UART: logic levels

The RS232 signal wire features two logic levels:
-10V (mark, binary 1)
+10V (space, binary 0)
0V is not a valid level
Bit in practice the threshold is usually +1 or +2 volts

Serial ports in all processors are 0/3.3V ("TTL levels")
You need an external level translator
the most common devices are the MAX232 and MAX3232
But in practice every vendor has its own 232 e 3232

A catch-all pinout for the 9-pin connector (DCE-DCE)
pin 1 shorted to pins 4 e 6
pin 7 shorted to pin 8
pins 2 and 3 crossed, pin 5 (GND) is shared.