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Will USB 1.1 devices run faster when connected to a USB 2.0 bus?
Unfortunately, no. The USB 2.0 specification is specifically written to allow
developers to design higher-speed peripherals that can take advantage of the
extra bandwidth. USB 1.1 devices, though, will still operate at 12Mb/s at
full-speed and 1.5Mb/s at low-speed on a USB 2.0 bus. Even though USB 1.1
devices won’t run any faster, they can work alongside USB 2.0 devices on the
same bus. However, if you plug in a USB 2.0 device to a USB 1.1 bus, the
speed of the USB 2.0 device will decrease to 12Mb/s.
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
backward 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 manage 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.
