Data Carrier Detect, abbreviated as DCD, or alternately Carrier Detect abbreviated as CD, is a control signal present inside an RS-232 serial communications cable that goes between a computer and another device, such as a modem. This signal is a simple "high/low" status bit that is sent "from DCE to DTE", or in a typical scenario, from the peripheral to the computer. It is present on virtually all PC serial ports - pin 1 of a nine-pin (DE9) serial port, or pin 8 over a 25-pin (DB25) port. Its purpose varies depending on the device connected, but the most typical meaning is to indicate when a modem is connected to another remote modem via telephone lines.
The word "carrier" is a reference to the analog carrier signal generated by a modem, which is modulated to carry the data. On a data modem, loss of the carrier equates to termination of the connection.
Much like the Ring Indicator signal, on a PC's serial port, changes to the DCD signal state generate a hardware interrupt that can be captured by the processor any time the DCD signal changes state, preventing the PC from needing to constantly poll the pin.
Although DCD is a one-way signal from the device to the computer, the DTR (Data Terminal Ready) is the corresponding line that carries the same status from the computer back to the device, when applicable.
Read more about Data Carrier Detect: As Used On Modems, As Used With Null Modems, As Used With Other Hardware
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