Simply put: RS232 level is -15~-3V is considered to be logic 0, 3~ 15V is considered to be logic 1; Any level not in this range is invalid; TTL level greater than 2.4V is considered logical 1, less than 0.4V is considered logical 0; Levels between.04 and 2.4V are invalid; The MAX232 is a chip that converts these two levels into each other. Building master typing is very tired, catch points well ~~~