Working of Pins

Abbreviation Full Name Function
TD Transmit Data Serial Data Output (TXD)
RD Receive Data Serial Data Input (RXD)
CTS Clear to Send This line indicates that the Modem is ready to exchange data.
DCD Data Carrier When the modem detects a "Carrier" from the detect modem at the other end of the phone line, this Line becomes active.
DSR Data Set Ready This tells the UART that the modem is ready to
establish a link.
DTR Data Terminal Ready This is the opposite to DSR. This tells the Modem that the UART is ready to link.
RTS Request To Send This line informs the Modem that the UART is ready to exchange data.
RI Ring Indicator Goes active when modem detects a ringing signal from the PSTN.