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I am noticing a bunch of a half-PCI Express Centrino chipsets that say "not for Lenovo/HP", what's the reason for this? Technically, PCI-Express is standardized. What could possibly be different?

Here is an example, and here is another example, and here is another example.

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With Lenovo, the problem is that the BIOS has a whitelist of permitted PCI-Express cards and will refuse to boot with any unauthorized PCI card (source), i.e., they want you to buy only their own PCI cards. There are often ways around this, including hacking the BIOS, hacking the PCI-Express card (replacing the ID information). If you boot a non-whitelisted card your bios will throw the 1802 error,

1802: Unauthorized network card is plugged in - Power off and remove the miniPCI network card.

I suspect the same is true for HP.

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Yea, it's true for HP. In fact, I'm struggling to find a compatible dual band card for an HP laptop right now... – Bob Mar 10 '13 at 19:04

Lenovo uses a WiFi antenna design that could exceed the maximum radiated power permitted under FCC regulations if used with a card that emitted the maximum power permitted. To avoid a configuration that violates FCC regulations, Lenovo designed the BIOS only to permit WiFi cards that were tested with its antennas to be used.

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Can't all antennas exceed a radiated power with an amplifier and a better method to handle excessive signal-to-noise? I thought it was the card itself that had the transmitter in it. – Evan Carroll Mar 12 '13 at 15:46
@EvanCarroll The card itself has the transmitter but not the antenna. The antenna determines the shape of the radiated power. With a high-power transmitter and a high-gain antenna, more power can be radiated in one direction than the FCC permits, theoretically posing a safety hazard if you fall asleep with your head resting on your laptop while you're downloading over WiFi. (These guys are paid to worry about these things.) – David Schwartz Mar 12 '13 at 17:01

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