more chording keyboards

From: Derek Peschel <dpeschel_at_u.washington.edu>
Date: Thu Apr 22 12:57:53 1999

> Some Apple brand keyboard allow half a dozen keys to be pressed at the same
> time with each up / down recognized. This allows stuff like cmd-shift-power
> (reboot) or some long ones I forget using about 5 keys to make some obscure
> choice. Cheap keyboards get confused when too many things happen at the
> same time.

I guess you're talking about the ADB keyboards which work with the Macintosh.
(The same keyboards work with the Apple //gs, so I guess this message IS on
topic after all!)

There's only one specification I've seen for them: You can press as many
modifier keys as you want at the same time, but only two letter/number/
punctuation keys. I think that means they have two-key rollover. The power
key is handled differently from the other keys. I wrote a program to show
key-down and key-up events, to test my keyboard (in case I could somehow
"cheat" and get more-than-two-key rollover) and found I couldn't cheat, and
the spec was correct. I forget if I had bought my non-Apple keyboard by then.

So most of the special combinations involve modifier keys. Command-control-
power is reboot (same as Ctrl-Open Apple-Reset on the //gs) and the trickier
combinations include letter keys as well. The "Zap the PRAM" sequence is
Command-Option-P-R. There are a number of combinations like that, and they
all have two letter keys.

I've always wondered how you make a keyboard that has two-key rollover, or
N-key rollover for that matter. I know that most keyboards these days scan
a matrix of switches, but I don't know where rollover comes into that process.

-- Derek
Received on Thu Apr 22 1999 - 12:57:53 BST