My keyboard and key rollover tester (posted 2020-12-24)
I have a new mechanical keyboard on order, which lets you hot swap the keyboard switches. When you do that, it's a good idea to do a quick test to see if all the keys still work. There are many tools to do this, but I decided to make my own web-based one. I think it came out pretty nicely.
You can see if the keys work by typing them one at a time, or (usually) by running your finger along each row.
In addition to that, you can also see how much key rollover your keyboard supports, i.e., how many keys you can type at the same time. Higher end keyboards usually support "n-key rollover" (NKRO), which means that any combination of keys will be recognized correctly. This is useful for some types of games.
Old keyboards usually don't support any additional rollover, so they can only correctly recognize arbitrary combinations of two keys. (This limitation doesn't apply to shift/control/alt or command/windows.) Additional keys may or may not be recognized. However, it doesn't really matter too much if you can't type Z F6 . For non-gamers, what matters is that when you type quickly, it can happen that one finger hasn't released the previous key yet while another finger is already pressing the next key. Turns out that on many keyboards that don't support real 3-key rollover, you can still type three (or more) keys at the same time, as long as those keys are all on different columns. Which means: you type them with different fingers. After all, how are you going to type E, D and C at the same time, as touch typists type all three of those with their left middle finger?
A common additional limitation (to avoid more complex USB communication) is a 6KRO limitation.
What the rollover tests (for 3 - 7 KRO) do is test combinations of all the regular alphanumeric keys and most punctuation and then tell you if it looks like you have full key rollover, enough for touch typing or no real rollover for that number of keys. You can of course also just press as many keys as you like at the same time and see if they register.
So have a look and let me know what you think.