Appendix 1 – USB 2.0 Questions & Answers
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What is a USB hub? Will USB 2.0 devices work on USB 1.1 hubs and vice versa?
You can connect up to 127 devices to a single USB bus. For more than two devices, you must make new connections using a
peripheral called a "hub.” A hub, which you hook up directly to a USB connector on your computer, usually has 4 or 7 output
connections enabling you to connect the same number of peripherals. One of the hub's main functions is to regenerate signals
that lose part of their strength as they are transmitted via the USB cable. By connecting yet another hub to a connector on the
existing hub, you can then plug in a new set of peripherals and so on, up to a maximum of 127.
You can use your USB 2.0 devices with USB 1.1 hubs, but the peripherals will be limited to USB 1.1 performance levels. In
the near future, USB 2.0 repeaters will be available, able to communicate in three modes: high-speed (480Mb/s), full-speed
(12Mb/s), and low-speed (1.5Mb/s). As USB 2.0 is backwards compatible, you will be able to connect USB 1.1 devices to
USB 2.0 hubs; however, the USB 1.1 devices will still maintain their normal performance levels (i.e. 12 Mb/s).
In practice, users who desire enhanced USB 2.0 throughput will need to connect their USB 2.0 hardware on both ends of a
2.0 hub to achieve the “high-speed” mode. On one end, a USB 2.0 compliant host controller resides on the host PC to man-
age the enumeration process and power management. On the other end, USB 2.0 hardware must be connected directly to
the root hub or through a USB 2.0 hub.
Some hubs have no power supply and others are self-powered. When you choose a hub, opt for the self-powered variety, as
they have their own AC adapter. The most powerful hubs provide 0.5A of power to each port.
If you buy a hub, make sure that it supports per-port switching. This function prevents the entire chain of peripherals from
freezing up if one of them is not working properly or is down.
Important Note
:
Avoid using USB connectors found on certain peripherals such as keyboards. These are passive (or
pass-through) connectors that lead to power loss and unstable operation.
Important Note
:
Only use USB cables shorter than 5 meters (approximately 15 feet). Using longer cables cause the
peripherals to malfunction due to excessive reduction in electrical signal strength.
You will find additional information at the following Internet address:
http://www.lacie.com/usb
LaCie Pocket CD-RW User’s Manual
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