Btw: Creality Sonic Pad and Alternatives
Generally, if you don’t know how to install a clean system and Klipper on your pad, I’m not sure it is usable or that we can help you with it.
That only means there is a USB2UART chip in between. I’m not sure where or what this means.
Generally, on old boards, there is no real USB 2 MCU connection.
Instead, there is CH341 USB2SERIAL converter.
So, if you try to connect to this type of motherboard, under the hood, it would connect like
Klippy host → USB → CH341 → UART → MCU.
Where on normal motherboards, it is:
Klippy host → USB → MCU.
Or
Klippy host → UART → MCU.
So, to flash that motherboard, the SD card is used, and mcu should be configured for UART connection.
Hope that explains something.