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.