9 & 25 Pin RS232 Null Modem Pinouts

      Null Modem DB9-DB9
RD   2 ---------------> 3   TD
TD   3 <--------------- 2   RD
SG   5 <--------------> 5   SG
RTS  7 <-+          +-> 7  RTS
CTS  8 <-+          +-> 8  CTS
DSR  6 <-+          +-> 6  DSR
DCD  1 <-+          +-> 1  DCD
DTR  4 <-+          +-> 4  DTR

Null Modem DB25-DB25     
TD   2 ---------------> 3   RD
RD   3 <--------------- 2   TD
SG   7 <--------------> 7   SG
RTS  4 <-+          +-> 4  RTS
CTS  5 <-+          +-> 5  CTS
DSR  6 <-+          +-> 6  DSR
DCD  8 <-+          +-> 8  DCD
DTR  20<-+          +->20  DTR

 ___________                    _______________________________________
 \1 2 3 4 5/  Looking at pins   \1  2  3  4  5  6  7  8  9 10 11 12 13/
  \6 7 8 9/  on male connector   \14 15 16 17 18 19 20 21 22 23 24 25/
   -------                        -----------------------------------
The numbering of the pins on a female connector is read from right to
left, starting with 1 in the upper right corner (instead of 1 in the
upper left corner for the male connector as shown above).