9 min
How to Test a Keyboard Properly
A key that seems dead is usually not a broken switch. Test in the right order — registration, rollover, chatter, latency, feel — and you find the real fault.
Why a USB Keyboard Stops at Six Keys
Hold down six letters and press a seventh, and on many keyboards the seventh never arrives. The keyboard is not broken and the key is not faulty. The limit is written into the simplest way a USB keyboard talks to a computer.
8 min read · updated 21 September 2026 · links to 5 tools
In short
Most USB keyboards can describe their state to the computer in a fixed eight-byte message: one byte for the modifier keys, one reserved byte, and six bytes for ordinary keys. Six slots means six ordinary keys at once, which is what 6-key rollover, or 6KRO, describes. The modifiers — the left and right Ctrl, Shift, Alt and Windows or Command keys — are packed as single bits into their own byte, so they do not use up slots, and the message can carry up to 14 keys if eight of them are modifiers. Press a seventh ordinary key and the keyboard fills every slot with an error code, which the computer ignores, so the new key does not register. Keyboards sold as N-key rollover, or NKRO, use a different, larger message that has room for every key, but some switch back to six keys for compatibility with a computer's start-up screens and some adapters. Ghosting is a separate fault, caused by the wiring inside the keyboard rather than the message it sends.
The message a keyboard sends
A USB keyboard does not report each press as it happens. Instead, the computer asks it for its current state many times a second, and the keyboard answers with a short message listing every key that is down. The simplest format for that message is called the boot keyboard report, because it is the one every computer understands before any special software has loaded, including the setup screens you see while a PC starts.
The boot report is exactly eight bytes long, and each byte has a fixed job. The codes in it are the keyboard's own numbers for each key; a browser translates them into names of its own, which the key code viewer shows as you press each key.
| Byte | Holds | Keys it can carry |
|---|---|---|
| 0 | Modifier bits: left and right Ctrl, Shift, Alt and GUI (Windows or Command) | 8, one bit each |
| 1 | Reserved, always ignored | 0 |
| 2 to 7 | One key code per byte for every other key | 6 |
What happens when you press a seventh key
With six ordinary keys already down, the report is full. The rules for USB keyboards say what to do next: instead of dropping one key and keeping the others, the keyboard fills all six slots with a special code, 01, called ErrorRollOver. It means “too many keys to report”.
The computer treats a report full of ErrorRollOver as no new information and keeps the keys it already knew about. So the seventh key simply does not appear, and neither does anything else pressed until one of the six is released. Nothing is lost permanently and nothing is damaged; the message was just too small.
Because the modifiers live in their own byte, you can hold six letters and still add Ctrl, Shift and Alt on both sides. A boot-protocol keyboard can report 14 keys at once when eight of them are modifiers, but never more than six of anything else.
6KRO and NKRO
6-key rollover, or 6KRO, is the name for exactly this limit: any six ordinary keys, plus modifiers, register together. For typing it is far more than enough, because nobody holds seven letters at once while writing. It matters in games that use many keys together, in music software played from the keyboard, and in stenography, where whole chords of keys are pressed at the same moment.
N-key rollover, or NKRO, means every key can be down at once and still be reported. Over USB that needs a different message: instead of six slots, the keyboard sends a map with one bit for every key on the board, so the size of the message no longer limits the count. The older PS/2 connection never had the six-key ceiling at all, because it sent each press and release as a separate message.
Why an NKRO keyboard can still stop at six
The larger NKRO message is only understood once the operating system has loaded its keyboard driver. A computer's start-up and setup screens, and some KVM switches and adapters, only understand the eight-byte boot report. To stay usable everywhere, some NKRO keyboards start in 6KRO mode, fall back to it when they are not sure what they are connected to, or offer a key combination to switch between the two.
So a keyboard advertised as NKRO can quite correctly behave as 6KRO on a particular computer or through a particular hub. If a test shows six keys where you expected more, check the keyboard's manual or software for a rollover setting before assuming a fault, and try it connected directly to the computer rather than through a hub or adapter.
Rollover is not ghosting
Ghosting is a different fault with a different cause. Inside most keyboards the keys are wired in a grid of rows and columns, and the controller finds a press by looking for a connected row and column. When three held keys form three corners of a rectangle in that grid, the fourth corner can look pressed too. Keyboards either report that extra “ghost” key, or, more often, block a key to avoid reporting one.
The practical difference: a rollover limit depends on how many keys you hold, whichever keys they are. A ghosting problem depends on which keys you hold, so a keyboard can manage ten keys in one combination and fail on three in another. Test both, separately.
| Rollover limit | Ghosting or blocking | |
|---|---|---|
| Cause | The size of the message sent to the computer | The wiring grid inside the keyboard |
| Depends on | How many ordinary keys are held | Which particular keys are held |
| Typical symptom | The seventh key never appears, whichever it is | One specific combination fails, often three or four keys |
| Changes with | A rollover setting or a different connection | Nothing; it is built into the board |
How to test your keyboard's rollover
- Open the key rollover test and click into it so it receives the keys.
- Hold down letters one at a time, without releasing any, and watch the count of ordinary keys. A 6KRO keyboard stops at six.
- While holding six, add Ctrl, Shift and Alt. They should still register, because modifiers do not use the six slots. The modifier key test checks the left and right versions of each separately.
- If the count goes past six, the keyboard is sending more than the boot report allows, which means it is working in an NKRO mode.
- Run the ghosting test separately for the specific combinations you use, such as W, A and Shift with Space.
A web page only sees the keys the operating system passes to the browser. Media keys, the Fn key and shortcuts the system keeps for itself never reach the page, so leave those out of a rollover test.
Frequently asked questions
Why does my keyboard only register 6 keys at once?
Because it is sending the standard USB boot keyboard report, which has room for six ordinary keys plus the modifier keys. Pressing a seventh makes the keyboard send an error code in every slot, so the computer ignores the new key until one is released.
What is the difference between 6KRO and NKRO?
6KRO registers any six ordinary keys at once, plus modifiers such as Ctrl and Shift. NKRO, or N-key rollover, registers every key at once, using a larger USB message with one bit for every key.
Do Ctrl, Shift and Alt count toward the 6-key limit?
No. The eight modifier keys — left and right Ctrl, Shift, Alt and Windows or Command — are single bits in their own byte, so a 6KRO keyboard can report six ordinary keys and all eight modifiers together.
Why does my NKRO keyboard only show 6 keys?
Some NKRO keyboards run in 6KRO mode for compatibility with start-up screens, KVM switches and some adapters, or have a setting to switch modes. Check the manual or software, and test it connected directly to the computer.
Is 6-key rollover enough for gaming?
For most games, yes; movement keys, a modifier and a couple of actions rarely exceed six ordinary keys. Games or music software that use many keys at once, and stenography, benefit from NKRO.
Is keyboard ghosting the same as rollover?
No. Rollover is a limit on how many keys can be reported. Ghosting comes from the wiring grid inside the keyboard and affects particular combinations, so a keyboard can pass a rollover test and still fail a ghosting test.
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