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PCMCIA (PC Card) and CardBus
The first laptop PCs had very few expansion options. They typically had serial and parallel ports, and some expensive units had add-on docking stations (port replicators) but these are quite limited in their capabilities. In 1989, a group of PC system and computer manufacturers created the Personal Computer Memory Card International Association or PCMCIA. The goal of this group was to define a standard for expansion devices--called PC Cards--to be used in special slots in mobile computing equipment. The group of course succeeded--probably beyond even their own expectations! Virtually all mobile PCs made in the last five years have the ability to accept these credit-card-sized PC Cards (often called "PCMCIA cards" after the name of the trade organization, even though they don't like it). In addition, this technology is also used in other types of electronic gear, outside of the PC world.
As the name "PCMCIA" suggests, one of the original main applications of PC Card technology was memory expansion. Ironically, today many laptops don't even need expansion slots for this purpose: they have separate user-upgradeable memory slots! The new PC Card slots however were quickly embraced, and are now used for many other purposes. Due to the explosion in mobile computing, manufacturers of devices of all sorts moved quickly to exploit this new interface--in addition to memory cards, you can get modems, network interface cards, sound cards, and other devices that fit into PC Card slots. Of course, this includes a rather wide variety of storage devices, or I wouldn't be bothering to tell you all of this. :^)
There are actually two different interfaces that are used with PC Cards today. The first is the older, "original" PC Card or PCMCIA interface, which is a 16-bit interface that runs at 8 MHz. If this sounds a bit like the regular ISA system bus found on desktop PCs, there's a good reason for that: the traditional PC Card interface is based on ISA. Of course the form factor and device packaging is different, but in terms of operation, they are similar. This means that conventional 16-bit PCMCIA cards have similar performance constraints to ISA: they are perfectly fine for modems and similar slow devices, but inadequate for performance-intensive applications such as networking--or storage devices.
To address the limitations of conventional PC Cards, in 1995 the PCMCIA announced a new interface called CardBus, which is a 32-bit interface running at 33 MHz. Say... doesn't that sound a bit like good old PCI? Bingo, you win again. :^) In some ways, CardBus is to PC Card what PCI is to ISA. Cards using this newer interface maintain the same physical packaging as older cards--they just run faster. Most newer notebook PCs have PC Card slots that will automatically support both 32-bit CardBus devices and also the older 16-bit PC Cards. CardBus support is essential for high performance devices such as modern hard disks or high-speed Ethernet.
There are in fact several ways that hard disks can be interfaced to a system using the PC Card or CardBus interfaces:
PC Cards continue to be a force in the growing mobile computing market, and I don't expect them to go away any time soon. While PC Card slots are standard equipment on notebooks, you can add support for them to desktop PCs, using hardware kits that include an interface card and slots that mount in a regular drive bay. While of little attraction to most PC users, these can be helpful to some people who want to share devices between a desktop PC and a notebook.
For more information on PC Cards and the PCMCIA, see http://www.PCMCIA.org.