Rollover, blocking and ghosting

These inter-related terms often come up in discussions of a keyboard's ability to register multiple keys at once. Often confusion exists over what exactly they correspond to, which is exacerbated by overzealous marketing departments using them in misleading ways, or using the terms interchangeably.

Key Rollover

Key Rollover is a measure of how many keys a keyboard can correctly register and subsequently transmit to the computer at once. There are three categories of rollover that can be found in keyboards.

Full n-Key Rollover - Any number or combination of keys can be pressed simultaneously, and the computer will receive them all correctly from the keyboard.

2 Key Rollover - Any two keys can be guaranteed to be registered properly when pressed simultaneously. Beyond that, certain combinations of keys will work, and others will cause blocking (i.e. the controller deliberately drops them to prevent ghosting). The maximum number of keys that can be registered by the computer is subject to the limitations of the interface.

Interface-limited n-Key Rollover - You can press any combination of keys simultaneously, as long as the amount of keys pressed does not exceed what is allowed by the interface between the keyboard and the computer. Subsequent key presses often cause previously pressed keys to be released in order to handle the newest key press.

Sometimes, xKRO (where x is some number) is used to describe the latter two arrangements, and is misleading because it implies that they are the same problem but with a different number of keys that can be handled at once. On a PS/2 2KRO keyboard, many more than two keys can be correctly registered at once, but certain combinations will fail. On a 6KRO keyboard, you have an absolute limit of 6 keys that can be pressed at once, but within that, blocking does not occur. The dropping of keys in 6KRO isn't blocking - it's implemented for different reasons, and has different effects.