Keyboard Ghosting vs N-Key Rollover Explained
Ghosting means an unpressed key is reported. Blocking means a pressed key is withheld, often to avoid an ambiguous matrix state. Rollover describes how many simultaneous keys can be reported correctly. These are related behaviors, but rollover is not exclusively a USB limit.
If keys work individually and fail together, use the ghosting test to record the exact combination. A missing key in the browser may also be caused by a shortcut or remapping, so compare more than one application before blaming the matrix.
2KRO, 6KRO and NKRO
| Term | Meaning | What a browser check can establish |
|---|---|---|
| 2KRO | Any two ordinary keys can be reported together; some larger combinations may work | Whether the particular pairs and groups you tried arrived |
| 6KRO | Six ordinary keys can be reported together, commonly with modifiers separate | Whether six ordinary keys and your actual shortcuts work |
| NKRO | All supported ordinary keys can be reported together in the relevant mode | A lower bound from tested combinations, not certification of every combination |
Rollover can be constrained by matrix design, scanning, firmware, report format and connection mode. A board can have different behavior over Bluetooth and USB. Check its documentation before assuming one mode represents the others.
Why USB boot keyboards often stop at six
The USB HID boot-keyboard report reserves six entries for ordinary keys and a separate bit field for modifiers. Other HID report formats can report more ordinary keys; USB itself is not limited to six.
This makes the counting method important. Six letters plus Shift and Control does not prove more than six ordinary keys are supported. Conversely, seven ordinary keys reaching the browser demonstrates more than 6KRO for that combination, not unlimited rollover.
Why combinations matter
In a matrix, several closed switches can create an ambiguous electrical path. Isolation or a suitable matrix design prevents unwanted paths; firmware may suppress a key when the state is ambiguous. Physical adjacency on the keycaps does not necessarily match electrical rows and columns.
Marketing terms such as “anti-ghosting” may describe only a selected group. They do not establish that every key combination works. A “19-key” claim needs the manufacturer's definition before it can be interpreted.
A useful test sequence
- In the rollover test, hold letters and numbers first, without modifiers.
- Add keys gradually and watch which ones remain held.
- Repeat with a different part of the board and a different press order.
- In the gaming keyboard test, try combinations you actually need, such as W + D + Shift + Space.
- Repeat using the same connection mode as your game or application.
Write down missing keys, not just the maximum count. Avoid operating-system shortcuts that can move focus away. If only one game fails, continue with WASD troubleshooting.
Does firmware fix ghosting?
Firmware can change report mode, mappings and blocking behavior. It cannot add missing physical isolation components. Whether an update helps depends on the actual limitation; test supported modes and consult the keyboard maker before replacing hardware.
Sources
- USB HID 1.11, Appendix B defines the boot-keyboard report and modifiers.
- QMK: how a keyboard matrix works explains scanning and diode isolation.
- Browser testing limits explains why observed combinations are evidence, not certification.