Rivaling the exquisite, miniature beauty of the Fabergé eggs made for the Russian czars, Personal Computer Memory Card International Association (PCMCIA) hard disks are tiny jewels crowning the hard-disk industry. Amazingly, disk makers squeezed the guts of a conventional, multiplatter hard disk into a 2.12× 3.37×0.41-in. (approximately 54×85.6×10.5-mm), Type III PCMCIA package. Even thinner Type II hard disks also exist. (PCMCIA recently changed the official name for PCMCIA cards to "PC Cards.")

Weighing 3 oz or less, the Type III disks store an astounding amount of data-as much as 420 Mbytes. Be careful, though. Some makers quote storage for compressed files. If you detect this dodge, simply divide the quoted capacity by two to derive the real capacity.

Makers are producing these marvels with some oddball markets in mind besides the disks' obvious use in laptop computers. These markets include personal digital assistants (PDAs), electronic games, dedicated word processors, and automotive information and navigation systems.

This list shows that disk makers are better engineers than they are soothsayers. PDAs failed in the market because potential customers could not figure out what the PDAs were or why anyone would want to buy one. This mystery persists to this day. Electronic games and dedicated word processors are price sensitive. PC Card hard disks cost as much as or more than these products themselves. However, PC Card hard disks are rugged enough to survive the automotive environment. Currently, a rental-car company is testing in-car navigation systems that use PC Card hard disks. Whether such systems will replace gas-station maps and the American Automobile Association's hand-customized route maps, only time will tell.

PCMCIA has done an excellent job of ensuring that designers have usable, workable standards. Because of PCMCIA's efforts, users can mix and match PC Cards from all makers and can simply plug the cards into any device and expect them to work without problems or conflict. All PC Card hard disks comply with the PCMCIA's ATbus-attachment (as in IBM PC/AT) specification. So, all PC Card hard disks will be compatible with all PCs, both new and old.

Although some PC Card disk makers at first resisted the PC Card 3.0 Standard's requirement for a card-information structure (CIS), all PC Card hard disks now have a CIS. The host computer can interrogate any PC Card's CIS to determine the card's functions-such as low-voltage operation or DMA-storage format, and software drivers. This information is necessary for disks to comply with the Plug-and-Play Standard. All PC Card hard disks are plug-and-play and "hot-swappable," which means you can plug them in and out with the power on and without rebooting.

As for software drivers for PC Card hard disks, PC Card hard-disk makers are sticking to the standard operating systems (OSs) for PCs: Microsoft Windows, OS/2, and the Macintosh OS. Drivers for workstation OSs are noticeably absent.

All PC Card hard disks require the host computer to have card-and-socket services software. SystemSoft's CardSoft PCMCIA device drivers are the most common, and Versa Technology also offers a package, called CCMSTR.SYS. PC Card hard-disk makers report that the software for their disks consumes 20 to 100 Mbytes of RAM.

Card-and-socket services software is not the only software you get with PC Card hard disks. Allowing for the worst case, Epson's $699 260-Mbyte PC Card hard disks come with a data-recovery program ominously named "DRoP."

Calluna formats its 260-Mbyte Type III disks at its factory in Scotland so that the user doesn't have to format them. The company's disks also have built-in, user-programmable software for data security and power management. The proprietary data-security software protects stored data with a password.

As storage devices, PC Card hard disks could hardly be more straightforward. You first run a setup program to install the device drivers. From then on, you simply plug the disk in and use it, just as you would any other drive. But the engineering that makes this smooth and simple performance possible is anything but straightforward.

The petite platters of PC Card hard disks are rotating so fast, and their magnetic domains are crammed so tightly together that the industry had to upgrade the read-channel electronics of higher capacity disks. At today's read-channel bit rates, the chances of less-than-perfect bits are much higher than with older, bigger, slower disks. All PC Card hard-disk makers have adopted partial-response, maximum-likelihood (PRML) decoding of bit streams of their higher capacity disks. PRML detects and corrects errors in these bit streams on the fly.

Alone among the dizzying swarm of new PC Cards, PC Card hard disks have electromechanical as well as electronic components. In general, electromechanical components are significantly less reliable than electronic components. In particular, a major manufacturer of redundant arrays of inexpensive disks (RAIDs) reports that the calculated MTBF that hard-disk manufacturers quote in their spec sheets are optimistic when compared to actual field experience. In contrast, MTBF for electronic equipment (using either Bellcore or MIL-SPEC methods) yields conservative figures compared to actual field-failure rates. (EDN is attempting to get permission to publish this disk research for publication in an upcoming article on disk technology.)

The disk industry is obviously concerned about these Lilliputian disks' ruggedness and resistance to shock and vibration. The disk makers expect users to bump, drop, and handle these precision devices with all the caution and dexterity customarily lavished on a $2 floppy disk. Today's PC Card hard disks can withstand as much as 200g shock during operation and up to 1000g when not in use. For reference, 200 and 1000g shocks are roughly equivalent to 4- and 8-ft drops, respectively, upon a medium-hard surface.

Makers have achieved these goals via different routes. For example, MiniStor Peripherals Corp's $499 260-Mbyte disk employs a positive mechanical latch to lock its head actuator in place when the disk is not in operation. Once engaged, the latch requires no further force. In contrast, some PC Card hard disks use an energy-consuming magnetic lock that is less positive than the mechanical latch.

The company characterizes its heads as "nanosliders" to signify that they are smaller and lighter than the "microsliders" that its larger disks use. These nanosliders have inherently lower mass and less inertia than the heads of larger drives, providing better shock resistance and fewer head crashes. Small heads allow for more tracks and higher storage densities. Smaller heads are also shorter than larger heads, permitting closer stacking of media platters.

MiniStor's PC Card hard disks' oversized spindle-motor bearing is the same component the company uses in its 3.5-in. disks. The large bearing protects the motor from shock.

Although some companies' disks admit filtered air to their head-disk assemblies (HDAs), MiniStor seals its HDAs and backfills them with dry nitrogen gas. The sealed unit should prevent dirt from fouling the disks' extremely tight mechanical clearances and allow the disks to operate at altitudes up to 40,000 ft.

PC Card hard-disk makers could not offset their platters down around their motors similar to the way automobile tire rims are offset back around their brake assemblies. To do so would reduce the already-minuscule writable area of the platters. Further, PC Card hard-disk makers often use glass platters, which tend to be perfectly flat, to enhance shock and vibration resistance. Consequently, the disk's spindle motors have to be especially thin. For example, Maxtor's new motor for its $595 (OEM evaluation units) PC Card hard disk measures 0.5 in. in diameter and is only 5 mil thick. The motor uses plain bearings instead of ball bearings to reduce the motor's size and to increase its strength.

Maxtor hasn't limited innovations to the disks' mechanical components. The disks use a piezoelectric-film accelerometer to sense and withstand mechanical shocks up to 2000g. Shocks can jar the head, causing it to skip to a wrong track during reads and writes. The shock sensor trips at shock levels that are only 10% of the shock that would knock the head off track. Once tripped, the disk suspends read/write operations, retaining the data in a buffer, until the shock stops. After the shock, the disk automatically writes the data.

Like many manufacturers, MiniStor had to "stiffen" and "embed" its disks' servo systems. A "stiff" servo is a digital servo having a high sampling rate. The servo rate is simply a function of the number of servo-reference patterns per revolutions recorded on the platters and the platters' rotational speed. A high sampling rate provides tight control of the heads' actuators, keeping heads on track even if the going gets bumpy.

An embedded servo is an alternative to using one complete platter surface for servo information. Instead, all surfaces have bursts of servo data separated by data fields. This scheme allows disk designers to strike the required balance between capacity and ruggedness for each application. Another advantage of embedded servos is that they are less vulnerable to thermally induced misalignment of mechanical components because each head tracks its own servo burst.

Intégral's PC Card hard disks look like other PC Cards on the outside but have fundamentally different insides. The company employs what it calls "dynamic head loading," which removes the heads from the disk when the power is off or when the disk is in standby mode. Other makers continue to use a downsized version of the "start/stop contact" method. In this method, the heads come to rest-perhaps in designated landing zones-on the platters' surfaces when the platters spin down.

Intégral says that, by parking its heads off the platters, the disks resist shock better than start/stop-contact disks do. Further, because the heads never wear the media down by physically touching the platters, the disks are capable of unlimited power cycling. In addition, because the heads are not resting on the platters at the instant the disks spin up, they avoid "stiction"-a nasty effect combining sticking and friction. High humidity and temperature aggravate stiction, causing disks to stop cold under worst-case conditions. Manufacturers of start/stop-contact disks intentionally roughen platter surfaces to avoid stiction.

Intégral PC Card hard disks' platters have especially smooth, polished surfaces that enhance the micro-aerodynamics of the disks' flying heads. The polished surfaces permit the heads to fly 1.2 µin. above the platter on an air bearing vs the 3 µin. that 2.5- and 3.5-in. disks require. This low flying height permits the disk to put down smaller magnetic domains, allowing denser packing of data and increasing disk capacity.

PC Card hard disks also have to make room for disk-controller electronics that once filled an entire pc board. IC packaging varies widely from maker to maker. Intégral, for example, uses no standard packaging. The company's disks retract the head actuator completely off the platter, and the retracted actuator takes up room inside the PCMCIA package that start/stop-contact disks can use for electronics. The company uses modern, strenuous packaging technologies, such as multichip modules and chips on boards.

Maxtor, on the other hand, rolled over the electronics it developed for larger disks into its PC Card disks because it had already shrunk its electronics. The company uses a Texas Instruments' DSP µP as the heart of its disk controller. The controller has only five ICs: the DSP µP, an interface adapter, a TI dedicated-servo IC, a ROM, and a read/write-channel preamp IC. The company puts these five ICs onto a dual-sided, six-layer, 18-mil pc board mounted in the same plane as the platters. However, the DSP µP and the interface IC have custom pinouts that exactly match the PCMCIA connector, easing pc-board design and improving signal integrity.