Understanding USB Limitations

If you installed the USB device's software correctly and you are using the most up-to-date version of the driver, you still might have problems because of USB's physical limitations. Limitations that can cause problems include:

• Insufficient power Many USB devices receive power from the USB port. Connecting too many unpowered devices to a USB hub can result in a power shortage, which can cause a USB device to not respond properly. This is particularly common when using an unpowered external USB hub. To quickly determine whether a problem is power related, disconnect other USB devices and connect each USB device directly to the computer one by one. If devices work when connected separately but fail when connected simultaneously, the problem is probably power related. Decrease the number of devices or add a powered USB hub.
• Excessive length USB devices can be no more than 5 meters (16 feet) away from the USB hub to which they are connected. Although USB devices will never ship with cable longer than 5 meters (16 feet), some users connect USB extenders to allow longer distances. Depending on the quality of the cable and possible sources of interference, you might experience problems with shorter distances. To determine whether length is the source of problems, remove any USB extenders and connect the USB device directly to the computer.
• Too many devices USB can support up to a maximum of 127 devices connected to a single USB host controller, which is more than enough for the vast majority of client computer scenarios. You can have a maximum of seven layers of USB hubs connected to the computer's USB host controller, and no more than five external hubs.
• Insufficient bandwidth Most USB devices are designed to work within USB bandwidth limitations. However, video cameras in particular might need more bandwidth than USB is capable of providing. If you receive a "Bandwidth Exceeded" message, first try disconnecting other USB devices. If the message continues to appear, attempt to reduce the bandwidth used by the device by lowering the resolution of the camera. For best results with a video camera, connect it to an IEEE 1394 (also known as Firewire or iLink) port.

Note If you see the message, "Hi-speed USB device is plugged into non-hi-speed USB hub," the USB device is USB 2.0, but the USB port is an earlier version. The device will probably work, but it will work slowly. You can improve performance by adding a USB 2.0 port to the computer.