Mehdi,

If you have a ready-made keyboard (possibly membrane type), you probably have no possibility to modify it and you have to live with the limitations of the plain matrix.

But if you build a matrix of switches, you can add a serial diode to each switch. Then you can detect each switch even if more of them is on. Try to make a drawing and see how the current flows when you "activate" the rows and more keys are pressed. Try it for the L-shaped 3-switch case which I described above in ASCII art, and you should see the difference.

In this way, for example, the pinball ("flipper") machines from Williams have connected their numerous inputs from the switchis around the playfield, reducing and gratly simpifying the overall wiring.

Jan Waclawek