Printer control interface pcb

Hello, in one of my printer disassembly adventure, i found a really nice keypad.
It's a long pcb filled with lot of buttons, some leds and a 16x2 display.
This board uses a HT48c70 microcontroller to handle the communication buttons etc
Searching doesn't give any useful information, except for a repair manual that reports this board to communicate using uart.
Connected with a serial converter at 9600 baud(pretty standard) and found that pressing buttons worked, printing in the serial terminal a double hex byte representing a button id and a control byte for ending communication, i suppose.
Now, where i'm stuck is how could i send data to the display and torn the leds on and off.
Is there a way to find the protocol or retroengeneer the controller? Should i swap it with a arduino?
I attach some pictures of the board.

Connect the keypad PCB to the main board of the printer, power it up, and capture some of the serial data sent to the display. Then you can figure out how it works.

// Per.