of USB. He wants to put a netcam in his backyard, which means he needs a connection that's about 100 feet long. Can he do this?

USB has a specified cable length limitation of 5 meters (16.4 feet). This doesn't mean that USB won't work on longer cables. It just means that 5 meters is the "guaranteed" length that USB will work. Go longer than 5 meters and you might encounter problems.

Back in January, we did an experiment on the show to see how long we can go with our USB cables. We used all of our USB cable extenders and got to a length of over 17 meters, and our USB connection was still working.

Leo recommends using passive USB extension cables until you get to your desired length. It may still work. You can also try using optical USB extension cables, but they're pricey ($165 to $245). You can also try using a USB extender, but they cost $299 to $369.

Thanks for calling, Tom, and thanks for helping out in the chat room. Now get back in there before we lose complete control of chat!

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USB Limits
      

How far can you go with USB?
By David Prager
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   Video Highlight   
   The Limits of USB


USBQ: Do you know if there is a USB cable length limit if you only hook one device to it, such as a netcam?

-- Richard, via email

A: According to the USB Implementers Forum (the designers of the USB spec), USB cables have a range of optimal operation no longer than 5 meters. In fact, they go out of their way to avoid approbation of using any kind of signal repeater to create a longer cable.

Of course, we decided to give it a shot anyway. We've overclocked far too many processors here on "The Screen Savers" to not test out USB's limitations.

First, we connected a generic USB netcam with the 10-foot cable it came with, and turned on the video so that we could see it on the monitor. The video looked great.

In order to create a longer cable, we used Belkin's 6-foot Pro Series USB extension cable (sells for under $20). This made our camera 16 feet long (equivalent to the 5-meter limitation), with the video still looking great.

Luckily, we still had several more of these extension cables at our disposal. After systematically hooking each cable up, we finally exhausted our supply, yielding a USB cable 56 feet long (17.07 meters). The picture still looked great, although the video's appearance on our monitor exhibited more latency than our original connection.

A low powered, low bandwidth single USB device such as a netcam can function at significantly longer distances than the spec designates, although no company would assume the liability of recommending it because of the potential problems you can encounter (much the same way no company recommends trying overclocking).

For more taxing USB devices, more expensive products exist that can extend the range of USB. One example is QVS's Active Repeater extension cable. Unlike the passive Belkin cables that we used, an active extension will actively regenerate signals to prevent data errors. These cables boast effective operation at up to 80 feet.

The best and most expensive option is using a product such as Icron's $270 USB Ranger that uses Ethernet to increase USB's range to 100 meters (328 feet).

Here is the USB Implementers Forum explanation to USB cable limits:

    The [USB] cable length was limited by a cable delay spec of 26ns to allow for reflections to settle at the transmitter before the next bit was sent. Since USB uses source termination and voltage-mode drivers, this has to be the case, otherwise reflections can pile up and blow the driver. This does not mean the line voltage has fully settled by the end of the bit; with worst-case undertermination. However, there's been enough damping by the end of the bit that the reflection amplitude has been reduced to manageable levels. The low speed cable length was limited to 18ns to keep transmission line effects from impacting low speed signals.

More questions about the USB spec can likely be answered in this FAQ.

Posted January 29, 2002

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[NiN^_^NiN]

15 feet is the max before it's deemed unreliable

i have a PC in the car and i'm running 15feet od usb cables for my touchscreen gps and flash reader all on a non-powered usb it runs fine

[Tahnok]

Hmm, I really don't know how I missed USB.org in my google search.

It sure looks like distance won't be a problem, but what about making the cable? I looked through the entire site and didn't see anything about severing the wire. I hope this means that it is not a problem, but it could also mean that it just doesn't come up.

[cholin]

Look for a USB wiring diagram.  I know that a USB has four wires, black, red, white and green.  I was told that black and red are negative and positive, so white and green are prolly clock/data

[ShinAce]

Close. USB uses a kind of differential data lines, so the color codes are:
black(sometimes doubled up) =  gnd
red = vcc (5 volt)
white = data+
green = data-

The + and - might be inverted, but that's the basic code. And yes, you can splice 2 cables together to make a single long cable.

[cholin]

Aww I was close lol.  I knew the red and black though because Im making a little powered light through the USB port   Good luck though with your project.

[NiN^_^NiN]

just buy some USB Extension cables.

I have to in my car to make the distance and they just plug into each other and they don't come apart at all

u can glue the suckers together to make sure they wont

[Tahnok]

And yes, you can splice 2 cables together to make a single long cable.

That is what I hoping to hear. Thanks!

Thanks for the color codes too, I didn't realize there was a standard. It will be greatly useful since I will be tapping power off the USB port. BTW, one more thing, does anybody know the maximum current and voltage that a USB port can supply? All the website says about it is that it will supply what is needed, but everything must have a limit.

[NiN^_^NiN]

I also think people have made USB cables out of cat5e to

[Trimoor]

5v@500mA is the max.  There is also a low current standard for bus powered hubs, but the main ports will all source 500mA.  Beware that longer distances will cause extra resistance in the cable, lowering the maximum current draw.

It's not a good idea to splice a USB cable.  It will lower the tolerances, resulting in shorter working distances.  Use extention cables (they are extremely cheap online), and tie a knot where the two plugs join, like people do with 120v extention cords.

There's not much of a chance you will damage anything (unless you stupidly splice the power to the data line), so go ahead and experiment.

[IntruderAlert]

Here.. just use this:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=44934&item=6750617502&rd=1

[ShinAce]

USB is expected to supply something like 100mA per port.
If you want more than a watt, use a relay.

[Tahnok]

It looks like it should be able to pull off what I want it to do then. I'm going to try tapping LEDs off the USB for my CP. I know this has been done before, but I am going to be running quite a few and wanted to be able to run the numbers.

I have been working on the plans a lot more lately, and I think I will only need to splice the cable in one point. And even then, I may be able to wire it directly to the PCB instead of just mid-wire.

Thanks for the help everyone! I am now much more confident in moving forward with my project!

[IntruderAlert]

well make sure to post some pics
i'd like to see it when you're done

[Tahnok]

Well, the lighting won't be done until I can get some transparent buttons. I'm hoping those transparent buttons come through so I can get a few of those.

[rdagger]

You can definitely push the USB length a little.