PCMCIA

PCMCIA Cards come in a few distict types, mostly based on physical size,:

• Type I
  3.3 mm thick. The first PCMCIA Cards, uses primaily for non-volitile memory and thus lacked any soft of real I/O Spec, so they were never used for anything more advanced than storage.
  
  
• Type II
  5 mm thick in the middle, still 3.3 mm on the edges. The most common form factor today. The extra millimetre gave a bit more headroom inside, and they were granted a generously design I/O spec that included the mythical beast Plug and Play. Nearly every kind if device is available in Type II form, including some hard drives. Also allowed for use of 3.3v, 5v and 12v power.
  
  
• Type III
  10 mm thick -- the same size as two Type II Cards stacked, but electrically identical to Type II. Mostly used for Hard drives and high-end network cards that provide 'real' plugs.
  
  
• Type IV
  15 mm thick. Intended for large capacity hard drives, the standard was never ratified and thus is seldom seen. No Type IV devices exist and only a handfull of card slots are large enough to accomidate them.

It is interesting to note that, although everyone calls them PC Cards now, the name conversion from PCMCIA only technically applied to Type II cards. No one really cares, I'm sure, but 'PC Card' means Type II PCMCIA.

There is also another advanced segment of cards, called CardBus, that really deserve their own descriptions.

In 1989, with the mobile computing industry kicking at the seams of its infancy, a number of portable storage cards were appearing on the market.

John Reimer an executive and engineer then working at Fujitsu on mobile computing solutions, recognized that different, incompatible cards were being produced by different companies. In order to avoid complete chaos and to insure that anyone buying a mobile device would be able to choose from a variety of companies for their removable media, Mr. Reimer decided, there would have to be an established standard to protect both consumers and businesses.

He soon found that he wasn't alone, and, with the assistance of his colleagues working at other companies, he successfully secured funding and agreements from a small number of corporations that would spearhead the non-profit organization PCMCIA (Personal Computer Memory Card International Association), run by a board and a group of elected officials, and chaired for a number of years by the founder, John Reimer.

The idea caught on quite well. "By 1991, PCMCIA had defined an I/O interface for the same 68 pin connector initially used for memory cards. At the same time, the Socket Services Specification was added and was soon followed by the Card Services Specifcation as developers realized that common software would be needed to enhance compatibility." Today, anyone would be hard pressed to find a laptop computer or a PDA without a PCMCIA slot or a Compact Flash slot, (Compact Flash] cards being adaptable to PCMCIA slots with a simple adapter). PCMCIA now boasts over 200 member companies and affiliates.

for more: http://www.pcmcia.org/about.htm