How USB (Universal Serial Bus) Ports Work?
Just about any computer that you buy for your home or office today comes with one or more Universal Serial Bus connectors on the back. These USB connectors let you attach everything from mice to printers to your computer quickly and easily. The operating system supports USB as well, so the installation of the device drivers is quick and easy too. Compared to other ways of connecting devices to your computer (including parallel ports, serial ports and special cards that you install inside the computer's case), USB devices are incredibly simple!
In this edition , we will look at USB ports from both a user and a technical standpoint. You will learn why the USB system is so flexible, and why USB is able to support so many devices so easily -- it truly is an amazing system! What Is USB?
Anyone who has been around computers for more that two or three years knows the problem that the Universal Serial Bus is trying to solve -- in the past, connecting devices to computers has been a real headache!
Printers connected to parallel printer ports, and most computers only came with one. Things like zip drives in need of a high-speed connection into the computer would use the parallel port as well, often with limited success and not much speed.
Modems used the serial port, but so did some printers and a variety of odd things like palm pilots and digital cameras. Most computers have at most two serial ports, and they are very slow in most cases.
Devices that needed faster connections came with their own cards, which had to fit in a card slot inside the computer's case. Unfortunately, the number of card slots is limited and you needed a PhD to install the software for some of the cards.
The goal of USB is to end all of these headaches. The Universal Serial Bus gives you a single, standardized, easy-to-use way to connect up to 127 devices to a computer. Each device can consume up to a maximum of 6 megabits per second of bandwidth, which is fast enough for the vast majority of peripheral devices that most people want to connect to their machines.
Just about every peripheral made now comes in a USB version. A sample list of USB devices that you can buy today includes:
Printers
Scanners
Mice
Joysticks
Flight yokes
Digital cameras
Webcams
Scientific data acquisition devices
Modems
Speakers
Telephones
Video phones
Storage devices like Zip drives
Network connections like Intel's AnyPoint home network.
A typical USB connector for a device, called an "A" connection
If it is a new device, the operating system auto-detects it and asks for the driver disk. If the device has already been installed, the computer activates it and starts talking to it. USB devices can be connected and disconnected at any time.
Many USB devices come with their own built-in cable, and the cable has an "A" connection on it. If not, then the device has a socket on it that accepts a USB "B" connector.
A typical "B" connection
The USB standard uses A and B connectors to avoid confusion. "A" connectors head "upstream" toward the computer, while "B" connectors head "downstream" and connect to individual devices. By using different connectors on the upstream and downstream end, it is impossible to ever get confused -- if you connect any USB cable's B connector into a device, you know that it will work. Similarly, you can plug any "A" connector into any "A" socket and you know that it will work.
Running Out of Ports?
Most computers that you buy today come with one or two USB sockets. With so many USB devices on the market today, you easily run out of sockets very quickly. For example, on the computer that I am typing on right now, I have a USB printer, a USB scanner, a USB webcam and a USB network connection. My computer has only one USB connector on it, so the obvious question is, "how do you hook up all the devices?"
The easy solution to the problem is to buy an inexpensive USB hub. The USB standard supports up to 127 devices, and USB hubs are a part of the standard.
A typical USB 4-port hub accepts 4 "A" connections
A hub typically has 4 new ports, but may have many more. You plug the hub into your computer, and then plug your devices (or other hubs) into the hub. By chaining hubs together, you can build up dozens of available USB ports on a single computer.
Hubs can be powered or unpowered. As you will see below, the USB standard allows for devices to draw their power from their USB connection. Obviously a high-power device like a printer or scanner will have its own power supply, but low-power devices like mice and digital cameras get their power from the bus in order to simplify them. The power (up to 500 milliamps at 5 volts) comes from the computer. If you have lots of self-powered devices (like printers and scanners), then your hub does not need to be powered -- none of the devices connecting to the hub need additional power, so the computer can handle it. If you have lots of unpowered devices like mice and cameras, you probably need a powered hub. The hub has its own transformer and it supplies power to the bus so that the devices do not overload the computer's supply
