Anyone knows the speed of expresscard slot, will it slow down USB 3.0 when it comes out?
"ExpressCard" is a physical slot that carries two different signals.
One is PCI Express, one lane, rated at 250 MB/s (1000 Mb/s) each direction.
The other is USB 2.0, rated at 480 Mb/s (60 MB/s) total.
The manufacturer of an ExpressCard chooses which interface they wish to use.
For example, I have an ExpressCard memory card reader (similar to, but not the same exact model as,
this one,) that connects via USB 2.0. So even if I had an SD card that was capable of more than 60 MB/s, I couldn't take advantage of it. (My card also isn't compatible with SDHC, which are the new higher-capacity, faster ones. The largest SD card it can handle is 2 GB.) When I look in System Profiler (or Device Manager when booted to Windows,) it appears as a USB device. As far as the OS is concerned, it is a USB device, period. It doesn't have any clue that it's an ExpressCard.
I also have an ExpressCard FireWire 800 card (again, similar to
this one,) that connects via PCI Express. That lets me take full advantage of the 800 Mb/s (100 MB/s) speed, I'm not limited to USB 2.0's 60 MB/s. System Profiler and Device Manager show it as a PCI Express device with ExpressCard extensions. (Hot-swap ability, for example.)
But the PCI Express x1 link does have a limit of 2.5 Gb/s (about 250 MB/s after overhead) each direction. That means that an eSATA card is marginally limited, since eSATA based on SATA 2.0 supports up to 3 Gb/s (about 300 MB/s after overhead; which is slower than PCI-e's 250 MB/s. Plus, most eSATA controllers have two 3 Gb/s ports.)
However, in the real world, eSATA's 3 Gb/s limit is never reached. In testing at work, I have used the fastest SSD's available, and have yet to hit a bottleneck caused by an eSATA ExpressCard. (At work, we have
this card.) Likewise, USB 3.0's theoretical limit is 5.0 Gb/s, noticeably over eSATA's 3.0, yet I doubt an ExpressCard interface will be a limiting factor.
Of course, we may see an ExpressCard 2.0 standard that implements PCI-e 2.0, which is 5 Gb/s each direction. In that case, USB 3.0 would be perfectly fine.
(Disclaimer to my disclaimer: I work at Intel; but not in any department that has anything to do with upcoming standards such as USB 3.0, ExpressCard, PCI Express, etc. My guess on ExpressCard 2.0 is purely a guess, based on no inside knowledge from anywhere. If there is no ExpressCard 2.0 ever, don't come crying to me...)
