how to send a signal to serial port? ( logic 1 or 0 ) can anyone give me a source code?

Answer 1

since you sent me an e-mail, I'll give you a little advice.

a serial port is a serial port is a serial port.
very similar to how you would access a modem you probably want to signal just one pin, and that would depend on your operating system.

so add to your question the hardware and software and os you are using!