Editing Rollover, blocking and ghosting (section)

==Key rollover==

Key rollover is a measure of how many keys can be simultaneously registered by the keyboard. There are a number of types of rollover that keyboards can have, and any limitations they have arise from different problems and have different effects.

===n-key rollover===

n-Key Rollover, usually called NKRO, refers to a keyboard that suffers from neither blocking nor ghosting. In a true NKRO keyboard, any number and or combination of keys can be pressed simultaneously, and all will be registered correctly.

===A technical explanation of NKRO===

Implementing NKRO involves preventing ghosting from happening on the electrical level, there are two ways in which this can accomplished -

*The keyboard can be built with switches that don't allow current to find its way through unintended paths, such as capacitive or Hall effect switches. 
*Insert diodes along the the switching matrix's columns to ensure that current can only flow in the correct direction.

Both of these were common practice up till the mid eighties, where cost cutting led to blocking becoming the standard way to prevent ghosting.

===Interface-limited NKRO===
The USB Human Interface Device (HID) protocol fully supports N-key rollover. However, the compatibility version of HID that all present systems implement limits USB keyboards to reporting a mere six regular keys together with four modifiers. Additional keys pressed beyond the limit will generally cause some of the other keys to be dropped.

Many USB keyboards implement workarounds to bypass this limit; the most common trick is to simulate multiple endpoints, e.g. the keyboard pretends to be a USB hub with several keyboards attached. When more than six keys are pressed simultaneously, the [[keyboard controller]] simulates up to six keys coming from one of its virtual keyboards, and the rest coming from its other virtual keyboards.

These sorts of tricks lead to compatibility issues with certain systems; see [[NKRO-over-USB issues]] for a workaround to this problem.

===2KRO, 6KRO and others===

Sometimes, key rollover is described using a number. This practise is somewhat confusing because it means different things in different contexts.
The convention used by all serious keyboard brands is to use a number that represents the ''worst case'' of all possible key combinations on a keyboard.

'''2KRO''' is often used to describe a keyboard with blocking, and is meant to be read as "any combination of two keys are guaranteed to work, and some combinations of three or more keys will work, but others will not".

'''6KRO''' is often used to describe a keyboard that is NKRO inside but which is limited by the variant of the [[USB]] protocol it uses. This variant is often chosen because it has the least problems with finicky hosts. It allows six arbitrarily chosen keys plus any combination of the four pairs of modifiers ([[Shift key|Shift]], [[Control key|Control]], [[Alt key|Alt]] and [[Windows key|Windows]]/[[Command key|Command]]).

Some rollover quantities greater than 6, such as 12KRO, 18KRO or 22KRO are used to refer to heavily optimised matrices, or interface-limited NKRO keyboards that have got around the 6 key limit.

'''10KRO''' and '''14KRO''' are sometimes used by brands that don't adhere to the industry-standard convention for counting rollover. It most often means 6KRO over USB but counted as 6 keys + modifiers (4 or 8)