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

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

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!