6 May
2012
6 May
'12
1:06 p.m.
On 05/06/2012 12:55 PM, Chuck Guzis wrote:
Don't almost all USB keyboards have 6-key
rollover, where PS/2
keyboards generally have n-key rollover?
Very few keyboards with either USB or PS/2 interfaces have more than
2-key rollover. There are almost always cases where a third key pressed
simultaneously will cause a phantom fourth key. This doesn't mean that
there aren't a lot of cases where three keys will work correctly, only
that there are some cases that it will fail. Usually they do wire the
"modifier" keys (shift, control, alt, etc.) so that they can't produce
phantom keys, but some combinations of three or more "normal" keys will
produce phantom keys.
The reason is that they don't want the expense of adding a diode in the
matrix along with each switch, to prevent the phantom keys.
The keyboards that do support n key rollover are mostly expensive
specialty keyboards, often sold for gaming.
USB device class "boot mode" can't support more than 6 key rollover, but
normal mode should be able to. However, I haven't found any USB
keyboard that actually does have it. I'm guessing that the code shared
between boot and normal modes has the limitation.