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I want to use a very long USB cable to place a webcam in a different room. The maximum length that I found was five meters.

  1. What is the maximum length of a USB cable?

  2. What is the maximum length achievable when using USB extension cables?

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As an alternative, there's a 10m wireless USB option. –  hyperslug Feb 13 '11 at 20:16

2 Answers 2

up vote 24 down vote accepted

5 meters is the maximum USB cable. You can get further by connecting USB hubs serially (maximum of 5 hubs chained this way). You can't just use USB extension cables due to timing issues.

From the USB 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.

Q4: I need to put a USB device X distance from my PC. What do I do?
A4: If X is less than 25m or so (see previous question), buy a bunch of hubs and connect them serially with 5m cables. If you need to go farther than that, put another PC, or maybe a laptop, out where you need the device to be and network it with the first PC using something that's intended to be a long-range connection, such as Ethernet or RS-485. If you need to use nothing but USB, consider using USB based Ethernet adapters to hook the PCs together.

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Just FYI, it's not really a timing issue, since using hubs doesn't mitigate the signal's propagation time. Rather, the maximum cable length is due to line capacitance/inductance issues, and the signal's voltage loss along the length of the cable. If the cable was too long, the digital logic levels might not be triggered properly at the receiving end. Each hub "resends" the signal so-to-speak, which is why this solution works. –  Breakthrough Aug 9 '11 at 16:21
    
@Breakthrough No, the signal voltage is easy enough to amplify; I think it is about the timing on the single link. The problems from capacitance and inductance are - if you look closely - exactly the timing issue; The signal is spread in time direction. But even the latency can be relevant inside the link protocol; It get's much easier if you can use a full round trip for each byte you transfer - no need to keep track what may need repeating on a glitch. And the latency over five hubs is just the same for the device to computer link ;) –  Volker Siegel Aug 29 at 7:02

if you want to go beyond 5 meters, use a booster.

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interesting. works by using CAT5/5e/6 instead of USB cabling for the long haul: "This plug-and-play device consists of a Base Unit and a Remote Unit connected together by a standard Cat5, Cat5E, or Cat6 network cable. Simply connect your cables to your device; then connect to your computer and USB peripheral and you're up and running. The Super Booster USB Extender has internal DC power conditioning circuits to maintain the proper voltage to the remote device so no external power is required for either the Base or Remote units." –  quack quixote Nov 3 '09 at 1:44

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