Following information obtained from Tomi Engdahl (then@delta.hut.fi),
 http://www.hut.fi/~then/mytexts/mouse.html

 Microsoft serial mouse

   Serial data parameters:
     1200bps, 7 databits, 1 stop-bit

   Data packet format:
     Data packet is 3 byte packet. It is send to the computer every time mouse
     state changes (mouse moves or keys are pressed/released). 
         D7      D6      D5      D4      D3      D2      D1      D0
     1.  X       1       LB      RB      Y7      Y6      X7      X6
     2.  X       0       X5      X4      X3      X2      X1      X0      
     3.  X       0       Y5      Y4      Y3      Y2      Y1      Y0

     Note: The bit marked with X is 0 if the mouse received with 7 databits
     and 2 stop bits format. It is also possible to use 8 databits and 1 stop
     bit format for receiving. In this case X gets value 1. The safest thing
     to get everything working is to use 7 databits and 1 stopbit when
     receiving mouse information (and if you are making mouse then send out
     7 databits and 2 stop bits). 
     The byte marked with 1. is send first, then the others. The bit D6 in
     the first byte is used for synchronizing the software to mouse packets
     if it goes out of sync. 

      LB is the state of the left button (1 means pressed down)
      RB is the state of the right button (1 means pressed down)
      X7-X0 movement in X direction since last packet (signed byte)
      Y7-Y0 movement in Y direction since last packet (signed byte)

    Mouse identification
      When DTR line is toggled, mouse should send one data byte containing
      letter 'M' (ascii 77).


 Logitech serial mouse

   Logitech uses the Microsoft serial mouse protocol in their mouses (for
   example Logitech Pilot mouse and others). The original protocol supports
   only two buttons, but logitech as added third button to some of their
   mouse models. To make this possible logitech has made one extension to
   the protocol. 
   I have not seen any documentation about the exact documents, but here is
   what I have found out: The information of the third button state is sent
   using one extra byte which is send after the normal packet when needed.
   Value 32 (dec) is sent every time when the center button is pressed down.
   It is also sent every time with the data packet when center button is kept
   down and the mouse data packet is sent for other reasons. When center
   button is released, the mouse sends the normal data packet followed by
   data bythe which has value 0 (dec). As you can see the extra data byte
   is sent only when you mess with the center button.


 Mouse systems mouse

   Serial data parameters:
     1200bps, 8 databits, 1 stop-bit

   Data packet format:
          D7      D6      D5      D4      D3      D2      D1      D0
     1.   1       0       0       0       0       LB      CB      RB
     2.   X7      X6      X5      X4      X3      X2      X1      X0
     3.   Y7      Y6      Y5      Y4      Y3      Y4      Y1      Y0
     4.   X7'     X6'     X5'     X4'     X3'     X2'     X1'     X0'
     5.   Y7'     Y6'     Y5'     Y4'     Y3'     Y4'     Y1'     Y0'

     LB is left button state (0 = pressed, 1 = released)
     CB is center button state (0 = pressed, 1 = released)
     RB is right button state (0 = pressed, 1 = released)
     X7-X0 movement in X direction since last packet in signed byte 
           format (-128..+127), positive direction right
     Y7-Y0 movement in Y direction since last packet in signed byte 
           format (-128..+127), positive direction up
     X7'-X0' movement in X direction since sending of X7-X0 packet in
             signed byte format (-128..+127), positive direction right
     Y7'-Y0' movement in Y direction since sending of Y7-Y0 packet in
             signed byte format (-128..+127), positive direction up

     The last two bytes in the packet (bytes 4 and 5) contains information
     about movement data changes which have occured after data bytes 2 and 3
     have been sent.