Probing the pads on the mainboard with an oszilloscope during bootup, especially the 1.8V (1.78V) ones and the ones right next to them.
if you get something like this: http://micsig.eu/media/wysiwyg/Micsig_Handheld_oscilloscope_UART_graphic_interface.jpg between 0 and 1,8V it’s propably a uart signal.
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