EIA-RS-232C specifies electrical characteristics, logic levels, and various signal line functions.
On TxD and RxD:
Logic 1(MARK)=-3V to -15V
Logical 0(SPACE)= 3 to 15V
On control lines such as RTS, CTS, DSR, DTR and DCD:
Signal effective (ON, ON, positive voltage) = 3V ~ 15V
Signal invalid (OFF, OFF, negative voltage)=-3V ~ -15V
The above provisions explain the definition of logic level in the RS-323C standard. For data (information code) : the level of logic \"1\" (mark) is lower than -3V, and the level of logic \"0\" (empty sign) is higher than 3V; For control signals; ON state (ON), that is, the effective level of the signal is higher than 3V, OFF state (OFF), that is, the invalid level of the signal is lower than -3V, that is, when the absolute value of the transmission level is greater than 3V, the circuit can effectively check that the voltage between -3 and 3V is meaningless, and the voltage below -15V or higher than 15V is also considered meaningless. In actual work, the level should be guaranteed to be between ±(3 ~ 15)V.
There is a \/-12V power supply on the PC, so the output voltage of the RS232 can reach a swing above \/-9V. Other power supplies converted by charge pumps, such as the max232 chip, have a no-load swing of about \/-7V.