Number 251 - April 2004

Understanding USB
by Vinny La Bash, vlabash@home.com Sarasota Personal Computer Users Group, Inc.
Hail USB! Until a few years ago if you wanted to add an external device to your computer, such as a scanner or a Zip drive, it needed your one and only parallel port. The trouble with this was that your printer had already staked out that territory. One way of getting around the problem was installing an A/B switch. However, if you needed more than one device, you had to install an A/B/C/D switch. Sometimes moving among these devices meant having to turn one off before you could use another, and often you had to reboot before your machine would recognize another device on the switch.    Early Palm Pilots and digital cameras sought your serial port. Computers had, and most still have two serial ports, but they were slow and almost always involved installation of controlling software.    There were also devices that came with their own controller cards. This meant you had to open the case and install the card in an expansion slot, provided you had one available. Things could get crazy quickly, and you had to handle IRQ conflicts, more cables, and additional power cords.    Rescue arrived with the introduction of the USB port (Universal Serial Bus) that lets you attach almost anything to your computer quickly and easily. Windows XP is designed to support USB so device conflicts are gone. The standard allows up to 127 devices on a single USB port. In practice, no one uses that many devices. USB connectors let you attach everything from TV tuners to modems. It's an amazingly flexible technology. If you had a toaster with a USB connection you could hook it up, but it's doubtful you could watch bread turn brown on the screen.    Installing a USB device is incredibly simple. Windows XP senses it through a process called auto-detection, and asks for the driver disk if it's needed. If you have previously installed the device, XP activates it, and it's ready for use. Part of the beauty of USB is that you can connect and disconnect devices at any time without having to reboot your machine or change any options. If a cable is built-in to a USB device it will connect to your computer with its own "A" connector. Otherwise it will connect with a "B" connector. "A" and "B" connectors are of different sizes and shapes so there is never a question of getting them mixed up.    Today, most desktop computers are built with least four USB ports. That is inadequate, but there are relatively inexpensive USB hubs available that act as expansion devices. The number of ports available on an expansion hub can vary from as few as two to as many as seven,	depending on your needs and how much you care to spend. Plug the hub into your computer, and then plug your devices into the hub. You can chain hubs together, and build dozens of available USB ports on a single computer.    The USB standard lets USB connected devices draw their power from their USB connection to the PC. This works well for mice, digital cameras, web cams, and other devices that use small amounts of power. Printers, scanners, and other high power accessories require their own power supply, and they can be plugged into hubs that do not have their own power supply. If you run out of USB sockets and you have a lot of low power devices, get a powered hub so you don't overwhelm the PC bus.    Inside your computer the USB bus lets the computer act as a host to all USB devices attached to it. If you have the up-to-date USB 2.0 standard, data can be transferred at up to 480 megabits per second. If you were moving text, that works out to about 30,000 pages of documents per second. That's a maximum rate, not typical of actual use. Nevertheless, it's impressive.    USB has other advantages. You can plug in or remove USB devices without having to reboot your machine, and the computer can put USB devices into an inactive state when conserving power. XP queries all USB devices when your machine powers on, assigns each one an address, and determines what kind of data it needs to send or receive. XP keeps track of the total bandwidth of all the attached USB devices. If the combined devices reach 90% of the 480 megabit maximum, XP denies access to any additional devices. The remaining 10% is reserved for transmitting control characters, stop and start transmission codes, error checking, and other overhead.    USB 2.0 encourages the development of innovative products that would be impossible to develop with the older standards. It's the solution for all PC users who want an instant, no-hassle way to connect new hardware like digital joysticks, scanners, digital speakers, digital cameras or a PC telephone to their computer. Plug and Pray has truly become Plug and Play.    Copyright 2004. This article is from the February 2004 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication.
Number 251 - April 2004