Книга: Ring Indicator: RS-232, D-subminiature, Interrupt, IBM Personal Computer XT, 16550 Uart, 8250 Uart, Data Terminal Ready

High Quality Content by WIKIPEDIA articles! Ring Indicator, abbreviated as RI, is a signal that exists inside an RS232 serial communications cable that goes 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 over a 25-pin (DB25) port. Its purpose is to indicate to the computer that the phone line is ringing. As implemented in modern PC hardware, a hardware interrupt is generated that can be captured by the processor any time the RI signal changes state. This functionality was originally present in the 8250 UART - the chip responsible for providing the serial port - which was present in the original IBM PC XT. Virtually all serial hardware for newer PC's, including the popular 16550 UART, has maintained this functionality. Having support for this hardware interrupt means that a program or operating system can be informed of a change in state of the RI pin, without requiring the software to constantly 'poll' the state of the pin.