Tech?Update!: SD Card Technology

SD cards are used as storage media in many different mobile devices, e.g. for photographs in digital cameras, for the storage of music files in MP3 players, in camcorders, GPS devices, mobile and smart phones and recently also in e-book readers. It would be more correct to use the term SD memory card or Secure Digital memory card, but the shorter "SD card" is widely used for convenience. Since SanDisk developed the first SD card in 1999, more and more card types entered the market under the supervision of the SD Association (SDA). SD cards come in different storage capacities, sizes and data transfer rates and are not interchangeable, i.e. they are not compatible with all devices. So users have to take care as it sometimes is not clearly apparent which SD card is the right one for their device.

SD Card Development and SD Association (SDA) 

Together with Panasonic (Matsushita) and Toshiba, SanDisk developed the first SD card on the basis of the older MMC (multimedia card) in 1999. "SD" for "Secure Digital" refers to the originally intended function of this type of card: It features DRM (digital rights management) functions based on CPRM (Content Protection for Recordable Media) standards because it was assumed that under the pressure of the music and entertainment industries DRM functions would be widely implemented to prevent data piracy.

One year later, at the Consumer Electronics Show CES, SanDisk, Panasonic and Toshiba announced the foundation of the SD Association. This syndicate has grown to about 1,000 enterprises and aims at the definition of industry standards and the wide dispersion of SD standards to various applications. However, so far none of the standard specifications have been standardized according to DIN-, EN-, IEC- or ISO. 

The first SD cards measured 32 x 24 x 2.1 mm and were available as 32MB and 64 MB cards soon after the foundation of the SDA. During the following years the SDA developed standards for smaller card forms (miniSD, microSD) as well as for cards with higher storage capacities. 

Card Design

SD cards have a specific asymmetrical design with one beveled corner. On the opposite side of the card an optional write protection tab can be found. When there is no tab, the card cannot be overwritten. The cards are equipped with an integrated controller, an SPI Bus and an SD-Bus (1 Bit or 4 Bit). For operation SD cards require 3.3 volt, SD cards of the SDHC and SDXC families can run on 1.8 volt if necessary. Typically the FAT file system based on MBR partitioning is used (mostly FAT32, but also FAT16 and exFAT). However, other formats and partitioning is possible.

The most common SD card families today are distinguished by their storage capacity. Every family comes in three sizes, which may lead to mix-ups. As not all devices are compatible with all SD cards, users need to pay attention when buying SD cards. 

Storage Capacity of SD Card Families

According to their storage capacity SD cards are differentiated into the following families which are downward compatible:

• SD – Secure Digital: Standard for capacities up to 2 GB, in SD-devices, can be used in SDHC- and SDXC-compatible devices. SD cards with a storage capacity up to 4 GB are available, too. On the other hand, older SD devices operate with 1 GB SD cards only. It is necessary to check compatibility of card and device.
• SDHC – Secure Digital High-Capacity: Standard for storage capacities from 4 GB to 32 GB, can be used in SDHC- and SDXC devices
• SDXC – Secure Digital Extended Capacity: Standard for storage capacities from 32 GB to 2 TB, only compatible with SDXC devices

Different Sizes and Special Features

All card types described here are available in up to three sizes: 

• SD, SDHC, SDXC, SDIO: 32 x 24 x 2.1 mm
• miniSD, miniSDHC, miniSDIO: 20 x 21.5 x 1.4 mm
• microSD, microSDHC, microSDXC: 11 x 15 x 1 mm

There are adapters for the smaller cards so they can be used in standard SD card slots, too.

SDIO – Secure Digital with I/O

SDIO is a special card type which combines SD technology with I/O devices, mostly portable devices such as notebooks, PDAs, GPS devices, adapters for Bluetooth, Ethernet, IrDA or barcode readers. These cards can operate in SD slots when the host devices are aware of SDIO.

In addition to these card types there exist some particular SD cards with special features, e.g. SD cards with a USB interface.

Data Transfer and Speed Classes

Just as SD card development has led to ever larger storage capacities, data transfer rates to PCs and writing speed increased with time. SDHC and SDXC cards, for example, achieve data transfer rates up to 104 MB per second with the UHS-I (Ultra High Speed) mode and a UHS-1-enabled device. For instance, this makes it possible to record HD videos tapelessly with camcorders or to shoot serial photographs with digital single lens reflex cameras (D-SLR). The UHS-2-mode, also for SDHC and SDXC cards, enables data transfer rates up to 312 MB per second.

The SDA defined the following speed classes for guaranteed minimum writing speed:

• Class 2: 2 MB/s
• Class 4: 4 MB/s
• Class 6: 6 MB/s
• Class 10: 10 MB/s

Digital cameras, camcorders and similar devices often list other additional speed and capacity declarations, such as maximum data transfer rates at optimum conditions, maximum recording times or the number of photographs that can be stored. 

Usually files on SD cards are read out locally at the work station (PC, notebook). This can be done via connecting the host device (camera, camcorder, mobile phone etc.) to the work station. Users can also insert SD cards into special adapters which are then connected to the work station. SD cards with integrated USB interface can be directly connected. 

SEH myUTN-120 SDCardserver

SEH has developed the myUTN-120 SDCardserver for making SD cards and their files available across the network. On top of a USB port for virtualizing USB devices via the network, myUTN-120 is equipped with an SD card reader for SD and SDHC cards and is compatible with all speed classes. You can use this to share SD card files easily across a network just as if the card were directly attached to your work station. Attaching SD cards in this way allows several users to access the connected devices and SD cards. A safe point-to-point connection provides security as only defined users may access connected cards and devices. Use the device assignment function to determine which cards will be accepted by the SDCardserver and to prevent the usage of unwanted SD cards in your network.

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