T420i ExpressCard USB adapter question

[hurricanejohn]

I have a T420i and I am wondering if i get a ExpressCard USB 3.0 adapter if i will actually get those kind of speeds or if I'm wasting my money? I've upgraded the CPU to a i5-2520m if that matters at all

[atagunov]

Reportedly X220 has got Intel QM67 chipset. Apparently same for T420i.
QM67 is said to feature PCI Express 2.0.
PCI Express 2.0 supports 500Mb/sec per lane.
USB 3.0 is said to support up to 650Mb/sec per port.

I believe ExpressCard connector offers 1 lane of PCI Express. Therefore it seems that while USB 3.0 won't run at maximum theoretically possible speed there's a chance to run close to it. The speed will probably be limited by the ExpressCard itself - and on the devices you plug into it - rather than by Thinkpad.

People have used the same ExpressCard connector on X230 to run an external GPU - and were reasonably happy. They even channeled video back to the internal monitor via the same link: command/textures/etc go out and video goes back via the ExpressCard slot. ExpressCard in a X230 is running on PCI Express 2.0 just like in a T420i. Being able to run external graphics card in my view shows that there is a lot of bandwidth.

[axur-delmeria]

Fellow forum member dr_st has a side-by-side comparison of various Expresscard USB 3.0 adapters: viewtopic.php?f=18&t=127819

[dr_st]

I believe ExpressCard connector offers 1 lane of PCI Express. Therefore it seems that while USB 3.0 won't run at maximum theoretically possible speed there's a chance to run close to it. The speed will probably be limited by the ExpressCard itself - and on the devices you plug into it - rather than by Thinkpad.

There is no USB device in existence, that is able to run at the maximum theoretical USB speed, and never has been. In PCI-Express the actual speeds of the interface can be quite close to the theoretical, so the interface will not be the bottleneck, but the USB 3.0 chipset will. There are differences and they are quite noticeable in those analyses that I did (which axur-delmeria linked to). However, it is still far better than the onboard USB 2.0.

[TPFanatic]

The nice thing about T420 is if you want to transfer data across SATA drives you get the built in eSATA port for your convenience.