The unit consists of four boards: Power supply; MCU; Registers and the dot matrix displays:
The MCU is a Dallas DS87C520 that receives commands through RS422. The processor board have one DS8921AM RS422 transceiver that converts the differential signal to single ended.
When the board is powered it shows this animation:
After a loooong time searching through the web I found a document in chinese that describes the protocol for this series of displays.
With the help of google lens translator I found that it should accept the commands via serial with 1 start bit; 7 data bits; even parity; 1 stop bit; and the baud could be between 300 to 38400.
The command start with the ID of the display, luckly it is displayed in the startup animation. After spending some time reading the doc I build this command: %14D%1SDISPLAY OK%Z
Then I got my FT232 USB to Serial and tried all the possible baud rates but it didn't work, the display remained blank.
I decided to remove the DS8921AM and solder a wire direct to the line that goes to the MCU, after that I tried all the possible baud and it worked with 38400.