USB Cable Length Limits

USB or the Universal Serial Bus was developed by Ajay Bhatt, an Intel employee as an interface that allows compatibility between external devices and host system, mostly a computer. It functions as a replacement for many serial ports (transfers data one bit at a time) and parallel ports (transfers data bits at the same time). A USB port allows you to connect peripheral computer devices like mice, keyboards, printers, USB flash drives, hard drives, etc. to the host system. Today USB has become almost a standard interface on most systems including computers, smart phones, DVD players, and the likes. There are several different USB cable types, which have different plugs and receptacles therefore allowing you to connect to a variety of peripherals. With its widespread usage, one of its main drawbacks is the USB cable length limits that are stipulated by the manufacturers.

Length Limits of USB Cables

One of the main problems that many people face with USB devices are the length limits of USB cables. USB cable length rules stipulate the length of the cable at 5 meters or 16 feet for USB 2.0 devices and 3 meters or 9.8 feet for USB 1.0 devices. The reason for these USB cable length limitations is that the electrical design of the USB model does not allow for cable lengths that are longer than 5 meters at the maximum. This is because with a USB connection if the information or host commands remain unanswered within the allowed time of 1.5 microseconds, the command is considered lost. Even after allowing for the maximum number of hubs the acceptable delay per cable only accounts to about 26 nanoseconds. USB was always intended to be used for desktop computers when they were designed and therefore, range limitations were accepted at large by users. The limitation was related to the electromagnetic field propagation that needed to be handled on the USB data lines. This causes a limit to the USB cable lengths. USB 3.0 which was unveiled this year, has no cable length specification but since it uses the same copper cabling as previous versions the length limits for USB cables are still pretty much in place. The only possible development on the cable length limits can be expected with a fiber optic cable design development.

Bypassing Length Limits for USB Cables

The one way to bypass these levels for length set for USB cables and meet USB cable length requirements is to use the maximum permitted USB hubs in a series. According to USB 2.0 specifications, the maximum number of USB hubs that can be connected to increase the range of the USB is 5. This will allow you to increase the USB cable length limits to about 30 meters for USB 2.0 devices and 15 meters for USB 1.0 devices. The distance between each USB hub is specified to be not more than 5 meters. Another option that you can use is to connect five active extension cables together or four cables and a USB hub together which will again allow you to increase the length limit of the USB to about 30 meters. To connect your PC to a USB device, in accordance to the limits of USB cable length, map the area and then decide how you can use either extension cables or USB hubs to connect the devices.

While the length limits for USB cables do act as a severe drawback where USB is concerned, it has in no way affected the near standardization of the interface. In fact, it has forced people to develop alternative methods to better the length depending on their USB cable length requirements.