Andy, it is not strictly true that a keyboard can cope with any number of keypresses at once. Using a scanned keyboard (which they all are), if a three keys are pressed which lie on the vertices of a rectangle (i.e. two share a row address and two share a column address) then it appears to the keyboard controller that the fourth key in the rectangle has also been pressed. There is no way to tell them apart. The spurious fourth keypress that appears is known as the ghost key (or phantom key), and most keyboard scanners will reject the third and fourth keypresses since they can't tell which one is real. Try it on your own keyboard - on mine for instance, pressing and holding "asd" results in all three keys being recognised, but pressing "asx" results in only "as" being registered because the "x" lies on the third vertex of a rectangle. The reason that keys like Shift and Control can be pressed with any other key is that they are deliberately assigned to an empty (or nearly empty) row or column, and cannot therefore cause a ghost key problem. Check out the keyboard scanners on the holtek website for more details.