The pinout of the usual 9 pin plug as used for serial mice is:
-12 V, Imax = 10 mA
+12 V, Imax = 10 mA
+12 V, Imax = 10 mA
This is the specification, in fact 9 V suffices with most
The mouse driver can recognize a mouse by dropping RTS to low and
raising it again. About 14 ms later the mouse will send 0x4D ('M') on the
data line. After a further 63 ms, a Microsoft-compatible 3-button mouse will
send 0x33 ('3').
The relative mouse movement is sent as dx (positive means
right) and dy (positive means down). Various mice can operate at
different speeds. To select speeds, cycle through the speeds 9600, 4800,
2400, and 1200 bit/s, each time writing the two characters from the table
below and waiting 0.1 seconds. The following table shows available speeds
and the strings that select them:
The first byte of a data packet can be used for synchronization
The Microsoft protocol uses 1 start bit, 7 data bits, no parity and one
stop bit at the speed of 1200 bits/sec. Data is sent to RxD in 3-byte packets.
The dx and dy movements are sent as two's-complement, lb
(rb) are set when the left (right) button is pressed:
Original Microsoft mice only have two buttons. However, there are some three
button mice which also use the Microsoft protocol. Pressing or releasing the
middle button is reported by sending a packet with zero movement and no
buttons pressed. (Thus, unlike for the other two buttons, the status of the
middle button is not reported in each packet.)
Logitech serial 3-button mice use a different extension of the Microsoft
protocol: when the middle button is up, the above 3-byte packet is sent. When
the middle button is down a 4-byte packet is sent, where the 4th byte has
value 0x20 (or at least has the 0x20 bit set). In particular, a press of the
middle button is reported as 0,0,0,0x20 when no other buttons are down.
The Mousesystems protocol uses 1 start bit, 8 data bits, no parity, and
two stop bits at the speed of 1200 bits/sec. Data is sent to RxD in 5-byte
packets. dx is sent as the sum of the two two's-complement values,
dy is send as negated sum of the two two's-complement values. lb
(mb, rb) are cleared when the left (middle, right) button is
Bytes 4 and 5 describe the change that occurred since bytes 2 and
3 were transmitted.
The MM protocol uses 1 start bit, 8 data bits, odd parity, and one stop
bit at the speed of 1200 bits/sec. Data is sent to RxD in 3-byte packets.
dx and dy are sent as single signed values, the sign bit
indicating a negative value. lb (mb, rb) are set when the
left (middle, right) button is pressed:
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