Hi everyone,
I resolved independently: the P2 jumper setting was wrong. They must be set to RS232 and not to TTL.
In summary, for benefit of those who want to check the communication of the display, it is necessary to use the usb to serial adapter powered at 12 V (jumper P1= PWR). Connect the PC through the USB port, setting jumpers P2 to RS232 (not TTL). The software sscom32e.exe or equivalent is used (for example AI Thinker). The “hexdata” box is activated to correctly see the hexadecimal codes coming from the display.