The pinout of the DIN-8 connector (female, on the CPU) is:

            7           1  GND              5  TxDA (Keyboard)
        8       6       2  GND              6  RxDA (Keyboard)
        5   4   3       3  Vcc              7  TxDB (Mouse)
          2   1         4  RxDB (Mouse)     8  Vcc

This is the pin assignment of the PS/2 mouse port on ASUS motherboard.
You can make the cable by yourself or purchase it from ASUS dealers.

Mouse Port       DB9-Connector(#)  PS/2 mouse-Port  Signal(*) Signal
  Asus                 (corrected)    Connector             (corrected)
  1                  5     5             2           GND       GND
  2                  9     4             3           CLK       DATA
  3                  -     -             -           NC        NC
  4                  8     8             5           +5V       +5V
  5                  1     7             1           DATA      CLK
  6                  -     -             -           -         -
  7                  -     -             -           -         -
  8                  -     -             -           NC        NC 
----------                              3 4
 1234              12345                + +
 ++++              +++++              2+   +5
 +  +               ++++               1+|+6
 5  8               6789                 |
                                    (front view)

Source: c't magazin fuer computertechnik, okt 1994. page 272.

(*) Please note that the connection from onboard PS/2 mouse port to the
    mouse connector is correct but the singal label is wrong.
(#) Based on information from Harald Gattringer. See following.

Jerry Lee
ASUS

------------------------------------------------------------------------
From: e8425814@stud1.tuwien.ac.at (Harald Gattringer)
Subject: PS2-Mouse port
Date: 22 Mar 1995 15:07:40 GMT
Organization: Vienna University of Technology, Austria
NNTP-Posting-Host: stud1.tuwien.ac.at

Signal definition of the DB9-MouseMan Adapter !
-----------------------------------------------

My Logitech MouseMan comes with an PS2/DB9 adapter. From this adapter I
got the DB9/PS2 pin definition (the other signal label from Jerry Lee):

      Pinout on the SP3g:       |            Pinout of a PS2-Mouse
                                |                   Socket
   --------------------------   |          -------------------------
     1 2 3 4   | Keyboard |     |
     5 o o 8   | connector|     |                  3      4
               +----------+     |
    PS2-mouse                   |                2          5
    connector                   |                     ||
    (o's are missing pins)      |                  1  ||   6
                                |                     []

    pin #     signal      DB9-connector     PS2
   (board)                  (MouseMan) 
   --------------------------------------------
      1      <--GND-->         5             2
      2      <--DATA->         4             3
      3         NC             -             -
      4      <--+5V-->         8             5
      5      <--CLK-->         7             1
      8         NC             -             -
                          /    4             2
                 not used \    6             3
                               \------v------/
                                   Adapter

With a cable wired like the above drawing, my MouseMan works very
well on the PS2 port.

------------------------------------------------------------------------
From ae873@torfree.net Mon Oct  2 20:30:21 1995
Date: Mon, 2 Oct 95 08:39 EDT
To: jerry@asustek.asus.com.tw
X-Personal_name: Geofffrey Sy
Subject: PS2 Mouse Connector update.

This is an update to the document:
   "http://asustek.asus.com.tw/FTP/ASUS/TechInfo/ps2-mouse.txt"

Mouse Port       DB9-Connector(#)  PS/2 mouse-Port  Signal(*) Signal
  Asus                 (corrected)    Connector             (corrected)
  1                  5     5             2           GND       GND
  2                  9     4             3           CLK       DATA
  3                  -     -             -           NC        NC
  4                  8     8             5           +5V       +5V
  5                  1     7             1           DATA      CLK
  6                  -     -             -           -         -
  7                  -     -             -           -         -
  8                  -     -             -           NC        NC

The DB9-Connector pins-out are both correct.  The original pins 5-9-8-1
connection is for older Logitech Mouseman, Serial-MousePort.  These are the
models that came with a DB9 connector and a DB9 to DIN adaptor.  The later
models of the MouseMan Serial-MousePort came with a DIN connector and a DIN
to DB9 connector.  The later models work with the pins 5-4-8-7 connection.