How Computers Work – Part 11 – USB [Mega Series]

The universal serial bus (USB) is the best thing that ever happened to computers since power buttons. If you take a moment to think about how your life would be like without USB ports on your computer, you’d quickly find yourself hugging that little box of integrated circuits without question. Believe it or not, there was a bygone era when USB didn’t exist, which makes you think about how little compassion for end users PC manufacturers must have had back in the day. Almost everyone today wouldn’t buy a computer without at least 2 USB ports.

Before USB

Back in the day, when people used to tell you they were going to prepare food with an axe and a chicken in their hands, everyone was using either serial or parallel connectors on the computer. They looked like this:

Serial and parallel connectors: Worse than having athlete's foot. The obnoxious long one's a parallel port.

So let’s say you’re back in the 90s with a PDA. You connect that sucker to the serial port. Computers usually have two of these, so that leaves room for a modem to connect to the web or something, right? And you don’t even sweat over adding a printer, since the parallel port is free. 20 years pass, and you have the iPod, wireless mice and keyboards, gaming pads, and tons of other devices. Where are you going to make room for all that without disconnecting something already on your PC? Things like these made IT guys panic and run around in circles back in the 90s.

The New Player in the Ballpark

USB was designed with the scope of saying “Why bother with all these different connectors that can only connect one device?” Seven companies sat in think tanks to come up with the design in 1994: Microsoft, Compaq (ugh…), DEC, IBM, Nortel, Intel, and NEC. Are you noticing a difference between banks and tech companies? Think “banks invented derivatives, and tech companies invented USB.”

What they came up with was much more elegant than anything developed before that day:

Behold, a thing... It's elegant...

Everything in its design says “You’re a winner.” The best part of USB is that you can connect up to 127 devices to each port. You also might be enticed by the fact that the majority of devices available that interact with computers support USB, making the feature extraordinarily convenient.

Types of Connectors

The following is a brief list of the types of USB conectors:

  • Type A

The Type A connector. This one connects to the computer's USB port. The other end of the cable differs more.

  • Type B

This is one of the most common USB connectors, and the first one to reach the public.

  • Mini USB

The Mini USB connector (right) is used for smaller devices like phones and digital cameras.

  • Micro USB

Micro USB is used for very compact devices, like slim phones that cannot waste any space.


If you want to know how you’d ever be able to connect 127 devices to your computer through USB, look no further than the “hub.”

This device contains different ports for peripheral connections and one USB port for connection to your computer. The device gets is power from the cable connecting to the computer, although it can sometimes get some of its juice from a wall adapter. Best of all, you can connect multiple USB hubs to each other, further extending the amount of USB devices you can add to one single port on the computer.

This is the typical USB hub. Some USB hub manufacturers make their hubs in exotic shapes, however.

Hubs usually have four ports, but there are many companies making USB hubs with more than 10.

Finally, Some Geek Talk!

The computer doesn’t just magically welcome every device and start the USB party. Each device has to be queried while the computer is booting and running. When it’s done, each device receives a number – a process better known as enumeration. During this process, the computer also “talks” to the device, trying to find out everything about it much like how that guy in the bar wants to know your number.

While chatting it up with the allegedly innocent little device, the computer negotiates how it will communicate with it, using either of three modes:

  • Interrupt – For keyboards, mice, etc. This type of communication simply halts everything else for signals to pass. When you press a key on the keyboard, I think you’d prefer not to wait for a page to finish printing before the computer responds to your requests, right?
  • Bulk - The computer sends data in 64-byte segments to a device that communicates in this fashion and verifies each segment for errors. Printers most commonly use this mode of communication.
  • Isochronous - This is pronounced “I suck-ronous.” Basically, the computer sends a streaming signal to the device and doesn’t even look twice at it. It comes useful in the case of speakers, which don’t need error correction and require a steady, timely stream of data.

The computer’s USB bus often has a maximum capability of 480 Megabits (Mb) per second in bandwidth available. USB 3.0 raises the bar to 10 times as much. Nevertheless, if your isochronous and interrupt devices use 90% of that bandwidth, the computer will not respond to requests from other devices. This means that your ideal dream of having 126 speakers and one keyboard/mouse connected to your computer might not work so well. The other 10% of the bandwidth is reserved for bulk communication devices.

Remember when I mentioned that the computer queries devices during and even after its boot process? What I meant is that you can connect any USB device to your computer and disconnect it as you please. The computer will detect it and, in most cases, install its software. This is known as “hot swapping.”

USB 3.0

The advantages of USB 3.0 over the run-of-the-mill USB 2.0 aren’t astronomical, but significant enough to be worth an upgrade.

For one, your devices receive almost twice as much power (900mA/5V) in comparison to USB 2.0 (500mA/5V). This jump up in power allows you to connect more non-powered devices to the USB ports and demand more from them.

USB 2.0 also has another flaw corrected in the 3.0 release: The cable only has 2 wires for communication. 3.0 implements six wires, allowing it to transmit and receive at the same time instead of waiting for either. Note that while these differences exist, all USB 2.0 and 3.0 devices & computers are compatible with each other. If you plug a USB 2.0 cable between two USB 3.0 ports, though, you’re not going to reap the benefits I just mentioned. The cable, the computer’s ports, and the device all have to match up.

I’m still having trouble, though, figuring out what version of USB this little bugger uses:


Stay tuned for another part of How Computers Work!

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