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  • Pins commonly used for RS-232 (serial):

        
     
    DB-25
    DB-9
    Signal Direction
    Signal Name
     
    1
    x
    Protective
    Ground
     
    2
    3
    DTE-to-DCE
    Transmitted Data
     
    3
    2
    DCE-to-DTE
    Received Data
     
    4
    7
    DTE-to-DCE
    Request To Send
     
    5
    8
    DCE-to-DTE
    Clear To Send
     
    6
    6
    DCE-to-DTE
    Data Set Ready
     
    7
    5
    x
    Signal Ground
     
    8
    1
    DCE-to-DTE
    Received Line Signal Detector (Carrier Detect)
     
    20
    4
    DTE-to-DCE
    Data Terminal Ready
     
    22
    9
    DCE-to-DTE
    Ring Indicator

    Commonly used RS232 Null Modem Pin out for DB25 and DB 9 connectors:

            
     
    Signal
    Name
    DB-25
    DB-9
    <->
    DB-9
    DB-25
    Signal
     
    FG
    (Frame Ground)
    1
    -
    -
    1
    FG
     
    TD
    (Transmit Data)
    2
    3
    2
    3
    RD
     
    RD
    (Receive Data)
    3
    2
    3
    2
    TD
     
    RTS
    (Request To Send)
    4
    7
    8
    5
    CTS
     
    CTS
    (Clear To Send)
    5
    8
    7
    4
    RTS
     
    SG
    (Signal Ground)
    7
    5
    5
    7
    SG
     
    DSR
    (Data Set Ready)
    6
    6
    4
    20
    DTR
     
    CD
    (Carrier Detect)
    8
    1
    4
    20
    DTR
     
    DTR
    (Data Terminal Ready)
    20
    4
    1
    8
    CD
     
    DTR
    (Data Terminal Ready)
    20
    4
    6
    6
    DSR