How to debug serial communication with RealTerm?
The display outputs 5V/0V signal but with RS232 compatible polarity, for direct connection into a RS232 input. The FTDI cable has 5V TTL input, so it does not have RS232 input. You need a different USB cable with RS232 interface, or you need a RS232 tranceiver to convert to TTL levels, or maybe an inverter to just invert the polarity.