Help identify MaskROM or UART on RK3288 POS device for Armbian installation

[System error]

Hello everyone,

I have an old POS machine that uses the Rockchip RK3288 processor. It has a 1920x1080 screen and works fine, but I don’t have any root or bootloader unlock permissions.

I would like to flash Armbian on it and make better use of the device.

So far I’ve tried:

ADB access (only recovery available)

Tried using adb reboot bootloader, but the device just powers off

Identified some test points like KEY, T3, T2, T52, T51, but not sure which one is for MaskROM shorting

Found no physical recovery button, only a reset and power button

There might be a UART port, but I’m not sure

👉 Can someone please help me:

Identify which test points or pads I can short to enter MaskROM mode

Locate a possible UART debug port on the board

Any advice for dumping original firmware or unlocking bootloader

I've attached front and back photos of the mainboard. Any suggestions or experience with RK3288 boards are greatly appreciated!

Thank you 🙏