| 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 Detect | When the modem detects a "Carrier" from the 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. |
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