Universal Serial Bus
Wilke Renwick
ITE 100 P. Healy
Biotechnology at it’s finest…
What is Universal Serial Bus?
USB is a single,
standardized way to
connect peripherals to
computers
The USB connectors, usually on the back of the PC,
allow the user to connect many different devices to the
computer from modems to digital cameras.
When was USB developed?
Some of the world’s leading technology industries got
together to develop a standardized way to connect
computers and external devices. In 1995, USB was the
resulting product released at a data transfer rate of 12
mbps.
USB 2.0 now runs at 480 mbps
Why Develop USB?
USB was a new technology developed in
hopes of helping standardize and simplify
the way external devices connected and
interacted with the PC.
Before USB, printers and zip drives both
connected to parallel printer ports. Zip
drives required high-speed connections to
the computer, which provided limited speed
and success. Also, because most
computers only came standard with one
parallel printer port, this was not a very
practical situation.
Why Develop USB? (cont.)
Like the parallel printer port, the serial port was also used by
multiple devices. First and foremost, modems were connected
through the serial port. A variety of other devices such as Palm
Pilots, digital cameras, and some printers, also required the use of
this particular connection which, in most cases, was very slow.
The Goal of USB: Accomplished
The release of Universal Serial Bus eliminated the need for
multiple ports of different sizes and shapes for all of the PC
user’s different devices, therefore rendering the slower parallel
printer ports and serial ports almost obsolete.
Not only does USB standardize the connection of external
devices, it is also very user-friendly, simple, and easy to use.
There are 120+ different devices for sale today that use this
technology including mice, speakers, modems, storage
devices such as zip drives and flash drives, printers, and many
more.
What about limited ports?
The solution to this
problem is simple: buy a
USB hub.
Relatively Inexpensive (as low as $15)
Hubs can be linked, potentially giving you
unlimited USB ports to your computer.
The biggest difference between types of
hubs that is important to know when
dealing with USB devices is between un-
powered and powered hubs.
Powered Hubs
Needed when connecting multiple
unpowered devices such as mice or digital
cameras.
These low-powered devices derive their
power source from the bus.
If too many are connected through a hub,
the computer may not be able to handle it.
Un-powered Hubs
Un-powered hubs can be used with any
number of high-power devices such as
printers and scanners that have their own
power supply, thus not requiring power
from the bus.
Safe to use with low-power devices (mice,
cameras, joysticks, etc.) as long as too
many aren’t connected as once.
Features (www.howstuffworks.com)
120+ different devices can connect to the host, either directly or
through a hub.
The maximum data rate for USB 2.0 is 480 Mbps
Hot-swappable
Many USB devices have a “sleep” feature in which the host computer
and the device go into a power-saving mode.
USB Connections
Just how simple is using USB? Very. Just
“Plug and Play”
If the device is new and has not yet been installed, your operating system will more
than likely auto-detect it and ask for a driver. Then, you simply install the peripheral.
If the device has already been installed, simply plug it in to the computer and they
will begin to interact.
The connection is simple and nearly impossible to get mixed up:
USB “A” Connector: USB “B” Connector: (heads downstream to the device)
(heads upstream towards the computer)
How It Works
The Universal Serial Bus process is simple:
When the host powers up, it queries all of the devices connected to the bus
and assigns each one an address. This process is called enumeration.
The Universal Serial Bus divides the available bandwidth into frames, and
the host controls the frames. Frames contain 1,500 bytes, and a new frame
starts every millisecond. During a frame, isochronous and interrupt devices
get a slot so they are guaranteed the bandwidth they need. Bulk and
control transfers use whatever space is left (at least 10%).
The Three USB Data Transfer Types
Interrupt - A device like a mouse or a keyboard, which will be sending
very little data, would choose the interrupt mode.
Bulk - A device like a printer, which receives data in one big packet,
uses the bulk transfer mode. A block of data is sent to the printer (in
64-byte chunks) and verified to make sure it is correct.
Isochronous - A streaming device (such as speakers) uses the
isochronous mode. Data streams between the device and the host in
real-time, and there is no error correction.
As devices are enumerated, the host is keeping track of the total
bandwidth that all of the isochronous and interrupt devices are
requesting. They can consume up to 90% of the 480 Mbps of
bandwidth that is available. After 90% is used up, the host denies
access to any other isochronous or interrupt devices. Control packets
for bulk transfers use any bandwidth left over.