PCMCIA Cards

Notebook computers use PCMCIA cards of two types for expansion: 18-bit PC Cards based on the ISA bus and 32-bit CardBus cards based on the PCI bus. No matter which standard a given card follows, however, it must follow exactly the same physical specifications as all PCMCIA cards. Every card will slide into every slot—well, almost. The standard allows for three thickness variations, but one of which is almost never found, and nearly all slots will accommodate the other two sizes. In other words, physically, PC Cards and CardBus cards are identical, and both use exactly the same connector.

Under the PCMCIA standard, PC Cards and CardBus cards come in three sizes, differing only in thickness. The basic unit of measurement—the size of the typical card slot—is based on the medium-thickness card, designated Type II. Measuring 54 by 85 millimeters (2.126 by 3.37 inches) and 5 mm (about three-sixteenths of a inch) thick.

PC Cards and CardBus cards physically follow the form factor of earlier memory cards (including the IC Card) standardized by JEIDA (the Japan Electronic Industry Development Association). The first release of the PCMCIA specification paired this single-size card with a Fujitsu-style 68-pin connector. Under the current PCMCIA specification, version 8.0 (and all versions since the 2.1 specification), this form factor is designated as the Type I PC Card.

The thinness of the Type I card proved an unacceptable limitation. Even without allowing for the PC Card packaging, some solid-state devices are themselves thicker than 3.3 mm. Most important among these "fat" devices are the EPROMs used for nonvolatile storage. (Most computers use EPROMs to store their system BIOS, for example.) Unlike ordinary, thin ROMs, EPROMs can be reprogrammed, but this requires a transparent window to admit the ultraviolet radiation used to erase the programming of the chip. The windowed packaging makes most EPROMs themselves 3.3 mm or thicker.

Fujitsu faced this problem when developing the firmware to be encoded on memory cards and therefore developed a somewhat thicker card that could be plugged into the same sockets as could standard memory cards. Modem and other peripheral makers found the Fujitsu fat card more suited to their purposes. To accommodate them, PCMCIA 2.0 standardized an alternative: the Type II PC Card. Essentially based on the old Fujitsu developmental EPROM form factor, Type II PC Cards are 5 mm thick but otherwise conform to the same dimensions as Type I cards.

The PCMCIA standard puts the extra thickness in a planar bulge, called the substrate area, in the middle of the card. This thicker area measures 48 mm wide and 75 mm long. Three millimeters along each side of the Type II card are kept to the thinness of the Type I standard so that the same card guides can be used for either card type. Similarly, the front 10 mm of a Type II card maintain the 3.3 mm thickness of the Type I standard so that the same connector can be used for either card type. Naturally, the actual card slot for a Type II card must be wide enough to accommodate the maximum thickness of the card.

In September 1992, PCMCIA approved a third (Type III) form factor for PC Cards. These still-thicker cards expand the bulge of Type II from 5 mm to 10.5 mm and are designed to accommodate miniaturized hard disks and similar mechanical components. As with Type II cards, Type III PC Cards remain thin at the edges to fit standard card guides and standard connectors. Although a number of hard disk drives appeared in this format, few are used today in notebook computers. However, high-capacity MP3 players have embraced the Type III card design.

In practical terms, a Type I card comes closest to being a truly flat, credit-card style card. Type II cards have small bulges at the top and bottom to accommodate circuitry. Type III cards have thick lumps to hold a disk drive. Figure 30.9 illustrates the apparent differences between the three card types.

Note that many notebook computers lay out their endowment of two PCMCIA slots with one over the other and without a metal separator. This design allows the two slots to hold two individual Type II or a single Type III card. Nearly all current PCMCIA slots are wide enough to accommodate Type II cards. The only place you're likely to encounter a slot that will accept only Type I cards is equipment manufactured before 1992.

Under the current PCMCIA standard, both Type I and Type II cards can be implemented in extended form. That is, their depth can be increased by an additional 50 mm (to 135 mm) to hold additional componentry—for example, the antennae of WiFi network adapters. Such extended cards project about two inches more from standard PCMCIA slots.

To ensure that all cards easily and securely mate with their connectors, the PCMCIA standard requires that card guides be at least 40 mm long and that the PCMCIA card connector must engage and guide the connector pins for 10 mm before the connector bottoms out.

The layout of a PC Card or CardBus card is essentially symmetrical, meaning that it could inadvertently be inserted upside down. The PCMCIA design allows for such cases of brain fade by eliminating the risk of damage. Although the cards do not work while inverted, neither they nor the computers into which they are plugged will suffer damage.

Because the size and placement of labels on the cards is part of the standard, when you are familiar with the layout of one PC Card, you will know the proper orientation of them all, and CardBus cards as well. Moreover, other physical aspects of the cards—the position of the write-protect switch (if any) and battery (if needed)—are standardized as well. The PCMCIA standard also recommends that the batteries in all cards be oriented in the same direction (positive terminal up).

In addition to the physical measures that facilitate getting the cards into their sockets, two pins—one on each side of the connector—allow the computer host to determine whether the card is properly seated. If the signal (ground) from one is present and the other is not, the system knows that the card is skewed or otherwise improperly inserted in the connector.

Connector

All types of PCMCIA cards use the same 68-pin connector, whose contacts are arranged in two parallel rows of 34 pins. The lines are spaced at 1.27 mm (0.050 inch) intervals between rows and between adjacent pins in the same row. Male pins on the card engage a single molded socket on the host.

To ensure proper powering up of the card, the pins are arranged so that the power and ground connections are longer (3.6 mm) than the signal leads (3.2 mm). Because of their greater length, therefore, power leads engage first so that potentially damaging signals are not applied to unpowered circuits. The two pins (36 and 67) that signal that the card has been inserted all the way are shorter (2.6 mm) than the signal leads.