The USB Interface
A USB interface has three essential components: a host, a hub, and peripheral devices. USB host A PC is the USB host device that carries the operating system,
chipset, and BIOS that support the USB interface.
USB hub USB interfaces can be built in a tiered fashion. A hub can be plugged
into the host. Other hubs can be plugged into that hub and USB devices can be
plugged into each of the second-tier hubs. As long as the whole bus has only
127 devices, including the hubs, there should not be any problems.
USB devices In most cases, you will have only one or two USB devices
plugged into your PC, and these will be directly connected to the PC itself.
However, as described in the preceding bullet, USB devices can be connected
to hubs as well. In fact, if a PC has two USB ports, one can have a directly
connected device and the other a hub.How USB Works
When a USB device is plugged into a USB port, the host or the hub detects a change in the
voltageontheinterface.Thehostasksthenewdevicetoidentifyitself,aprocessUSBcalls
enumeration. The device replies with its type, its manufacturer, what it does, and the
amount of bandwidth it requires. The device is given an address code that identifies it
uniquely from any other USB devices already on the bus. Each USB device attached to the
bus, even two of the exact same device, gets a unique address ID so it can be referenced
and addressed by the host.
Once the device has its ID, its device driver is loaded. If one cannot be found, the user
is asked to supply a disk or CD-ROM with the driver. Unlike a serial or parallel port, any
resource conflicts are resolved by the host, which frees the user from configuring IRQs,
I/O addresses, or DMA channels. Each USB channel uses only one set of system re-
sources. If the USB port is supporting more than one device, the devices all share the sys-
tem resources of the USB port. When a USB device is unplugged from the system, the
reverse takes place. Once again, the host detects the voltage difference, retires the address
ID, and notifies the operating system to unload the device driver.
For more information on the Universal Serial Bus, visit the official USB homepage at
http://www.usb.org.The FireWire Interface
Another of the newer high-speed serial interface buses is the IEEE 1394 standard that de-
fines a serial bus protocol with data transfer speeds of between 100Mbps to 400Mbps
(around 12 to 50 megabytes per second). Newer versions of the 1394 standard, which are
being developed by the 1394 Trade Association (www.1394ta.org), will provide data
speeds of 800Mbps to 1.6Gbps.(^496) PC Hardware: A Beginner’s Guide