DE9 DB25     PC  
    pin pin sig dir
    --- --- --- ----
     nc  1  GND <->
     3   2  TXD -->
     2   3  RXD <--
     7   4  RTS -->
     8   5  CTS <--
     6   6  DSR <--
     5   7  GND <->
     4   20 DTR -->