The Basics of USB  

What is USB? 

USB, which stands for Universal Serial Bus, has been quite successful in 
the computer market in the last couple of years. It started to show in PCs 
sometime in 1996 and started to really take off in 1998. USB is another way 
to connect external devices to your PC. Popular examples for USB devices 
are printers, scanners, keyboards, mice, joysticks, UPSs, speakers, 
monitors, network kits, Zip drives, digital cameras, modems, Personal 
Digital Assistants (PDA) and more. 

Every PC comes already with at least one serial port and a parallel port to 
connect devices such as modems, mice, printers, etc. to your computer. The 
disadvantage of those two are speed as well as expandability: The serial 
port can only transfer one bit of data at a time, it transfers data as a 
series of bits, hence the name. The parallel port can only transfer 8 bits 
of data at a time. A PC comes only with a limited number of serial and 
parallel ports, therefore you're out of luck once you used them all up. 
Another inconvenience of serial and parallel ports are the fact that in 
order to connect a device to them, you have to turn off your computer, 
connect the device, reboot, then manually install the driver. USB was 
designed to eliminate all those shortcomings. 

Speed 

USB has two data transfer modes: The low speed mode transfers data at a 
rate of 1.5 Mbps, and is used for slow devices like keyboards, mice and 
joysticks. The high speed mode, used for printers, scanners, etc. can 
transfer data at a maximum rate of 12 Mbps, making it a lot faster than 
serial and parallel ports. For some applications USB might not be the best 
solution though. For example, you would use SCSI to connect fast drives, 
and Ethernet for 100Mbps networking. 

Expandability 

All new ATX style motherboards now come with two USB ports right on the 
board, I have also seen some boards that come with 4 USB ports. But with 
USB you can chain many devices, theoretically up to 127. You'll probably 
run out of desk space and power plugs long before you max this limit out. 
And you can mix and match devices of all types. The downside is that most 
older PCs do not have a USB port, but you can add them via a PCI expansion 
cards. 

Plug and Play 

One great thing about USB is that it is fully Plug and Play compatible. 
Installing a USB devices is extremely easy and the closest to Plug and Play 
as anything has ever come. Plug in the device, install the driver, and 
you're ready to go. An even nicer feature is that USB is hot-swappable, 
meaning you don't even have to turn your computer off to install a new USB 
device. Plug it in while Windows is running. As soon as the device is 
connected, Windows will detect it, prompt you for the driver, install the 
device and make it available to you to use. With USB, there are no jumpers 
to set, IDs to select, or IRQ conflicts to resolve. And after installing a 
USB driver for a USB device once, you normally won't have to reinstall the 
driver, even if you disconnect and later reconnect the device. 

Things you need to know about USB 

While USB is great for the reasons explained earlier, there are several 
things you need to be aware of in order to use USB devices successfully and 
to avoid damage to your computer equipment or malfunction of the devices. 

Hubs 

In order to use more than one or two USB devices to your PC, you need a USB 
hub. You can combine multiple hubs to cover all your USB port needs. Most 
USB hubs have 4 or more ports and cost between $20 and $40. There are two 
types of hubs: unpowered and powered. To understand the difference, you 
need to know that USB devices still need power in order to operate. Some 
devices like keyboards and mice only need a very low amount of power, which 
they get from the USB port through the motherboard. Other USB devices, 
however, need a lot more power, such as printers and scanners. If they 
would use the USB port as their sole power source, it would drain too much 
power from the motherboard and could lead to flaky system behavior all the 
way to system failure. For that purpose, high-power USB devices come with 
their own separate power source, usually an AC adapter. If you buy a USB 
hub to connect multiple USB devices, even if they are low-power ones, you 
should purchase a powered USB hub, meaning a hub that comes with its own AC 
adapter and therefore supplies all USB devices with power from its own 
source instead of draining it from the computer and putting an additional 
load on the power supply. 

Operating System 

There are only certain operating systems that offer support for USB 
devices. You won't be able to use USB with DOS, Windows 3.1 or Windows NT 4 
or the original version of Windows 95. While Windows 95b and c offer USB 
support, it is not very reliable and often does not work at all. Also, a 
lot of times the installer for software used by USB devices will first 
check to see if you have an OS that supports USB and refuses to install if 
it does not find a qualifying OS. You need to have either Windows 98, 
Windows Millenium Edition, or Windows 2000 in order to successfully use 
USB. Most newer distributions of Linux offer USB support as well but check 
first to make sure. The reason for this is that the OS needs to be able to 
support and use the USB port. The drivers you install for each device are 
solely for the device itself and cannot add USB support to the OS. 

Hot-swapping 

While USB is hot-swappable, meaning you can connect and disconnect USB 
devices while the computer is running, you still need to exercise caution 
and make sure you never connect or disconnect a USB device while data 
transfer is in progress. This can result in damage or at least corrupted 
data. 

USB keyboards and mice 

Keep in mind that since USB devices normally won't work in DOS, you won't 
be able to use your USB keyboard or mouse when booting into a pure DOS 
environment unless your PC has a motherboard that offers USB support in the 
BIOS. This means that if you ever have to boot from a startup disk, or 
enter the BIOS to make changes, or use DOS versions of programs like Drive 
Image or Partition Magic, you can't use a USB keyboard or mouse. For these 
situations you might want to check your motherboard/BIOS documentation to 
find out whether it offers USB support in the BIOS. If not, you should keep 
a spare non-USB keyboard and/or mouse handy.

Cable length 

You can buy extension cables for USB if you need to connect a printer or 
scanner that sits across the room. However, don't forget that signal 
quality degrades the longer the cable is. For that reason, there are 
limitations on cable length for USB devices, just as there are for network 
cables, printer cables, etc. The maximum safe cable length for a USB device 
is about 15 feet or 5 meters. If you need to cover a longer distance, you 
should consider using a powered USB hub or repeater to keep the signal from 
degrading or being corrupted. 

Troubleshooting USB 

As explained earlier, USB is extremely user friendly and easy to use. IN 
case you ever run into problems using a USB device, here's what you want to 
check first: 

Power 

Is your computer experiencing crashes and/or your USB device acting flaky? 
Make sure that you are not overloading the USB ports on your motherboard 
with too many high-power USB devices. Remove multiple USB devices and try 
them out one at a time. If they work when hooked up individually, but not 
when all connected, you should consider getting a powered hub. 

Device Driver 

As a general rule you should assume that the driver that came with the USB 
device, or any other device for that matter, is outdated the moment you 
open the box. Before you install it, go online and make sure you get the 
latest version of the device driver. If a device acts flaky while others 
work fine, chances are you might need an updated driver. It's easy to 
correct and free, you should try a driver update as one of the first 
troubleshooting steps. 

USB Port Driver 

If you can't get USB to work properly with any device, the problem might be 
with the USB driver in your operating system. Visit the web site for your 
motherboards' manufacturer and make sure you have the latest drivers. While 
you're there, you also might want to check for a newer version of the BIOS. 
Of course flashing your BIOS should be a last resort for resolving the 
problem, but it might be the solution you need. 

Conclusion 

If your PC has one or more USB ports, you're running an OS that supports 
USB and you go shopping for a peripheral for your PC, go for USB if you 
have the choice. It is easy to install and use, and faster and more 
flexible than some alternatives such as serial or parallel ports. Want to 
hook up three joysticks, two printers, two scanners, a web cam and a 
digital camera to your PC and use them all at the same time? USB makes it 
possible. 


http://www.PCNineOneOne.com