Number 225 - February 2002

USB Cable Length
by Keith Aleshire, January 2002 The Digital Viking, Twin Cities PCUG
Recently, I had to string a USB cable about 20 feet to reach and X-ray camera in a dentist's office. Unfortunately, USB cables are typically 6 to 19 feet. In practice, the USB specification limits the length of a cable between full-speed devices (e.g. a scanner) to 5 meters (a little under 16 feet 5 inches). For a low-speed device (joystick or mouse) the limit is 3 meters (9 feet 10 inches).     Why can't you use longer cables? The electrical design of USB doesn't allow it. When USB was designed, a decision was made to limit the electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable. Since USB was intended for a desktop environment, the limited range was considered acceptable.     So how far away from a PC can you put a USB device? With the maximum of 5 hubs connected with	14-foot cables and a 14-foot cable going to your full-speed device, this will give you 80 feet of cable. With a low-speed device, you will be able to be able to get a range up to 80 feet, depending on how long the device's cable is. With a straight-forward cable route, you will probably be able to reach out 75 feet or so from the PC.     If you need to put a USB device a considerable distance from your PC, buy a bunch of USB hubs and connect them serially with 10 foot 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 computers together.
Number 225 - February 2002