On 1/19/2011 3:05 PM, Tom Metro wrote: > n-key rollover? What does that mean?
A really cheap keyboard (never seen on a PC) would respond to only one key at a time; you would have to release the first one before you could press a second key. It was quickly discovered that people could type faster if you could type a second key before the first one was released; that's two-key rollover. Further development allowed three, four, etc. Eventually keyboards were build that would allow any number of keys to be pressed (entering them in order) before any were released, which is known as N-key rollover because N can be any number. The keyboard handling on PCs is unusual by historical standards (though it's now commonplace, Macs do the same thing) in that there are two layers of it. The keyboard has its own CPU (which may implement any level of rollover) that sends codes to the computer that tell both when keys are pressed and when they are released. Unusually (compared to older systems) there are press and release codes for EVERY key on the keyboard, allowing any key to be handled as a special modifier key (like Control, etc.) The actual "cooking" of the press and release codes into keystrokes is done by the keyboard driver in the BIOS or the OS (or in the case of Linux possibly in the GUI, X has its own keyboard processing). Even the sticky modifiers like Caps Lock are handled by the computer and then a signal is sent back to the keyboard to turn on the LED. Because the low-level keyboard scanning is done by the microcontroller in the keyboard, the code in that microcontroller can affect the quality of your keyboard experience. Poor debouncing is obviously a problem that is occasionally seen. Low levels of rollover (the standard doesn't require any particular degree of it) will making typing harder. I could also imagine a keyboard that was hard at sorting out the order of nearly simultaneous keystrokes that would cause dyslexia-like errors. PC keyboards can be connected to a variety of things other than PCs. Back when I took a digital circuit design class at Harvard Extension I did a project on the microcontroller-based computer that we build in the class (an 8051) that used a PC keyboard for input. The standard AT keyboard protocol is a simple serial interface; similar to RS-232 but using logic levels for signaling. (You can hook it into a UART.) USB is a little more work but there are lots of microcontrollers with USB interfaces now. Back to the original topic... I don't like loud clicky keys myself, nor keyboards that require a lot of force. To my taste the classic IBM keyboard was too heavy, too loud, and required too much typing force and too much key travel. My all time favorite PC keyboard was one made by NMB; sadly they don't seem to be made any more, my long loved one finally broke, and nothing else I have tried since QUITE matches the magic key feel of that one. But keyboards are a matter of personal taste; if the IBM-style keyboard does it for you, enjoy... just not in a laptop! _______________________________________________ Discuss mailing list Discuss@blu.org http://lists.blu.org/mailman/listinfo/discuss