Secure Digital (SD) Cards – popular digital camera memory card format

Secure Digital (SD) is a memory card format used to transfer data between PCs and smaller devices, such as digital cameras, PDAs, MP3 players and mobile phones. Because the memory used is solid state, i.e. it remembers what’s been written to it when the power is removed, the information stays on the card when it’s unplugged. In order to get the data onto a PC, a card reader, either integrated into the PC or connected via a USB port, is required. Laptop users can also use an adapter that takes SD cards and fits into a PC Card slot. A consortium of Matsushita (aka Panasonic), Toshiba and SanDisk developed SD because they had been outflanked by Sony’s introduction of Memory Stick, their own proprietary card format, in 1998. This had two features that neither MultiMediaCard (MMC) nor CompactFlash, (CF) the other major player in memory cards at the time, possessed: a write-protect switch and integrated copyright protection. SD came onto the market in 1999 and was based, at least physically, on the MMC format. SD cards are slightly thicker but this allows the contacts to be recessed to protect them from damage. The other major physical change from the MMC design was an asymmetrical profile, which prevents SD cards from being inserted the wrong way round. The closeness of the two designs means that MMC cards can be used in SD slots, but not the other way round. Multi-function card readers are available with a slot than can accept SD cards as well as MMC, SmartMedia, Memory Stick and xD-Picture Card. One interesting and innovative adaptation is SD Plus,...

MultiMediaCard (MMC) – SanDisk and Siemens NAND memory card for phones and smaller devices

Ten years after releasing the CompactFlash card and watching it become very successful, SanDisk had a go at repeating the trick, this time in collaboration with German company Siemens, releasing the MultiMediaCard (MMC). This was designed to take over from CompactFlash in most markets but also reach newer, smaller devices that the CF format is simply too big to fit, such as mobile phones, smartphones and PDAs. An MMC card is about the size of a postage stamp whereas a CF card is closer to a book of matches. One disadvantage of the form factor is that it is possible to insert an MMC card into a card reader slot the wrong way round. As the pins won’t connect there won’t be any damage, but it can be frustrating. The size reduction was made possible by the adoption of NAND flash memory developed by Toshiba and introduced in their SmartMedia card format in 1995. NAND flash memory allows greater storage densities, is cheaper to make and the data states are retained for longer than NOR flash memory, the method used by CF. NAND flash memory also has to be written to and erased in block mode, rather than bit-by-bit. This property is a disadvantage for directly accessing memory, as you do with a PC’s RAM, but makes it a good match for emulating secondary storage such as hard disks or optical disks. MMC came on the market in 1997 and the available memory sizes have constantly increased since then, although not to the same extent as cards that are available in CF form. There is a theoretical limit of...

Guide to SanDisk’s CompactFlash memory card

Memory manufacturer SanDisk introduced CompactFlash (CF) in 1994 as a small format memory card, which used the existing PCMCIA-ATA specification to communicate with PCs. These cards are roughly a quarter the size of a PC Card and use flash technology that allows data to be stored indefinitely, without any power to the card. CF cards come in two sizes, Type I and Type II. The only difference between the types is the thickness of the cards, 3.3mm for Type I and 5mm for Type II. This reflects the fact that there were different thickness PC Cards and slots at the time, and this allowed CF cards to take advantage of the thicker Type II PC Card slots. However, these thicker versions are now obsolete. CF had been devised primarily for devices smaller than PCs such as digital cameras and PDAs, and other memory manufacturers were attracted to the format by the standard interface. As CF slots began appearing on cameras and PDAs, as well as other more specialized devices, prices came down. At the same time, the available RAM on the cards increased until cards of 32Gb and more became available. The CF standard allows for a theoretical maximum size of 137Gb. The CF specifications are regularly being updated to take account of increased hard disk sizes, memory access speeds and upgrades to hard disk specifications such as IDE Ultra DMA 133 and SATA Because all types of CF cards can operate at both 3.3 and 5V, and they conform to the PCMCIA-ATA and TrueIDE functionality (compatible with ATA/ATAPI-4 disk interface specs) they can use almost any PC operating...

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