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USB Cable length?


RABid

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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.

 

Robert

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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.

"Music expresses that which cannot be put into words and that which cannot remain silent." - Victor Hugo
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