Guest column: USB for me . . . USB for you

Despite the explosion of PCs with Universal Serial Bus (USB) support, Gearhead has been intrigued by how little people know about this new technology. Indeed, Gearhead must admit that our own knowledge was a little sketchy, hence this column.

USB came about because of the need to develop a workable, simple-to-use and low-cost external expansion bus for PCs. This need arose because, although the Peripheral Components Interconnect (PCI) bus solved many performance and compatibility problems that had plagued the old Industry Standard Architecture bus, difficulties related to opening the case to add or change an expansion card and then reconfiguring the system remained.

Furthermore, while PCI made the Microsoft Windows Plug-n-Play system possible, it didn't guarantee that Plug-n-Play worked (hence the other term for Plug-n-Play: Plug-n-Pray).

USB was built around a serial bus (parallel busses are expensive) and designed to allow dynamic configuration. This means that add-on components can be "hot-swapped" (added and installed or removed and uninstalled while the system is running and without rebooting - now there's a step forward).

USB uses a master-slave design with the PC acting as the master and controlling the flow of all traffic. Such a design means that slave devices (peripherals) can be less expensive and simpler. Power can also be supplied to USB slave devices through the USB interface, again simplifying the design of peripherals and improving usability.

In a USB system, there are four types of devices: I/O devices, hubs, compound devices and composite devices.

An I/O device is the piece of equipment you want to connect to your PC: a modem, keyboard, mouse, scanner, speakers, etc. A USB system can support up to 126 devices. In Gearhead's experience, a few devices (two or three), will operate just fine, but don't push your luck, particularly if your processor is less than a Pentium II.

Hubs connect multiple USB ports to allow a number of USB slave devices to communicate. Hubs can be cascaded to a depth of five levels and here's where choosing the right type of hub can be crucial: If you use a self-powered hub, the USB specification says that 500 milliamps (mA) will be supplied to each connected device or hub. If the hub is self-powered, then only a total of 500 mA will be available to all connected devices. As the hub itself will count as one device, the true total available for all attached devices will be only 400 mA.

As each port demands 100 mA, other hubs that are connected to an unpowered hub will have to be self-powered if they are to function (while cascaded unpowered hubs might work, devices connected to them might not work even if the devices are, in turn, self-powered). In real life, this can be incredibly confusing to end users and support staff.

Compound devices are peripherals that also offer hub functionality: USB-enabled monitors are a good example. Composite devices are the over-achievers of the USB world - they are devices that combine several input/output functions in one peripheral. For example, a keyboard with speakers or combo toaster/MRI scanner (just kidding, Gearhead knows of no such product).

Next week, plug and play with gh@gibbs.com.

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