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MOUSE(4) Linux Programmer's Manual MOUSE(4)
NAME
mouse - serial mouse interface
CONFIGURATION
Serial mice are connected to a serial RS232/V24 dialout line, see ttyS(4) for a description.
DESCRIPTION
Introduction
The pinout of the usual 9 pin plug as used for serial mice is:
pin name used for
2 RX Data
3 TX -12 V, Imax = 10 mA
4 DTR +12 V, Imax = 10 mA
7 RTS +12 V, Imax = 10 mA
5 GND Ground
This is the specification, in fact 9 V suffices with most mice.
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 ta‐
ble shows available speeds and the strings that select them:
bit/s string
9600 *q
4800 *p
2400 *o
1200 *n
The first byte of a data packet can be used for synchronization purposes.
Microsoft protocol
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:
byte d6 d5 d4 d3 d2 d1 d0
1 1 lb rb dy7 dy6 dx7 dx6
2 0 dx5 dx4 dx3 dx2 dx1 dx0
3 0 dy5 dy4 dy3 dy2 dy1 dy0
3-button Microsoft protocol
Original Microsoft mice only have two buttons. However, there are some three button mice which also use the Microsoft protocol. Pressing or releasing the mid‐
dle 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 protocol
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 mid‐
dle 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.
Mousesystems protocol
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 (mid‐
dle, right) button is pressed:
byte d7 d6 d5 d4 d3 d2 d1 d0
1 1 0 0 0 0 lb mb rb
2 0 dxa6 dxa5 dxa4 dxa3 dxa2 dxa1 dxa0
3 0 dya6 dya5 dya4 dya3 dya2 dya1 dya0
4 0 dxb6 dxb5 dxb4 dxb3 dxb2 dxb1 dxb0
5 0 dyb6 dyb5 dyb4 dyb3 dyb2 dyb1 dyb0
Bytes 4 and 5 describe the change that occurred since bytes 2 and 3 were transmitted.
Sun protocol
The Sun protocol is the 3-byte version of the above 5-byte Mousesystems protocol: the last two bytes are not sent.
MM protocol
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:
byte d7 d6 d5 d4 d3 d2 d1 d0
1 1 0 0 dxs dys lb mb rb
2 0 dx6 dx5 dx4 dx3 dx2 dx1 dx0
3 0 dy6 dy5 dy4 dy3 dy2 dy1 dy0
FILES
/dev/mouse
A commonly used symbolic link pointing to a mouse device.
SEE ALSO
ttyS(4), gpm(8)
Linux 2020-08-13 MOUSE(4)