On Wednesday 22 October 2003 15:46, Carl Cerecke wrote:
> My keyboard, both at home and at work has the behaviour that, when I
> hold down left-shift and press t and r keys simultaneously (or nearly
> simultaneously), neither (or only one) of T and R appears, but not both.
> Right shift has no problems. Other pairs of letters seem to work fine
> with left-shift, but some don't: u and y, and p and [
I have an old AT keyboard (which I keep lying around because I have a couple
of old Pentium 1 class machines still in use)... and it actually types
incorrect things, rather than just plain omitting things (trying to 1-up Carl
here ;-) If I type a lower case 'd' followed relatively quickly by a space, I
get a backslash ("\") instead of the space. When typing at a shell I always
get "cd\" when I go to change directory. PITA alright ;-)
I would speculate that the switches/contacts under the keys are bouncing or
sticking a little bit. There must be some sort of encoder that converts the
100+ individual keypresses into something that can be communicated using just
5 wires (5? I think I counted right). So maybe on old keyboards this encoder
is crap and does something "undefined" when 2 or more keys are pressed at
once. Like (as a sideeffect of the way the logic is wired perhaps) adding the
encoded output for the two keys together. So for example if, when one key is
pressed, pin 3 should be high (and all others low), and for another key, pin
4 should be high (and all others low), then when both keys are pressed it may
cause both pin 3 and 4 to be high, which could be the code used for a
different key (or in Carl's case, an unused code perhaps, resulting in
nothing being typed). And if you type two keys quickly enough after each
other, due to keybounce or whatever, they are actually both on at the same
time at some point.
All just wild guessing of course :-) I'd love to know what actually causes
it.
Cheers,
Gareth
