Pin functions of the RS232 chip include:
1: DCD: carrier detection. The Modem notifies the computer that it is online. That is, the Modem detects a dial tone and is online.
2: RXD: This pin is used to receive data sent by external devices; When you use the Modem, you will find that the RXD indicator is blinking, indicating that data is coming in from the RXD pin.
3: TXD: This pin sends data from the computer to external devices; When you use the Modem, you may notice that the TXD indicator is blinking.