Hacking a USB Keyboard

Step 3 A note on pin layouts and shift registers

Basically a keyboard is a specialized shift register. It takes in a lot of inputs and sends out one output to the computer (that being a an ASCII code or control command).

When a switch is closed, the shift register processes which two pins are connected and interprets it as one particular output.

So, if you had ten pins going to each plastic sheet, then you would have one-hundred possible combinations. This is because every single pin on one sheet can be comined with every single pin on the other side. This would produce ten rows of ten possible combinations. In other words, you have just produced a "10 X 10" 2-dimensional array.

For instance, if you connect "Pin 4" on SIDE A and "Pin 6" on SIDE B you will produce the letter "M" on the computer (see picture).

If you connect "Pin 8" on SIDE A and "Pin 7" on SIDE B you will produce the letter "I" on the computer (see picture).

It's really quite simple.

Comments

[cspoelstra]

Thanx man, I was wondering for a wile to make such a thing as a cheap replacement for a midi controller for my DJ setup.. How about the switches you can use, is there an resistance problem ?

[smoak]

I'm trying to put this into a custom controller. Simply using 12 keys, random if need be. Which side is side A and which is side B. My guess is that the top layer is side A and bottom later is side B. Would I be right to assume this? I'm hoping it would save me the trouble of having to trace, I have terrible eyes.

[dimitrisa10]

what will happen if you connect three pins will it display 2 characters because i'm making a hand-top out of a pico itx board and im wondering if i can use this to make a thumb keyboard

[randofo]

If it is a function key or the shift key or something like that, it might do something. Most likely, it will not do anything (or nothing you would want it to do). It won't make two characters.

[dimitrisa10]

thx for the info do you think it would work though for a tiny keyboard

[currently_awake]

keyboards send two scan codes for each key hit. One for each key press, a second for the key release. (the second has a bit set to mark it as a release). All the keys on the keyboard work this way, even shift and control.

[mrsayao]

is it safe to assume the shift keys work the same way? I'm trying to incorporate this into a typewriter to text file project... shift is held down for special characters, then key is pressed. I have to take a look at the keys on the typewriter, but I assume mapping the special characters (like punctuation) are the same right?

[jochem]

Do you have any idea what will (generally) happen when you connect two pins that aren't normally connectible? Will the keyboard send a different control code to the PC, or will it just do nothing?

[randofo]

sometimes it will generate completely new commands. sometimes it will do nothing. It is trial and error.