A designation for a particular type of connector.

DB-9M Function Abbreviation
Pin #1 Data Carrier Detect CD
Pin #2 Receive Data RD or RX or RXD
Pin #3 Transmitted Data TD or TX or TXD
Pin #4 Data Terminal Ready DTR
Pin #5 Signal Ground GND
Pin #6 Data Set Ready DSR
Pin #7 Request To Send RTS
Pin #8 Clear To Send CTS
Pin #9 Ring Indicator RI
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