Which part of looking at the binary to understand how the device works and then writing a driver from scratch that talks to the device would violate copyright?

Which part of looking at the binary to understand how the device works and then writing a driver from scratch that talks to the device would violate copyright? One would not have copy the implementation. Just use the binary to understand how the hardware device works. Like "Aha, it sends point data as x/y/pressure triplets, each being 16 bit values" etc.