Adding PCMCIA (Personal Computer Memory Card International Association) card slots to a desktop computer or an embedded system is pretty easy. You just plug in one of the scores of different add-in and add-on PCMCIA card drives that have recently become available. Most of the drives are for PCs, but you can also find units for VME, STD, and PC/104 systems. Prices begin at under $300.

Making sure a card drive will operate all the different types of PCMCIA cards is not so easy, however. The numerous available drives have different numbers of slots, different slot sizes, and widely varying support for the many different types of PCMCIA cards. And a few drives, according to word-of-mouth reports, are plagued with noise problems and aren’t reliable.

The majority of PCMCIA card drives will do everything you want them to. They accept cards of the three standard thicknesses and possibly a fourth nonstandard thickness. They come with system software that operates any kind of PCMCIA device—a memory card, a hard disk, a LAN adapter, and more. They are well designed and have been adequately tested for reliable operation.

But some card drives do have limitations, some of which may be important to you and some of which may not. For example, some drives will operate memory cards or hard-disk cards, but not other peripheral cards. Some drives may not support flash-memory cards. Most drives for PCs allow hot swapping (changing cards with the power on), but this capability may be missing and possibly unimportant for embedded applications.

Beware of bad designs

Apparently, at least a few PCMCIA drives don’t incorporate all the good design practices that make for reliable operation. Solid evidence is hard to come by, but some manufacturers claim unnamed competitors either don’t have the expertise to design reliable drives or are cutting corners and costs to sell in the price-sensitive retail PC market. Noise, ground bounce, and crosstalk are the reliability culprits, these manufacturers say. Reliability suffers primarily from poor circuit-board layout and from cables that are improperly terminated and inadequately driven.

Most of the available PCMCIA card drives are for use with desktop personal computers, thus the emphasis on reducing costs. These products are either internal units, which install in a drive bay, or external units, complete with housing and power supply. In either case, they connect to the PC with a cable. In most cases, the cable connects the drive to an adapter board that plugs into the PC’s expansion bus. The drive may, however, connect to the PC’s parallel port, serial port, SCSI port, or IDE interface.

You have fewer variables and fewer options when adding PCMCIA slots to embedded systems. Most PCMCIA card sockets for VME, STD, and PC/104 systems are on a plug-in board, so you don’t have reliability concerns related to cabling. However, because most PCMCIA applications have centered around PCs, adaptations for other buses tend to be ad hoc creations that don’t have the full range of capabilities outlined in the PCMCIA specification. Most embedded-system add-ins support only memory cards, a notable exception being Ziatech’s ZT 8921, a board for the STD-32 bus with two card sockets for PCMCIA peripherals.

Of the many differences in PCMCIA card drives, quite a few merely reflect PCMCIA’s relative newness. The PCMCIA standard is still evolving and hasn’t yet been fully implemented. The standard also allows many different types and several different sizes of cards. (Indeed, much of PCMCIA’s value stems from that diversity.) Consequently, PCMCIA cards and drives aren’t, and won’t be, quite the commodity items that floppy disks and floppy drives are. You have to make an informed selection based on your needs.

But you will also find that seemingly similar PCMCIA card drives actually have quite dissimilar designs. Some design variations stem from differences in the various socket-controller ICs (Refs 1 and 2) that are the heart of ISA-bus based card drives. Other variations relate more to noise issues, both in connecting cables and in a drive’s controller board.

Noise is inherently a problem in PCMCIA designs. Socket-controller ICs, which establish an interface between a PC’s ISA bus and a PCMCIA socket, generate quite a bit of noise by simultaneously switching numerous signals. If the design of a card drive’s controller board doesn’t take that noise into account with proper layout and good grounding, the drive may not operate reliably.

Cables only exacerbate the noise problem. Unlike notebook computers, which have PCMCIA card sockets on a circuit board, add-in and add-on PCMCIA card drives must use ribbon cables between an adapter board and a drive bay. These cables contain 80 to 100 pins per card socket, depending on how many ground and power connections they have, and can be up to 2 ft long. According to research performed by Vadem Corp (Ref 3), a manufacturer of socket-controller ICs, you must terminate these cables properly and drive them adequately for reliable operation.

Buffers on PCMCIA address, data, and control lines help deal with cable noise. Some PCMCIA socket-controller chips have built-in buffers; others rely on external devices—usually 244-type line drivers and 245-type bus transceivers. In either case, the buffers not only isolate PCMCIA card sockets from the computer’s expansion bus (a requirement for hot swapping), but also serve as line drivers on the connecting cables. On-chip buffers can drive most cables, but external devices offer greater flexibility for special situations.

But buffers aren’t a panacea for cable noise. According to Kasturi Gopala- swamy, Cirrus Logic’s marketing manager for PCMCIA ICs, a designer’s expertise in dealing with a cable’s transmission-line effects is equally or more important. A smart designer who matches impedances and looks after other variables, says Gopalaswamy, can guarantee successful operation over cables up to about 14 or 16 in. long.

In general, buffers are a requirement for longer cables. External buffers can deliver more driving current to cables, but other factors for improving signal transmission are also important. For example, says Gopalaswamy, Cirrus’ socket-controller chips perform signal rounding to prevent cable ringing that would be induced by sharp transitions.

Some PCMCIA card drives apparently suffer from inadequate use of buffers. Some drive manufacturers omit buffers from the drive-bay assembly, at the cable end opposite the socket-controller IC. And you need buffers at both ends to run at full ISA-bus speeds, says PCMCIA design consultant Michael Mori. Otherwise, says Mori, author of a recent book on PCMCIA design (Ref 1), you have to put up with wait states.

In short, the issues of noise in general and cables and buffers in particular are complicated. There are no hard and fast rules for judging whether a particular card drive is adequately designed, but be aware that some may not be. If you’re in the market for a PCMCIA drive, be prepared to ask some tough questions.

Consider, too, that the socket-controller IC’s location determines the type and amount of cabling. In most add-in products, the socket-controller chip is on the adapter card that plugs into the PC’s ISA bus. Cables connect the chip, either directly or through external buffers, to the card sockets in the drive bay. In a few products, however, the controller IC is on the drive-bay assembly. In these products, cables actually bring the ISA bus to the drive bay.

If a PCMCIA card drive has two or more card slots, having the controller IC on the drive-bay assembly reduces the amount of cabling needed. If space is tight, the difference could matter, and a single-cable approach should—theoretically, at least—reduce costs. In either case, reliable operation depends on a design that addresses noise issues.

If you’re concerned about the compatibility of a card drive with many new and future PCMCIA cards, make sure the drive supports both 5 and 3.3V cards. The first release of the PCMCIA specification supported only 5V cards, and some products still adhere to that release only. Also, be aware that some flash-memory cards require 12V for writing and programming. If you anticipate using these cards, be sure your drive provides 12V on the proper socket pin.

For compatibility well into the future, you may want to ask drive manufacturers about their plans for supporting a newer, more flexible voltage scheme that will be in the next revision of the PCMCIA specification. The new arrangement will include the use of voltage-sense connector pins and mechanical keys to identify cards’ capabilities and to prevent damage.

Don’t forget to consider power in your assessment of card drives, either. It’s probably safe to assume that a drive can supply as much power as any PCMCIA card needs, except that—as we all know—it’s never safe to assume. Bear in mind that a 1.8-in. disk drive in a PCMCIA card can draw 600 mA of current or even more during spin-up.

Finally, be sure you know what software you’re getting with a card drive and, just as important, what software you need. For PCs, PCMCIA system software is very complex, providing all kinds of ease-of-use features related to hot swapping and automatic configuration. For embedded systems, those features are largely unnecessary—which is fortunate, since they’re also unavailable.

PCMCIA system software for PCs consists of two main modules—Socket Services (a BIOS extension) and Card Services, which provides high-level functions to application programs. These modules are developed by only a handful of companies, mostly BIOS vendors who sell the software to hardware OEMs. Some hardware companies develop their own system software, which may well suit your purposes. However, if you’re concerned about broad compatibility, you’ll want to be very thorough in determining just what the software will and will not do.

Is the software adequate?

A key question about software is whether it will work with all the different card types you anticipate needing. Memory cards, for example, will work with software that conforms only to PCMCIA Release 1.0; most peripheral cards, however, require Release 2.0 software. Flash-memory cards have their own software requirements. Unless a flash card emulates a disk drive, it will usually require flash-file-system software. This software, like Socket Services and Card Services, is provided to drive manufacturers by only a handful of companies.

Your choices in PCMCIA card drives are numerous. At least 30 companies make or sell drives, and many of these companies have numerous configurations that cover just about every

combination of connectivity, card slots, and capabilities. For example, most card reader/writer units have one or two slots, but some have as many as eight.

Some add-in units for PCs put slots in a drive bay and additional slots on the rear of the computer. The front slots are for frequently used devices, such as memory cards, and the rear slots are for PCMCIA cards that are seldom removed and may require external connections, such as LAN-adapter cards. The rear PCMCIA sockets are on the adapter board that plugs into the ISA bus.

As a starting point for selecting a PCMCIA card drive, see box, "For free information... ." The listings there indicate the types of systems that each manufacturer’s drives connect to and the types of connection. However, because so many different card drives exist, with new ones appearing all the time, check with the manufacturers themselves for information on their latest offerings.

1. Mori, Michael T, The PCMCIA Developer’s Guide, Sycard Technology, Sunnyvale, CA, 1994.

2. Legg, Gary, "Special components simplify interface to PCMCIA cards," EDN, June 10, 1993, pg 61.

3. Fong, Henry, "Adding PCMCIA sockets to desktop computers," EDN Products Edition, November 15, 1993, pg 59.

4. Legg, Gary, "PCI local bus gathers momentum," EDN, February 3, 1994, pg 25.

5. Richter, Bryan, "Integrating PC card slots in desktop computers," IC Card Systems and Design, January 1994, pg 27.

6. "Buyer’s guide: PC card reader/writers," IC Card Systems and Design, January 1994, pg 32.