The PC Card and CardBus Standards:
Compact, Duable, and Portable

The Personal Computer Memory Card International Association (PCMCIA) is a standards committee formed in 1989 by leading manufacturers of PC components. Initially intended as a standard for removable memory cards, the PC Card standard quickly expanded to include fax modems, LAN and serial interfaces, celluar uplinks -- and now, data acquisition cards and other interfaces.

The basic PC Card and CardBus standards define ultra-compact, business-card-sized accessory cards measuring 2.125" wide by 3.37" long. There are three different types of PCMCIA cards, defined by the maximum allowable thickness of the card:

• Type I cards can be a maximum of 3.3mm thick, and are usually memory expansion or solid-state disks.
• Type II cards, the most common, are just 5mm thick.
• Type III devices are the thickest, at 10.5mm (less than 7/16").

Most notebook PCs now come with two Type I or II slots. They accept either a single Type III card or two Type I/II cards.

PC Card and CardBus

In addition to the physical specs, there are now different PCMCIA data-transfer specs. The original spec. called for 8-bit data transfers. The next generation, still popular, called for 16-bit data transfers. The new "CardBus" spec calls for 32-bit data transfers, for much faster throughput.

The PC Card and CardBus devices are generally designed to be "hot-swappable" (you can insert or remove the card without turning off your PC).

Interface to the Real World

External devices interface to the PC Card or CardBus via an external cable, terminal panel, or box that plugs into the card. These connection accessories can be as simple as an ethernet connector or a phone jack required to attach standard connections or as complex as an analog multiplexer and gain amplifier or a digital I/O interface to a rack of control relays.

With the proliferation of notebook and even smaller computers, the demand for ultra-compact add-ons has grown.