USB 2.0
What is USB?
USB means „Universal Serial Bus“ and is a industrial standard to plug peripheral
devices to the computer.
In comparison with normal interfaces, USB has a very well performance and gave the
advantage of “hot-plug” and “hot-unplug”.
Another advantage is the 5-Volt voltage bus power.
So many USB-devices don’t need a separate current supply, so you only have one cable
the USB-cable.
USB was developed by a group of computer and telecommunication-companies and 1995
introduced in the version 1.0. Today USB is standard and sonly every computer will be
delievered with two USB-interfaces. Peripheral devices like mousse, keyboards, modems,
printers, scanner and so on, you find in great quantities.
Hubs
A USB-Hub is a device that can connect 4 resp. 7 additional USB-devices, about one
cable. By combining several Hubs it is possible to add till 127 devices.
The Hub on the one hand work as repeater and on the other hand as data-transmitter.
Data transfer
When the computer exchange data with one device that pass by a direct
connection(named pipe)
Over this connection the data can flow in four types:
- the control transfers serve co-operation between computers and the USB-device. You
dispatch these data, if you configure a device. Or the device displays that it is available.
- The bulk-transfers are used, if large quantities of data go over the line. That is for
example the case with printers or scanners. With such data communication all available
transfer capacity is used. So it can happen that the data line is clogged then the transfer
simply waits, until there is any place.
- Interrupt transfers are in demand with small, regularly occuring quantities of data. The
PC gets in fixed distances data of keyboard or mouse.
In these three transfer possibilities, there is always a error correction.
Isochronous transfers transmit the data continuously in regular intervals. It doesn't give
any error correction.Language and music is typical for these transmittal mode.
For a good sound it is very important to transfer the data immediately, but an error is not
so bad, because in the worst case it rushes.
How USB prevents mistakes
There is one protocol that prevents mistakes mistakes between an USB-device and the
computer. This protocol controles who will get which data and that all data will be
received correctly. When data will not be send in the right way the protocol arranges a
new transfer.
The protocol puts the data in packages. There are two different types of packages: One
transfers control information and the other one transfers the data. But both are built
equally:
The first part of the package exists of an information, called token, which
specifies the data in the package.These are normal data, which are for example
send between a computer and a printer, or internal controlling information, e.g.
the address of the following data package.
This address specifies to which one of the 127 devices the data will be sent. USB
can additionally save up to 16 sub addresses within each device (one is reserved
by USB)
The next one is a checksum that helps the receiver to check if all data are received
correctly.
When all address information are sent the first data package can follow. The protocol
announces a package and its content - the data that could be up to 1023 bytes. Then the
next checksum follows.
After the data package a handshake follows. There the receiver tells the sender if he
accept the data. If the checksum would not be correct the device will refuse the
handshake and the package has to be sent once again.
Within the isochronous transfer of speach data no handshake exists because there is no
error correction.
The appearance of the cables
Every USB cable exists of four wires. Two of them transfer the data and two of them
provide the connected devices with power.
There are two kinds of USB cables. For fast transfers with 12 MBits the cables are
twisted (Twisted Pair) and isolated.
Devices which work with slower 1.5 MBits (192 kByte per second) need other inds of
cables. They also have four wires but the two data cables are not twisted. Besides that the
cables are not isolated in a special way and every time connected with the devices. These
kind of cables are e.g. at mouses, joysticks or keyboards. These devices don't need high
speed transfer rates. These slower cables have a maximum length of three metres because
of their bad shielding.
What is USB 2.0?
USB 2.0 is the next-generation, high-speed interface to be embraced by the PC industry.
Major leaders in the PC industry support USB 2.0 such as Apple, Compaq, Dell, Hewlett-
Packard, Intel, Microsoft, NEC, Panasonic, Sony and Toshiba and are members of the
USB Implementers Forum.
Q: Why use USB 2.0?
It's a very fast connection standard that speeds up the performance of your peripherals
like dramatically decreasing the amount of time it takes to burn a CD. With maximum
data transfer rates of up to 480Mbps, USB 2.0 technology is forty times faster than the
USB 1.1 specification and also maintains backward compatibility with all USB 1.1
devices. USB 2.0 is also user-friendly because it offers "plug and play" features that
allow you to connect/disconnect devices without restarting the system and provides the
ability to connect up to 127 devices
Will USB 2.0 replace USB 1.1?
Not entirely, because many products such as generic keyboards, mice, joysticks and audio speakers do not
require the faster speed of the new USB 2.0 technology. Only bandwidth-hungry devices, such as webcams
and high-capacity storage systems, will need all the speed. However, next-generation systems will come
with USB 2 ports rather than USB 1.1.
What happen if
The entire bus under the USB 1.1 root hub will slow to 12Mbps. The operating system will probably notify
the user about the sub-optimal configuration and recommend for a better course of action.
If several USB 1.1 hubs are connected to a USB 2.0 bus, then each of the USB 1.1 hub will get a full
12Mbps bandwidth.
What is the max. length of a USB 2.0 cable?
5m. However, if you cascade 5 hubs with 5m USB cables, this will allow you to connect a device 30m
away.
What do I need to use a USB 2.0 device?
The requirement is similar to that of USB 1.1, but all components will have to be USB 2.0 compliant. A
successful USB 2.0 connection requires a USB 2.0 host controller card, a USB 2.0 driver and a USB 2.0
peripheral.