Use These Design And Test Methods To Create Robust USB Devices

Industry-Wide "Plugfests"
In theory, there shouldn't be any significant differences among USB implementations on a variety of different PC platforms and configurations. However, this may not be the case in reality. Subtle variations in host software are a result of detailed differences in the USB chips themselves, including differences between UHCI (Universal Host Controller Interface) and OHCI (Open Host Controller Interface) implementations.

As USB evolves and component problems are corrected, the potential for subtle but significant differences between revision levels of the same chips can lead to incompatibilities at the system level. There also are issues with BIOS support of USB, since BIOS software is still in the process of being refined to optimally address USB host controllers. Essentially, as with most new markets, USB has taken off before the underlying infrastructure has been completely solidified.

To counter this built-in potential for variability, the industry has established the use of periodic USB compliance workshops, also known as "plugfests," to give manufacturers of hosts, hubs, and devices a forum for interconnecting, stressing, and qualifying their device designs. Plugfests are held approximately every three months under the sponsorship of the USB Implementers Forum. Host systems are typically set up in hotel suites. Peripheral device manufacturers then move about, testing their items' compatibility with all of the different system configurations.

In the real world, it's not enough for a USB device or hub manufacturer to simply prove that it has correctly followed the specification, and therefore demand that everyone else conform to its design. You can never be sure what revision level of machines or subtle differences a device will have to contend with in the field. So, it becomes vitally important to collect as much empirical information as possible on potential incompatibilities.

The bottom line is that you don't want to spend all of your time debugging other manufacturers' designs. But you do need to know what's out there in order to defend your own design against potential failures in the hands of real-world users. After all, those highly variable, messy, and unforgiving user environments are the ultimate forums in which your USB designs will be judged as either failures or successes.