Keyboards: the computer doesn't write what I typed. Why?

Keyboards usually check the key presses by contact of wires in a matrix. The controller only sees contacts at matrix coordinates and, therefore, can reliably handle in worst case two independent keypresses. Two-key rollover (2KRO) means that: minimum two keys at once recognized. 2KRO is a must for fast typers that may press the next key before releasing the first. But all gets worse when it comes to typing with Shift pressed for uppercase; while manufacturers should keep the modifiers in separate contact lines to the MCU (Microcontroller Unit), most of them will have the modifiers in the matrix for low quality keyboards. This makes the keyboards type wrong when the fast typist writes uppercase characters with shift pressed, where letters effectively work as a only-one-key-at-once (1KRO) keyboard.

Also many keyboards sample the key presses at 125Hz which is quite low for fast typing. Even if sustained typing may be about 600presses/minute (100Hz) often two specific keys can be pressed in less than 125Hz period. Then the keyboard doesn't know which key was pressed first.

Many keyboards also implement only a basic USB keyboard protocol (named boot protocol) that allows reporting at most 6 pressed keys at once (modifiers apart in a bitset), but this is a problem only for gamers, not for fast typers. Keyboards can implement a HID device beyond the boot protocol and report more keys at once.

One problem is inherent to the USB protocol though; only regular the keypresses are communicated to the computer in order. The press of the modifiers is a bitset that is communicated in every message and, therefore, cannot be ordered with the other keys.

Tests to evaluate your keyboard

Test bad keyboard 1. Press Shift + two letters at once.

Example sequence from Logitech K120. Type TR uppercase like this:

Other keyboards hit this problem in different letter pairs, like BV, FG, etc. Some others will be able to respect the uppercase for the second letter.

Test bad keyboard 2. Press two keys in sequence very fast

Drop two fingers fast to two letters, caring to know what key you hit first. Repeat it many times and check the reliability changing which you press first.

Example with the Logitech k120:

1. Type MK many times in a row, with the two fingers drop down
2. Result on screen: mkmkmkmkmkmkmkmkmkmkmk (good)
3. Type KM many times in a row, with the two fingers drop down
4. Result on screen: mkkmmkmkmkmkmkmkmkmkmk (often swaps to mk)

Test bad keyboard 3. Check USB HID Report frames

PS/2 keyboards send key change one after the other, either press or releases, so they are always received in a particular order. But unlike the PS/2 protocol, the USB messages allow to notify more than one keychange (press/release) in a single frame, at the expense of somewhat unclear order. Non-modifier presses are always ordered, but releases or modifier press/release are optionally without order. Some devices (keyboards or PS/2-USB converters) choose to clarify the order by guaranteeing a single key change between any message. As a note, most notebook keyboards use a PS/2 link, not USB, so this would not apply to them.

Use the GNU/Linux usbhid-dump program to dump the frames and see if they send a report for every key change or they integrate multiple into one. Run it with timeout 10 usbhid-dump --entity=stream so after 10 seconds you will be able to rule your computer again.

Report format: The first byte indicates the state of modifiers; every bit is a modifier. The second byte is reserved. The next bytes indicate keycodes pressed, in the order they were pressed.

Example from Logitech K120. Frames at most every ~16ms, unclear order of some key changes.

Example from Genesis RX55. Frames every ~2ms, clear order of all key changes:

Test yourself

• The number of keys you held pressed at once during your typing (Max rollover).
• The WPM uses the standard of (final letters typed)/5. The more you backspace to correct, the less WPM.
• The presses/min includes backspaces, shifts, etc. used in your typing.
• The "fastest interkey time" reports the shortest time you had between two keys pressed.

Test bench

Note if you use Tridactyl: Use shift-insert to avoid commands. ESC to reset.

Reports of what I tested

Reports I heard

• Logitech wireless: ~5KRO even with right shift pressed.
• Apple wired keyboard: 2KRO modifiers apart fine, and proper order on keys pressed quickly..
• HP Slim kbar211, two tests good.
• HP 1156, two tests good. (Unmatches my experience)
• Dell SK-8165, 2KRO with shift fine but bad on fast two keypresses
• Microsoft Natural Ergonomic 4000 v1.0, 2KRO with shift fine and good with fast two keypresses
• Logitech K400, 2KRO with shift fine but bad with fast two keypresses

Based on: gadzikowski.com/nkeyrollover.html

Also related: Scan rate estimator for presses of same key