Inside the PCMCIA Storage Standard
By Bruce Schneier
January 11, 1993
Originally a nonvolatile storage standard, PCMCIA has grown to be a much more versatile interface. With its small size and low power draw, it has gotten a lot of attention from computer developers looking to reduce both bulk and power on their portable products.
Apple is evaluating PCMCIA for its PowerBook line, and Newton will ship with a PCMCIA slot; the slot also will support an Apple-proprietary 32-bit bus called TrimBus. Using PCMCIA, users can plug in cards containing everything from interactive maps to network connectors.
A PCMCIA card is a removable device about the size of a credit card (2.126 by 3.37 inches). It has a 68-pin interface along the short edge that works with eight- and 16-bit computer buses and supports physical access of up to 64 Mbytes of memory. (Apple's 32-bit TrimBus can address up to 256 Mbytes on a single card.)
The standard supports three different card thicknesses: a 3.3-millimeter Type I card, a 5-millimeter Type II card and the recently approved 10.5-millimeter Type III card.
PCMCIA cards fit into a slot that looks something like a floppy disk slot. PCMCIA slots can be found on some IBM PC-compatible notebook computers; some pen-based computers, such as EO Inc.'s 440 and 880; laser printers; and some MIDI (Musical Instrument Digital Interface) instruments.
There are two main kinds of PCMCIA cards: memory cards and I/O cards. Memory cards use one of several different solid-state memory technologies: static RAM; dynamic RAM; EPROM; and EEPROM (electrically erasable programmable ROM), also called flash memory. These cards are almost all Type I. The current generation of memory cards can hold up to 20 Mbytes of data, with 64-Mbyte capacities coming by 1995.
Memory cards can be used for data storage in place of floppy disks or for read-only storage. Some software vendors, such as WordPerfect Corp. of Orem, Utah, sell versions of their MS-DOS software on PCMCIA cards.
The performance benefits of PCMCIA memory cards are substantial. Since the cards are basically an array of memory chips, access time is about 10 times faster than a hard disk. Additionally, since there are no moving parts, power consumption is dramatically lower.
All this performance doesn't come cheap: 20-Mbyte cards currently cost between $900 and $1,200 each.
The Type III card case, as well as the recently approved PCMCIA-ATA (AT Attachment) standard, will allow developers to put subminiature rotating disk drives (1.8 inches or smaller) onto a PCMCIA card. Although slower and more power-hungry than memory cards, drive cards will be cheaper and have higher capacities; 32- and 42-Mbyte drive cards already have made their debut.
When Version 1.0 of the PCMCIA standard was released in August 1990, the cards were designed primarily to be used as extensions of system memory.
PCMCIA Version 2.0 enhancements included dual voltage and an execute-in-place (XIP) mechanism. Dual voltage supports 3-volt technology to minimize power consumption; XIP allows applications to execute directly from the card, without first having to be transferred into system memory.
With the introduction of PCMCIA-ATA, users will be able to take the memory card from a Newton and use it on an HP-95LX or a PowerBook, something that couldn't be done with Version 1.0 cards.
Version 2.0 also added support for I/O devices. A Version 2.0 PCMCIA card can contain almost any peripheral device, as long as it fits on the card type. Among the first PCMCIA 2.0 I/O cards to appear are 2,400-bps modems, fax cards, LAN adapter cards and bus interface cards. PCMCIA cards with SCSI ports, Ethernet and token-ring connectors as well as remote pager cards also have been announced. Most I/O cards are Type II.
It is the ability to mix and match PCMCIA cards that makes them attractive expansion options for notebook-computer users. "I can use a modem card when I'm on the road, an Ethernet card when I'm in the office, or a flash card when I need extra storage," said Ben Liberman, an independent Mac consultant in Chicago.
Photo of Bruce Schneier by Per Ervland.
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