1   2   3   4   5   6   7   8   9   10  11  12  13     1   2   3   4   5
o   o   o   o   o   o   o   o   o   o   o   o   o      o   o   o   o   o
                   25pin sub-d                            9pin sub-d
  o   o   o   o   o   o   o   o   o   o   o   o          o   o   o   o
  14  15  16  17  18  19  20  21  22  23  24  25         6   7   8   9

 (pinout shown looking from soldering side)

pin  pin  sig  signal name           purpose
25p. 9p.

 2    3   TXD  Transmit Data         outgoing data bits
 3    2   RXD  Receive Data          incoming data bits
 4    7   RTS  Request to Send       incoming handshake
 5    8   CTS  Clear to Send         outgoing handshake (remote RTS)

 6    6   DSR  Data Set Ready        (remote DTR)
 8    1   CD   Carrier Detect        (remote carrier)
20    4   DTR  Data Terminal Ready   terminal ready status

 7    5   GND  System Ground         signal ground
 1   na   FG   Frame Ground          signal shielding

note: FG should be connected only on one end of the cable!


wiring for NULLMODEM (DTE <=> DTE, ie 2 Computers):
---------------------------------------------------

variation1: ("3-wire", no handshake)

RXD ----- TXD
TXD ----- RXD

GND ----- GND
 FG ---(   FG

this is not sufficient for most applications. if you really think this
is what you want to use, you probably know what you are doing.

variation2: ("5-wire", hardware handshake)

RXD ----- TXD
TXD ----- RXD
RTS ----- CTS
CTS ----- RTS

GND ----- GND
 FG ---(   FG

this works stable for simple applications, use this for prototyping and
alike :)

variation3: ("full", hardware handshake, status)

RXD ----- TXD
TXD ----- RXD
RTS ----- CTS
CTS ----- RTS
DTR ----- DSR
DSR ----- DTR

GND ----- GND
 FG ---(   FG

full "modem" status, except CD. not practical. (may not work for
programs that expect a modem, ie terminal-programs)

variation4: ("full local", hardware handshake, local status)

RXD ----- TXD
TXD ----- RXD
RTS ----- CTS
CTS ----- RTS
DTR ---+- DSR
       +- CD
DSR -+
 CD -+--- DTR

GND ----- GND
 FG ---(   FG

note: DSR + CD are bridged to fool programs to think that they are
      online.

this is the suggested cable for endusers, it should work for any
program written for rs232 communications.