What is the maximum length of a USB cable?

Written by maxwell payne
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What is the maximum length of a USB cable?
(Julo, Wikimedia Commons)

A USB cable connects a wide variety of devices to a computer or laptop. Some examples include MP3 players, printers, keyboards, computer mice and external hard drives. While the USB cable is quite versatile in its uses, it does have limitations. The primary limitation of the USB cable is that it is restricted to a maximum length for a connection.

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Maximum Lengths

According to USB.org, USB operating specifications limit the maximum length of the cable to 5 meters for full speed devices and 3 meters for any low speed devices. 5 meters is around 16 feet 5 inches, and 3 meters is around 9 feet 10 inches.

Full Speed vs. Low Speed

Full speed devices are devices that typically require an external power supply and are sending and receiving larger amounts of data. Low speed devices can be powered by the USB connection itself. An example of a full speed or externally powered USB device would be an external hard drive. An example of a low speed or USB-powered device is a USB mouse or an MP3 player.

Reason for Length Limitation

The electrical design of a USB cable is the primary cause for the length limitation, according to USB.org. Both the design regarding electromagnetic fields and the concept of the transmission line theory contribute to the reasons for limiting the length of a USB cable.

Essentially the transmission line theory states that the electrical wave must start from one end of the parallel conductors that make up the transmission line inside the USB cable and travel down the line. If the wave isn't sent out in this method, electrical energy can leak out from the line and disperse into the surrounding area.

Solutions to Limitation Problem

If a USB device needs to be connected to a computer but the distance is further than the maximum length of the cable, there are a few solutions to extend to connection.

For a shorter distance that needs to be reached, a USB hub can be connected to the USB port and placed as far away from the computer as the hub's USB connection line is. Any devices can be connected to the USB hub using a 3 to 5 meter long USB cable.

For longer distances up to 30 meters a line of USB hubs and USB cables can be connected together leading away from the computer. The first hub is connects to the USB port on the computer. A USB cable connects to the first hub and is connected to the second hub. This chain repeats for up to 5 USB hubs and 30 meters of USB cable to create a connection to a USB device that is further away.

Warning

Two computers can connect directly via USB connection. However this kind of connection must be combined with a USB to USB adaptor that connects the two USB cables together between the computers. Directly connecting a USB cable between two computers is a fire hazard and can short out the computers. An adaptor must be used to handle this kind of direct connection.

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