Re: [Plib-users] Don't Do This!

[Steve B. <sjb...@ai...>]

Christian Mayer wrote:

> Have you tried a USB keyboard? I could imagine that they aren't using
> the old style method. And if that doesn'T work it's possible to try 2
> USB or 1 USB and 1 PS/2 keyboard, etc.

I don't think they'll be any different from the regular kind.

The problem isn't in how the data is transmitted from the keyboard to the
PC (that's basically just a serial protocol).  The problem is that the
decoder chip in the keyboard has only a limited number of pins (not one
per key) so it has to scan the keyboard using a row-column setup.

When you have two keys held down, two row lines are connected to two
column lines.  When you press down a third key that shared the same row
as one of those keys and the same column as the other, there is no way
for the keyboard controller chip to know which of the two keys were pressed.

Since the controller doesn't know which you pressed, it sends neither of
them (unless it's a really old keyboard controller chip - in which case
it sends *both*...Eeeek!)

I wrote up an Omniverous Biped FAQ item about this:

  http://www.sjbaker.org/steve/omniv/index.html

...check out "Keyboards Are Evil".


----------------------------- Steve Baker -------------------------------
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