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USB Cable length?


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What is the limit on length of USB cables? Does it very depending on use and if it is supplying power? My real need for a long cable is when using a USB keyboard controller. Last night I finally stuck a 10' extention onto a 6 foot cord for a total of 16'. It seemed to power my controller just fine as well as transmit MIDI data. I'm not sure how that would be for Audio, or a dense MIDI stream.



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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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