Data Carrier Detect

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.