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 Full Name FUNCTION
Pin 8 Pin 1 CD Carrier Detect When the modem detects a "Carrier" from another modem at the other end of the line.
Pin 3 Pin 2 RD Receive Data Serial Data Input (RXD)
Pin 2 Pin 3 TD Transmit Data Serial Data Output (TXD)
Pin 20 Pin 4 DTR Data Terminal Ready Opposite to DSR. The UART is ready to communicate.
Pin 7 Pin 5 SG Signal Ground
Pin 6 Pin 6 DSR Data Set Ready Tells the UART that the modem is ready to establish a link.
Pin 4 Pin 7 RTS Request To Send Flow Control. Informs the Modem that the UART is ready to exchange data.
Pin 5 Pin 8 CTS Clear To Send Flow Control. Indicates that the Modem is ready to exchange data.
Pin 22 Pin 9 RI Ring Indicator Indicates a ringing signal