RS-232

Old point to point serial communication

Device A                  Device B
TX ---------------------> RX
RX <--------------------- TX
GND --------------------- GND

It uses different voltages unlike modern microcontrollers: 0v = 0 5v = 1

Polarity is inverted: negative voltage is 1 positive voltage is 0

Also uses much larger voltage: +3V to +15V = logic 0 -3V to -15V = logic 1