Figure 2. USB topology.
Figure 3.A USB hub.
each end of the cable to permit the detection of
attaching and detaching a device at each port; they also
differentiate between the data rates of the signals.
As you see in Figure 5, the standard USB cable comes
with “A” and “B” plugs at the ends. The “A” plug is always
connected toward the host — or upstream. The “B” plug is
connected toward the USB device — or downstream. Figure
5 also shows the receptacles for the “A” and “B” plugs.
attachment ports for devices. Peripherals like printers,
keyboards, scanners, drives, cameras, and others are
attached to the USB ports, each of which has a status bit
that is used to report the attachment or removal of a device.
The host retrieves these bits from the hub and — if the
status bit indicates an attachment of a USB device or
another hub — the host will enable the port and give an
address to the device. Then the device becomes ready for
operation by the USB system. If the status bit indicates the
removal of a USB device, the hub disables that port and
tells the host that the device has been removed from the
port. If a hub is removed from a port, the USB system
software will remove all of the USB devices that were
previously attached to the system through that hub.
Two kinds of USB peripherals can be attached to the
ports: a bus-powered device that relies totally on the power
from the USB cable and a self-powered device that
supplies its own power for its operation. A hub also
supplies power to the USB devices connected to its ports.
USB Interface
The interface is the manner by which the USB devices
connect and communicate with the host. The physical
topology of the USB host and devices is shown in Figure
2. As you can see, you are only allowed to attach five
external hubs in series due to the hub’s timing constraints
and cable propagation delays.
A USB cable is used to connect the peripherals, hubs,
and host together. This four-wire cable is used to transfer signal
and power in a USB system (Figure 4). A twisted signal pair
is used on each point-to-point segment for the signaling data
rates of high speed (480 Mbs), full speed ( 12 Mbs), or low
speed (1.5 Mbs). The diverse data rates allow a number of
different bandwidth devices to be attached to the system.
The other two wires are used to deliver + 5 V DC power
to bus-powered devices. The USB cable can have variable
lengths of up to several meters. Terminations are used at
MAY 2004
USB Data Flow
We will now look at how data is moved across the USB
system. To get a better understanding of this concept, we
will look at the USB in a layered fashion. A simple view of
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