Hm, you're right, a Thunderbolt adapter PCIe card also requires the onboard GPIO header (along with an output port). Sorry, I misunderstood.
The TB_Header or TB2_Header (and now I guess TB3_Header) is Intel's interface to Intel chipset. "Technical documentation, specifications and datasheets are unavailable to the public."
From what I've recently been reading, Intel also has a "Thunderbolt Ready" certification. And, for whatever reason, ASUS had difficulty obtaining this certification on some of their mobos in the past, even after competitors already had other Thunderbolt mobos on market.
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