Decoding data from the MicroSoft Serial Mouse

Written by Paul Bourke
April 2003

The old MicroSoft serial mouse, while no longer in general use, can be employed to provide a low cost input device, for example, coupling the internal mechanism to other moving objects. The serial protocol for the mouse is 1200 baud, 7 bit, 1 stop bit, no parity. The pinout of the connector is standard serial as shown below.

Pin	Description
1	DCD	Data carried detect
2	RD	Receive data
3	TD	Transmit data
4	DTR	Data terminal ready
5	SG	Signal ground
6	DSR	Data set ready
7	RTS	Request to send
8	CTS	Clear to send
9		Ring

Every time the mouse changes state (moved or button pressed) a three byte "packet" is sent to the serial interface. For reasons known only to the engineers, the data is arranged as follows, most notably the two high order bits for the x and y coordinates share the first byte with the button status.

	D6	D5	D4	D3	D2	D1	D0
1st byte	1	LB	RB	Y7	Y6	X7	X6
2nd byte	0	X5	X4	X3	X2	X1	X0
3rd byte	0	Y5	Y4	Y3	Y2	Y1	Y0

Where

LB is the state of the left button, 1 = pressed, 0 = released.
RB is the state of the right button, 1 = pressed, 0 = released
X0-7 is movement of the mouse in the X direction since the last packet. Positive movement is toward the right.
Y0-7 is movement of the mouse in the Y direction since the last packet. Positive movement is back, toward the user.

Sample C code to decode three bytes from the mouse passed in "s", the button and position (x,y) are returned.

/*
   s should consist of 3 bytes from the mouse
*/
void DecodeMouse(unsigned char *s,int *button,int *x,int *y)
{
   *button = 'n'; /* No button - should only happen on an error */
   if ((s[0] & 0x20) != 0)
      *button = 'l';
   else if ((s[0] & 0x10) != 0)
      *button = 'r';
   *x = (s[0] & 0x03) * 64 + (s[1] & 0x3F);
   if (*x > 127)
      *x = *x - 256;
   *y = (s[0] & 0x0C) * 16 + (s[2] & 0x3F);
   if (*y > 127)
      *y = *y - 256;
}