The venerable PC Card has been around for at least 16 years. In dog years, which I believe are roughly equivalent to computer years, the PC Card is a staggering 112 years old.

Rather than being taken into the woods and put out of its misery, the PC Card is doing a slow fade-out. Fading into its place is the ExpressCard, which is designed to be thinner, lighter, and faster.

With ExpressCard slots becoming more commonplace in notebooks, I thought it was time for a primer. This week I've got the backstory on ExpressCards and how they differ from PC Cards. Check back next week to find out if you can use an ExpressCard in a PC Card slot and vice versa.

What Is an ExpressCard?

In 2003 the ExpressCard standard was introduced by the trade group behind the PC Card as a successor to that standard. The first ExpressCard modules appeared in 2004.

Similar in function to a PC Card, an ExpressCard allows you to easily add capabilities such as extra storage and wireless wide area networking to a notebook or desktop PC. Because they're faster than PC Cards, ExpressCards are particularly well suited for TV tuners, Gigabit Ethernet adapters, FireWire ports, and other devices that depend heavily on fast data transfer rates.

ExpressCards are available in two sizes. Both are 5mm thick and 75mm long and use the same 26-pin connector. One version, called ExpressCard/34, is 34mm wide. The other, ExpressCard/54, is 54mm wide. By comparison, all PC Cards are 5mm thick, 86mm long, and 54mm wide.

Why Are There Two Form Factors?

Many ExpressCard developers are opting to design products to fit the ExpressCard/34 format, in the belief that smaller is always better. However, some products, such as CompactFlash readers and hard drives with 1.8-inch platters, won't fit on the smaller card, so a larger format is needed.

Will Either ExpressCard Work in the Same Slot?

ExpressCard/34 cards will fit either an ExpressCard/34 or ExpressCard/54 card slot, but ExpressCard/54 cards will fit into an ExpressCard/54 slot only. Many new notebooks include an ExpressCard/54 slot.

How Do ExpressCards Differ From PC Cards?

PC Card technology uses a special I/O system to communicate with the host PC. The ExpressCard standard, however, was designed to interface directly with USB 2.0 and the emerging PCI Express buses that are standard in nearly all current PCs. This makes it easier and less expensive for developers to support the ExpressCard standard than it is to support the PC Card standard. As a result, some desktop computer makers--which traditionally didn't bother offering a PC Card slot--are incorporating ExpressCard slots into their designs. The upshot is that you can use one ExpressCard device on either a notebook or desktop PC, as long as each one has an ExpressCard slot.

The two types of device also differ in throughput speeds. In theory, the maximum throughput for the PC Card standard is 132MB per second. ExpressCard throughput tops out at 2.5GB per second for PCI Express and 480MB per second for USB 2.0 interfaces.