Software flow control sometime is called XON/XOFF flow control
● Software flow control does not use extra wires.
● Only 3 wires are required (RX,TX,GND).
● Transmission is started and stopped by sending special control flow characters.
● The control flow characters are sent over the normal TX and RX lines. The control flow characters are typically the ASCII codes XON and XOFF (0x11 and 0x13).
● If device A sends XOFF to device B it means that B should halt transmission to A until B receives an XON character from A.