High Quality Content by WIKIPEDIA articles! Ring Indicator, abbreviated as RI, is a control signal that is used in RS-232 serial communications cables between a computer and a modem. The signal is sent from the modem to the computer, typically over pin 9 of a nine-pin (DE9) serial port, or pin 22 of a 25-pin (DB25) serial port. Its purpose is to indicate to the computer that the phone line is ringing.