Key rollover

Key rollover is the ability of a computer keyboard to correctly handle several simultaneous keystrokes.

Keyboards that lack full rollover will register an incorrect keystroke when certain combinations of keys are pressed simultaneously.

Rollover has applications for stenotype, electronic music keyboards, gaming, and touch-typing generally.

[3] Full n-key rollover is required for stenotype, which relies on chording to input text far faster than conventional typing methods, allowing it to keep pace with the speed of human speech.

[4] For the user to get the benefit of the full n-key rollover, the complete key press status must be transmitted to the computer.

The (mandatory) HID report protocol, which is what operating systems use, imposes no restrictions and supports full n-key rollover.

However, the signal also routes through the blue and green switches to terminal 4, falsely indicating that key 1/4 has been pressed.

When these three keys are pressed simultaneously, this fourth keypress is erroneously registered by the keyboard controller.

Key jamming is often noticed when using a keyboard to play computer games where many keypresses combine to movement vectors and other simultaneous activities rather than typing text.