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CompactFlash (CF) was originally developed as a type of data storage device used in portable electronic devices. For storage, CompactFlash typically uses flash memory in a standardized enclosure. This form was first specified and produced by SanDisk in 1994. The physical format is now used for a variety of devices.
There are two main subdivisions of CF cards, Type I and the thicker Type II (CF2) cards. The CF Type II slot is used by Microdrives and some other devices. There are four main speeds of cards including the original CF, CF High Speed (using CF+/CF2.0), a faster CF 3.0 standard and a yet faster CF 4.0 standard that is being adopted as of 2007.
CF was among the first flash memory standards to compete with the earlier and larger PC Card Type I memory cards, and was originally built around Intel‘s NOR-based flash memory, though it switched over to NAND. CF is among the oldest and most successful formats, and has held on to a niche in the professional camera market especially well. It has benefited from having both a good cost to memory size ratio relative to other formats for much of its life, and generally having larger capacities available than smaller formats. Note: flash memory supports only a limited number of erase/write cycles (around 1,000,000 write cycles for NAND flash memory) before a particular "sector" can no longer be written. Typically the controller in a CompactFlash attempts to prevent premature wearout of a sector by mapping the writes to various other sectors in the card – a process referred to as wear levelling.
CF cards can be used directly in PC Card slot with a plug adapter, used as an IDE hard drive with a passive adapter, and with a reader, to any number of common ports like USB or FireWire. As it has a bigger size relative to the smaller cards that came later, many other formats can be used directly in a CF card slot with an adapter (including SD/MMC, Memory Stick Duo, xD-Picture Card in a Type I slot, and SmartMedia in a Type II slot, as of 2005) (some multi-card readers use CF for I/O as well).
NOR-based flash has lower density than newer NAND-based systems, and CompactFlash is therefore the largest of the three memory card formats that came out in the early 1990s, the other two being Miniature Card (MiniCard) and SmartMedia (SSDFC). However, CF did switch to NAND type memory later on. The IBM Microdrive format, which used CF Type II, was not solid state memory.
CompactFlash defines a physical interface which is smaller than, but electrically identical to, the PCMCIA–ATA interface. That is, it appears to the host device as if it were a hard disk of some defined size and has a tiny IDE controller onboard the CF device itself. The connector is about 43 mm wide, and the case is 36 mm deep and comes in two standard thicknesses, CF I (3.3 mm), and CF II (5 mm). Both types are otherwise identical. CF I cards can be used in CF II slots, but CF II cards are too thick to fit in CF I slots. Flash memory cards are usually CF I.
CF cards are much more compact than the even earlier PC Card (PCMCIA) Type I memory cards, except for its thickness which matches PC Card Type I and Type II thicknesses respectively. CF has managed to be the most successful of the early memory card formats, outliving both Miniature Card, SmartMedia, and PC Card Type I in mainstream popularity. SmartMedia did offer heavy competition to CF in smaller devices, and was more popular than CF at its peak in terms of market penetration, but SmartMedia would cede this area to newer card types (during the period of roughly 2002-2005).
The memory card formats that came out in the late 1990s to the early 2000s (SD/MMC, various Memory Stick formats, xD-Picture Card, etc.) offered stiff competition. The new formats were significantly smaller than CF, in some cases by an even greater fraction than CF had been smaller than PC Card. These new formats would dominate the memory card market for PDAs, cell phones, and consumer cameras (especially subcompact models).
However, a CF interface continues to be offered on many devices, and remains the main standard for professional cameras, as well as a number of consumer models as of 2005. Key features remain having a relatively low cost per megabyte, offering a greater capacity than smaller cards, the ability for the CF II to use MicroDrive, and the availability of adaptors which allow many other smaller card formats to be used in a CF slot. And CF cards can also be used in PC Card slots with very inexpensive plug adapters.
CF (and other formats) have not managed to totally replace PC Card Type I and II Memory cards in a number of industrial applications.
Flash memory devices are non-volatile and solid state, and thus are more robust than disk drives, and consume around 5% of the power required by small disk drives, and yet still have good transfer speeds (up to 40 MB/s write and 40 MB/s read for the SanDisk Extreme IV). They operate at 3.3 volts or 5 volts, and can be swapped from system to system. CF cards with flash memory are able to cope with extremely rapid changes in temperature. Industrial versions of flash memory cards can operate at a range of -45 to +85 °C.
CF devices are used in handheld and laptop computers (which may or may not take larger form-factor cards), digital cameras, and a wide variety of other devices, including portable audio recorders and desktop computers.
As of 2006, CompactFlash cards are generally available in capacities from about 32 megabytes to about 64 gigabytes, with perhaps the most popular choices being between 256 MB and 2 GB (in Europe and North America). Lower capacity cards (below 128 megabytes) are becoming rare in stores because higher capacity cards are readily available at the same price. The largest CompactFlash Type II cards commonly available currently are the 16 GB models from various manufacturers – SanDisk launched its 16 GB Extreme III card at 2006’s Photokina trade fair. These cards (and indeed almost all cards over 2 GB) require the host camera to support the FAT32 file system (if the camera is using a FAT file system). These largest cards, however, are not the fastest.
Main article: Microdrive
Microdrives are tiny hard disks—about 25 mm (1") wide—packaged with a CompactFlash Type II form factor and interface. They were developed and released in 1999 by IBM in a 170 megabyte capacity. Then the division was sold to Hitachi in December 2002 along with the Microdrive trademark. There are now other brands that sell Microdrives (such as Seagate, Sony, etc), and, over the years, these have become available in increasingly larger capacities (up to 8 GB as of Oct. 2006).
While these drives fit into any CF II slot, they take more power on average (500 mA maximum) than flash memory (100 mA maximum) and so may not work in some low-power devices (for example, NEC HPCs). Being a mechanical device they are more sensitive to physical shock and temperature changes than flash memory.
The popular iPod mini is a device which uses a compact microdrive to store music.
 CF+ specification
When CompactFlash was first being standardized, even full-sized hard disks were rarely larger than 4 GB in size, and so the limitations of the ATA standard were considered acceptable. Since then hard disks have had to make many modifications to the ATA system to handle ever-growing media, and today even flash memory cards have been able to pass the 4 GB limit. However, CF cards since the original Revision 1.0 have been able to support capacities up to 137 GB.
The CF+ standard, revision 2.0, added an increase in speed to 16 Mbyte/s data-transfer, according to the CompactFlash Association (CFA).
The CF 3.0 and 4.0 standards have also been released which supports up to 66 Mbyte/s data transfer rates, and a number of other features. The CF 4.0 standard supports IDE Ultra DMA 133 for a maximum data transfer rate to 133MB/sec.
 Compared to other portable storage
CompactFlash lacks the mechanical write protection switch that some other devices have, as seen in a comparison of memory cards. CompactFlash does not have any built in DRM or cryptographic features like on some USB flash drives and other formats such as Secure Digital. Such features are rarely used on other cards, however, and are therefore mostly superfluous.
CompactFlash has a disadvantage in its physical design. The pins for electrical contact are long and thin, and not located on the card itself. An improperly inserted card or poorly designed card slot can lead to easily bent and damaged pins, which are nearly impossible to straighten and can effectively destroy the device in which the card is being inserted, leading to high repair/replacement costs.
 Other devices conforming to the CF standard
The CompactFlash format is also used for a variety of Input/Output and interface devices. Since it is electrically identical to the PC Card, many PC cards have CF counterparts. Some examples include:
- Digital Camera
- Barcode scanner
- Magnetic stripe reader
- Super VGA display adapter
- Serial port and USB 1.1 host adapters
- readers for various other Flash media
 Caveat emptor
Compact Flash memory is relatively expensive compared to some other implementations of flash storage, and as such counterfeiting has become a problem. For this reason the credibility of retailers should be taken into account when purchasing cheap CF cards. However, genuine CF cards made by major manufacturers are often available legitmately from online retailers for much lower prices than on the highstreet.
Counterfeiting is not unique to just CompactFlash – some manufacturers have been using ‘read only’ chips for their cards that can only be written to about 10 times after which they fail.