The schematic has all the information about the connections between components, and normally the PCB debugging starts from locating desired signals from the schematic. However, for some simple PCBs, it's not that difficult to recognize which signal a pad holds, and the pads picked by the system may not be proper or the ones the users want, so we add another pad-selecting approach: users can pick the pads to be probed directly from the PCB layout in the GUI.