ExpressCard slot
The ExpressCard slot is a PC Card slot, that is not backwards compatible with the CardBus or 16-bit PCMCIA standard.
There are 2 formfactors, ExpressCard/54 and ExpressCard/34. An ExpressCard/54 slot can accept ExpressCard/34 cards, but not the other way around.
All current ThinkPad and Docking station implementations use ExpressCard/54 slots.
Linux support
ExpressCard adapters are handled either by the Linux PCI subsystem as hotplug PCI-Express adapters, or by the USB subsystem (depending on the adapter card).
