SolutionBase: Sorting out the differences in memory cards for PDAs and digital cameras

With all of the different options in flash memory cards available, it's hard to keep them all straight. This article points out the different kinds of cards and what you can use them for.

Over the past few years, memory cards have made their way into devices ranging from digital cameras to PDAs to printers and even to PCs in a quest to provide a ubiquitous storage mechanism. While some look slightly similar and others appear worlds apart, each type of card features its own technology and limitations. Even inside each category, there are a lot of choices. For example, SanDisk, a large supplier of memory cards, has no less than three types of SD cards alone: the "Ultra II SD", the "Extreme SD", and the "Extreme III SD". This article will highlight some of the significant differences in the various types of cards available on the market.

Secure Digital (SD)

Measuring just 24 mm (0.94") x 32 mm (1.26") and at only 2.1 mm (0.08") thick, the SD card has become common in many devices that demand good storage capacity and decent read/write performance. Even better, an SD card can be write-protected, just like a floppy disk, to prevent accidental erasure of content.

With 9 pins connecting the card to the device in which it's being used, typical SD cards consume between 2.7 and 3.6V of electricity and weigh just 2 grams. SD cards are commonly available in 1GB capacities, and 2GB and 4GB storage are available from some vendors, making SD one of the leaders when it comes to the amount of storage the device is capable of providing.

SD cards also help to protect the rights of content providers by providing copyright protection right on the device. Even more, SD cards can be expanded upon beyond storage. For example, SanDisk sells a combination 256MB/Wireless-B card that provides storage and wireless networking.

At capacities up to 512MB, the typical SD card transfers data at about 10 MB/s. Beyond that, the transfer rate usually increases to 20 MB/s. SD leads the pack in this regard. SD formats are determined by the SD Association, made up of a number of companies that provide SD cards.


miniSD cards are newer and smaller than typical SD cards. Weighing in at just 1 gram and with dimensions of 20 mm (0.79") x 21.5 mm (0.85") x 1.4 mm (0.06"), miniSD cards consume the same amount of electricity as their large counterparts, but use 11 pins to provide a connection to their host device.


TransFlash is a reduced size version of miniSD that is not designed to be handled on a regular basis. In fact, the SD Association considers this to be a "semi removable" media format designed primarily for mobile phones and PDAs.

TransFlash cards are 11 mm x 15 mm x 1 mm thick and come in capacities up to 128MB.

Figure A

A TransFlash 128MB module. That's a fingertip. Very small!

CompactFlash (CF)

Since 1994, CF cards have provided a solution for people needing reasonable amounts storage, but that didn't need the size of a full PCMCIA card to carry around. Even though, the current CF spec calls for capacities in excess of 130GB, you won't find cards with this kind of capacity on the market at this point. I don't even want to think of the price tag on something like that! You'll be somewhat hard pressed to find anything with more than a few to a dozen or so GB of capacity on the market presently.

CF specifications call for a maximum data transfer rate of 16MBps. This isn't a breakneck speed, but is good for devices like lower end digital cameras and wireless networking.

There are two major classes of CF storage devices available today: Type I and Type II. Both are similar in size, with Type II being just slightly thicker than Type I. Both use 50 pins to provide the electrical connection to the host device and both support either 3.3V or 5.0V host current. Both card types measure 43 mm (1.7") by 36 mm (1.4"). However, Type I cards are 3.3 mm (0.13") thick and Type II cards are 5 mm (0.19") thick.

With these dimensions, it's probably obvious that a Type II card won't fit into a Type I slot. That said, a Type I card will generally work in a Type II slot.

CompactFlash cards do more than just provide storage, though. In fact, many handheld PCs include a CF slot for use with a CF wireless network adapter.

SmartMedia (SM)

Once called Solid State Floppy Disk Card (SSFDC), SmartMedia cards measure 37 mm x 45 mm and are 0.76 mm thick. SmartMedia cards don't hold as much data as newer types of memory, though. In various searches, vendors indicate that they have cards available supporting up to 512MB, but with other cards commonly available in 1GB and even greater capacities, SmartMedia falls short.

SmartMedia cards do have one significant advantage over some other types of cards however; they are able to read and write data to and from the card in 256- or 512-byte blocks providing much more granular data access. However, SmartMedia cards aren't as sturdy as some other technology and require careful handling. In fact, if you look at a SmartMedia card, you'll see a wavy gold connection strip across the leads. This isn't just for show; rather, it helps to reinforce the physical integrity of the card.

Like CompactFlash cards, SmartMedia cards come in two voltages--3.3V and 5V. While some equipment supports cards with both voltages, not all does, so care if required when purchasing new cards. To tell which voltage your current cards use, hold the card in your hand with the gold pins facing up. If the notch on the card is in the upper left hand corner, you're holding a 5V card. If it's in the upper right, it's a 3.3V card.

SmartMedia cards aren't speed demons and max out at around 3.5MB/s at 64MB and greater capacity. Below that, transfer rates in the 1.3MB/s range are typical.

As for weight, or lack thereof in this case, SmartMedia cards weigh in at just 2 grams, making them ideal for portable applications.

Memory Stick (MS)

Adding such capabilities as copyright protection on some models and even an erasure protection switch a la a floppy disk, MemoryStick appeals to companies that want to limit the distribution of their works. It should be noted that not all MemoryStick cards include these features.

MemoryStick media is 50 mm x 21.5 mm with a thickness of 2.8 mm. According to the designers of MemoryStick, it's very resistant to shocks from a drop and from vibration. MemoryStick cards feature a 10-pin connection to the host device as well as a design meant to help remove dust from the contacts during insertion and removal.

MemoryStick is a very versatile card and is used in cameras, video cameras, PDAs and more.

MemoryStick supports a reasonable read and write speeds of 2.5MB/s, although the newer MemoryStick Pro product (below) vastly improves these figures. MemoryStick products, with the exception of the DUO line, weight 4 grams. The DUO line, a smaller version discussed below, weighs half this amount.

There are four major types of MemoryStick products available:

  • Memory Stick
  • Memory Stick DUO
  • Memory Stick Pro
  • Memory Stick Pro DUO

The "Pro" products improve upon the standard MemoryStick specification by providing much higher capacity and faster read/write speeds. In fact, to qualify for the Pro designation, a MemoryStick product must provide a minimum write speed of 15MB/s. MemoryStick Pro provides a read speed of up to 20MB/s.

MemoryStick DUO cards measure 31 mm (1.22") x 20 mm (0.79") x 1.6 mm (0.06") thick. Whereas the potential storage capacity for a regular MemoryStick card is 4GB, DUO cards support up to 2GB. Weighing in at just 2 grams, DUO cards also support speeds of about 2.5 MB/s and have 10 pins for a connection to the host device.

MultiMediaCard (MMC)

The slowest of the bunch, the MultiMediaCard reads data at about 1 MB/s and writes it at a paltry 200 KB/s. However, the MMC comes in as the smallest of the bunch at 24.0 mm x 32 mm x 1.4 mm thick and at a weight of just 1.5g.

The low data transfer rate for these devices isn't necessarily a bad thing, though. The cards easily support applications such as voice recording and some storage. For devices that continue to use these cards, they remain readily available at prices near those for other types of storage cards. The Multimedia card has been largely replaced by the newer xD-Picture Card, and the Reduced Size MMC, both discussed below.

Reduced Size MMC (RS-MMC)

RS-MMC is used primarily in mobile phones, handheld PCs, and digital music players and provides users with high storage capacities up to and exceeding 1GB. RS-MMC cards are sized at 18 mm x 24 mm x 1.4 m thick.

xD-Picture Card (xD)

The xD-Picture Card replaces the MultimediaCard. Developed by Olympus and Fuji, the xD card supports capacities of up to 8GB and sports a 3MB/s data transfer speed. Some specifications list a write speed of 5MB/s as well.

Capacities less than the full 8GB are commonly available. Through some research, I did find xD cards with 1GB capacity, but 512MB was much more common. xD cards are 20 mm x 25 mm x 1.8 mm thick.

From the name, it's easy to determine the primary reason for xD's existence, but it doesn't seem to be doing nearly as well as other types of cards on the market.

Remembering Memory options

This is one market where variety is the phrase of the day. For example, in the SD card line, there are storage devices doubling as wireless nodes, cameras and more. CompactFlash cards commonly provide wireless access for handhelds and more. Between that and vendors that provide multiple lines with multiple specifications, be sure to do your homework before you venture out to buy cards!