Here is the common PC serial port pinout and what the role of each pin is as specified by the RS232 standard.

D-Type-9 Pin No. (MALE)

1             5
  \ o o o o o /
   \ o o o o /
    6      9

D-Type-25 Pin No. (MALE)

1                            13
  \ o o o o o o o o o o o o o /
   \ o o o o o o o o o o o o /
    14                    25

25 Pin No.	9 Pin No. Abbreviation
Pin 8 Pin 1 CD
Full Name

Pin 2 Pin 3 TD
Carrier Detect

Pin 3 Pin 2 RD
When the modem detects a "Carrier" from another modem at the other end of the line.
Receive Data
Serial Data Input (RXD)

Pin 20 Pin 4 DTR
Transmit Data
Serial Data Output (TXD)

Pin 7 Pin 5 SG
Data Terminal Ready
Opposite to DSR. The UART is ready to communicate.

Pin 6 Pin 6 DSR
Signal Ground

Pin 4 Pin 7 RTS
Data Set Ready
Tells the UART that the modem is ready to establish a link.

Pin 5 Pin 8 CTS
Request To Send
Flow Control. Informs the Modem that the UART is ready to exchange data.

Pin 22 Pin 9 RI
Clear To Send
Flow Control. Indicates that the Modem is ready to exchange data.
Ring Indicator
Indicates a ringing signal

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