Much like CompactFlash was not developed in a bubble, and rather was a ‘new’ standard based upon previous work, so too is Secure Digital an evolution of an older standard. Unlike CF though, SD is not based on a hard drive standard and rather in 1999 the Secure Digital Card Association took the existing MultiMediaCards (MMC) standard and modified it to suit their needs.
Though it is more accurate to say that they took the ancient MMC standard and ‘evolved’ it to make a more modern standard, rather than say they revised it – as the SDA extensively reworked the standard until very little was left beyond core ideals. Basically, both standards are serial based designs, both were originally designed with Digital Rights Management baked right into the standard, and both were meant to compete against Sony and their Memory Stick standard.
The SD standard was developed with future performance growth and development in mind. So much so that ‘SD cards’ of today share little in common with their Millennium counterparts. For example, revision 2.0 (in 2006) of the SD standard introduced SDHC which included the High Speed bus interface, and capacities of up to 32GB. Then in 2009 SDXC (aka version 3.01 of the SD standard) was released and introduced the Ultra High Speed bus (AKA UHS-I) to the world. This standard increased capacity to 2TB, and boosted speeds from 25MB/s to either 50MB/s (SDR50/DDR50) or 104MB/s (SDR104/DDR104).
The latest standard is version 4.0. Dubbed UHS-II this new standard boosted speed to 312MB/s. However much like ‘MMCplus’ standard that it bares more than a passing resemblance too, in order to boost performance to this new level the interface itself had to be modified. In a wise decision backward compatibility was kept and instead of an entirely new interface, an additional row of contacts was simply added behind the existing UHS-I contacts. Much like in USB 3.0, or even MMCPlus storage devices, this double row allows both the host system and the device itself to instantly know which mode to use (High speed / UHS-I, or UHS-II).
This of course means that devices will have to have this additional row of contacts in order to make use of USH-II performance potential and this time this is a rather limited pool of devices! On the positive side all UHS-II cards can be used in any device that is capable of handling its capacity size.
There is however one complication, when the SDA introduced version 4 of the standard they also tweaked the UHS-I standard that UHS-II devices make use of. Basically, instead of 104MB/s compatible UHS-II cards are capable of running at 156MB/s over the UHS-I bus. On the surface this should be a good thing, but many first-generation UHS-II cards used controllers which were unable to step down to the ‘normal’ highest UHS-I mode; instead they were capable of UHS-II speeds, ‘improved’ UHS-I speeds, or limited to the slower UHS-I DDR50 bus standard of 50MB/s.
In the case of the Silicon Power Superior Pro series it is not only some of the fastest UHS-II cards on the market they are also some of the fastest UHS-I models. This card works on UHS-II, UHS-I (revision 4), UHS-I (revision 3 and 3.01), and High Speed (revision 2) devices.
