The PC recognizes a serial mouse if it gets a "M" character after raising
DTR.

The following will be clearer if viewed with a  fixed pitch font...

;===========================================================================
=
; Mouse protocol data format:
;
;  Serial data parameters:
; 1200bps, 7 databits, 1 stop-bit
;
;  Data packet format:
; Data packet is 3 byte packet. It is sent 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
;
; The byte marked with 1. is sent 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)
;
;Graphical description how the data is contained in the packet
;              1st byte        2nd byte         3rd byte
;          ================  ===============  ================
;           - 1 ? ? Y Y X X  - 0 X X X X X X  - 0 Y Y Y Y Y Y
;          ================  ===============  ================
;               | | \ / \ /      \---------/      \---------/
;               | |  |   |            |                |
;               | |  |   \----\       |                |
;               | |  \--------|-------|--------\       |
;               | |          / \ /---------\  / \ /---------\
;               | |         ================ =================
;               | |          0 0 0 0 0 0 0 0  0 0 0 0 0 0 0 0
; Left Button --/ |         ================ =================
;Right Button ----/            X increment      Y increment


Sending at 8 data bits will also work fine.


Bob Ammerman
RAm Systems

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