Energy and product review

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Tuesday, April 13, 2010

Expresscard to PCMCIA Can’t Kill PCMCIA

The original PCMCIA standard, before CardBus, was a physically smaller but electrically identical version of the old ISA bus. That's a 16-bit-wide interface, and those cards are often called 16-bit PCMCIA. Those cards may still work on modern laptops - the physical connector is the same for CardBus and 16-bit PCMCIA - but they won't work in expresscard to pcmcia adapter or USB-to-PCMCIA or pretty much anything-else-to-PCMCIA adapters.

expresscard to pcmcia adapter

And now we've got ExpressCard, which lets little laptop expansion cards use a PCI Express (PCIe) x1 connection, which has more bandwidth than PCI and - more importantly - doesn't have to share that bandwidth with other components in the computer, like the hard drive. Standard expresscard to pcmcia gives you a 133-megabyte-per-second theoretical ceiling speed, shared with other components; a single PCIe version-1-point-anything lane gives you around 250 un-shared megabytes per second.

This is actually more than fast enough to run a graphics card. There are several cardbus expresscard cards for desktop PCs, which work fine for anything but serious 3D. The ExpressCard power supply and the tiny size of the cards limit what they can do, though; an ExpressCard graphics adapter would be hard to power and impossible to cool. But there's at least one laptop docking bay that has graphics hardware built in, which connects to the computer via the ExpressCard slot. There are third-party ExpressCard-tethered port-replicator doodads, too. You can watch the youtube video for expresscard to pcmcia card:

Anyway, because PCIe is faster than PCI, it looked to me as if it should be impossible to make a device that lets you plug an ExpressCard into a PCMCIA socket. Well, OK, technically it's possible to adapt any interface into any other interface, provided you don't mind that your Frankenstein's-monster adapter will need lots of expensive brainpower, and don't need it to operate any faster than the slowest interface involved. But practically speaking, an ExpressCard-into-PCMCIA adapter ought to be impossible.

Then the penny dropped, when I noticed that expresscard cardbus adapter is one of two interface standards that ExpressCard supports. The other is USB, which has its own small allowance of pins on the ExpressCard connector. USB 2.0 can deliver a bit more than 40 megabytes per second of actual user data, with a following wind and no other devices competing with it. If a laptop expansion card doesn't need more bandwidth than that - which is very probably the case - the manufacturer can cut costs by making a card that ignores the PCIe pins on the ExpressCard connector and just hooks up via USB.


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