USB: Universally Altering Your Peripherals - The Standard Itself
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There are two versions of the USB standard, 1.1 and 2.0, although USB 1.1 devices have basically been superseded by their 2.0 counterparts. They are also purely pin compatible, and follow the same technology.
USB 2.0 was merely brought along to allow more data dependant devices to be connected to a PC and not be bogged down by v1.1's 12MB/s bandwidth limit. And of course, with the coming of IEEE 1394 (commonly known as "FireWire"), they also had competitive reasons to revise the specification.
USB is meant as a peripheral connectivity protocol. In other words, devices connected to the hub are slaves--mindless drones. They respond to control signals dictated by the host PC. All of this communication is standardized in the USB protocol, allowing just about any device to be connected to a USB hub, provided it's capable of following those data transmission standards.
Unfortunately, this strength is also a weakness. Unlike FireWire, there must be that host PC present. This design brings down on the complexity, however, which is one reason why USB has become the dominant standard for "dumb" things like Human Interface Devices (HIDs); keyboards, external sound cards, and mouses (yes, that is the proper term) fit into that category.
That "hub" I mentioned is the key part of the USB architecture. By isolating the devices from the host PCs hardware and software, a hub allows quite a few advantages over more direct connection. The most obvious is "hot swapping," or the ability to unplug a device and plug in another in the same port, and have the new device work without having to have turned off the computer. In the days before USB, if you wanted to plug your joystick in to play a game, you had to turn the PC off, plug it in to the gameport, then boot up again for it to be recognized. With USB, the hub actively recognizes a new device connected to it, gives it an address, configures all of its routing internally, and life goes on. The host PC is made aware of the new peripheral, and the address used to communicate to it through the hub, and any device drivers required.
Another advantage is that no longer with USB connected devices do we have to deal with IRQ (interrupt request) issues. Anyone who built computers before USB devices became prolific will remember the fun of getting your printer, keyboard, mouse, scanner, joystick, modem, sound card, SCSI, RAID, video card, and all other manner of devices to play nice together with 16 IRQs available by seeing which ones don't mind sharing. Now, one is given to that USB hub, and you hook up everything through that.
One more advantage of having the hub in between the devices is in the ability to update the protocol itself. When USB 1.1 and its 12MB/s data limit was updated to 480MB/s in USB 2.0, the host PC did not have make any changes to what it did when catering to 1.1 or 2.0 devices. The hub takes care of buffering and data speed matching with the streams from high speed and normal devices. All of this is invisible to the end user, allowing us simple folks to use our iGrills and electric blankets with not a thought as to the technology behind it.
What's interesting is that the standard itself allows a cable length of 5m. So while USB is not a "long haul" communication standard, it's certainly much more useful than that of Parallel ATA, which allows a maximum length of 50cm. In short, USB allows you to place devices all over a normal room, or at least your desk, and connect them to your computer. For those of us living in a studio apartment, that means we can safely use our USB noodle strainer.
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