What's a USB?

What's a USB Got to do With it?

Since the mid to late 90's the Universal Serial Bus, more commonly known as USB has become the gold standard in computer peripheral connectivity. For those of you who are latter day experts, and don't go back as far as us old timers, before USB, connecting stuff to your computer could be a bit of a nightmare.

Older computers typically came with only one parallel and two serial ports. If you had a printer (and who doesn't), that took up the parallel port, cutting by 1/3 the amount of connections available. Fortunately, the keyboard and mouse had their own connections, or we'd never have gotten anything connected to our computers. Oh, there was a game port to connect the joystick to as well. Of course, all of these connections ran at a much slower speed than even USB 1.0 did.

Wire mess.jpgIf you had to connect to more than that, you were in trouble. Let's say that you needed an external hard drive, you'd have to disconnect your printer, in order to plug it in. But, you couldn't "hot swap" with a parallel port, so connecting that external hard drive meant shutting your computer down. Other peripherals, which needed higher speed connections, required the installation of a special interface card into the computer. I remember having to install an interface card for my first flatbed scanner. Of course, if you were out of expansion slots, you were in real trouble.

I guess the only saving grace we had back then was that we really didn't have all that much to connect to our computers. Most of the things we connect through a USB today didn't even exist, except in the imagination of a few engineers and science-fiction writers.

USB to the Rescue

In the midst of this, seven of the computer industries heaviest hitters got together to develop a new way of connecting devices to computers. They had four primary goals:

  •          Making it fundamentally easier to connect to computers, by replacing the multiple connectors on the back of PCs

  •          Addressing the usability issues of existing interfaces, which were extremely easy to damage

  •          Simplifying software configuration of all devices connected to the USB

  •          Permitting greater data transfer rates

The first USB, 1.0 was released in January, 1996, but it didn't gain wide recognition and popularity until version 1.1 was released in September, 1998 to fix the problems with the original interface. Since then, USB 2.0 has taken over (October 2000) and the newly released USB 3.0 (released November 2008) will soon be taking over the market.


USB Version

Release Date

Low Speed Transfer Rate

High Speed Transfer Rate

Max Cable Length



Jan 1996

1.5 Mbit/s

12 Mbit/s

16 ft.



Sep 1998

1.5 Mbit/s

12 Mbit/s

16 ft.



Oct 2000

60 Mbit/s

480 Mbit/s

16 ft.



Nov 2008

60 MB/s

5 Gbit/s

9 ft.

What really makes the difference between USB and the earlier connection systems is the connection speed. Earlier systems couldn't maintain that high a connection speed, because the computers themselves weren't that fast. High amounts of data transfer required parallel connections, of which there weren't enough.

USB ConnectorThe USB connector is an engineering masterpiece in and of itself. The external shell of the connector, which is connected to ground, makes contact before any of the contacts do. This dissipates any electrical shock to ground, protecting the equipment. Next, the power and ground connection are made, and finally the data connection is made. This staged connection is what gives the USB connector the ability for "hot swaps" (connecting equipment while systems are on).

At the same time, the USB connector was designed to be pinless, eliminating the constant problem of bent pins with the old D-Sub Miniature connectors. Instead of pins, the contacts "wipe" against each other, both insuring a good electro-mechanical connection between the contacts, and cleaning them of any corrosion.

Finally, the USB connector is dummy-proofed. The famous USB symbol is required to be molded into the top side of the connector, to insure that people know how to install it. Even if someone messes that up, it is physically impossible for a USB connector to be connected backwards, short of beating on it with a hammer.

The one thing I don't understand about all this is, if USB was to make connecting simpler by standardizing connections and allowing everything to be connected to the same type of connector, why do we have more types of connectors on the back of our computers now, than we did way back then?

Okay, So How does it Work?

It takes a little more than adding a USB connector to a motherboard to make a computer USB compatible. When you connect a device to a USB port, the computer automatically senses it, assigns it an address and installs the necessary driver. That's why Microsoft had to come up with "Plug and Play" to make Windows compatible with the USB standard.

Finding, addressing and locating the driver for the device is all performed automatically by the computer, between the USB host controller on the motherboard and the USB driver loaded into the computer's software.

USB LogoLet's back up a minute. When that device is connected to a USB port, the first thing that happens is that a reset signal is sent from the computer to the USB device. As part of the response to this reset signal, the computer reads the data rate (speed) of the device. It then assigns the device a unique 7-bit address, which is used for all communication until the device is disconnected from the computer.

The next thing that happens is that the computer queries the device to find out what class and type of device it is. There are 19 different types of devices, covering such things as webcams, human interface devices (keyboards, mice, joysticks), mass storage devices, USB hubs and audio devices (speakers microphones, sound cards, MIDI). The computer searches for the driver for that specific device, and loads it. If the exact driver is not available, and the computer can't locate it, it loads a generic driver for that class of device.

USB device tree.gifA USB host controller can control up to 127 separate devices, including hubs, in a tiered tree structure, up to 5 tiers deep, with the hub installed at the controller is called the "Root Hub." That 127 device number really doesn't mean what it seems like, because one physical device can be several "devices" to the system. Take a webcam with a built-in microphone for example. The video is one device and the microphone is another. Theoretically, a compound device like this can contain up to 16 separate devices, although none have been made to date which are that complicated.

The five tiers deep means that you can connect a USB hub to your computer's USB connector, which is the root hub. Then you can connect another hub to that, becoming the third tier. Another hub can come off of this hub, which is the fourth tier, and you can finally connect your latest electronic widget to that fourth tier, making your device five tiers away from the computer. Of course, if you have that many devices actually connected to your computer, you won't be able to find any of them, due to the rats nest of cables all over your desk.

The USB connector is also able to pass power to the devices connected to it, although the amount of power is limited. Each USB host controller can only provide a total of 500 mA of power (that's 1/2 amp) to all the devices connected to it. For this reason, many USB hubs come with power supplies, providing their own power to all devices connected to them. With a power supply, a USB hub can provide 500 mA of power to each device connected to it. If a device needs more power than that, such as the case of a printer, it will have its own power supply.

Where'd Those Extra Connectors Come From?

Although the original USB specification only provided for two types of connections, called "A" and "B" (probably for originality). Type "A" connectors were originally intended to be used on host devices that supply power and Type "B" connectors on target devices that receive power. The intent was to avoid the possibility of two USB power supplies being connected together, leading to dangerously high current levels, damage to equipment and even fire.

Manufacturers of small electronic devices, such as PDAs and digital cameras, quickly saw a need for further connector types, especially smaller ones, leading to the creation of the "mini" connectors. Cell phone manufacturers needed a still smaller connector, developing the standard for the "micro" connectors. We now have 7 basic types of USB connectors, just enough to drive everyone crazy trying to find the right cable to connect a device.

USB PinoutThe original USB cable is a four conductor cable, providing two conductors for power and two for the data signal. The data wires are a twisted pair, insuring that it is a balanced line, to eliminate signal loss and noise on the line. The power supplied was 5 VDC.

All USB connectors, up through USB 2.0 have maintained the same 4 conductor cables and pinout. However the new USB 3.0 is being made as a 10 pin cable, allowing for three separate signals to be passed through the cable at the same time.



Type A Type Mini A Type Micro A
Type B Type Mini B Type Micro B

All USB connectors are "robust" connectors, intended for long life, with constant connection and disconnection. That is why they have been designed as pinless connectors, traditionally the most fragile part of any connector.

  •          The original connector was designed for a life expectancy of 1500 connection cycles

  •          The mini connector was designed for a life expectancy of 5000 connection cycles

  •          The micro connector was designed for a life expectancy of 10,000 connection cycles