Yeah, let's talk about rollover. Rollover and ghosting, two
different concepts that are often confused.
A keyboard has N-key rollover if you can press N keys down without
letting up on any of them and they will all register in the correct
order before any of them is released.
CAPTION: 2KRO, 6KRO, NKRO
The most common rollover limits are 2KRO, 6KRO, and NKRO. Very old
keyboard designs often have 2KRO. 6KRO is characteristic of USB
keyboards when they power up into what's called "boot mode". NKRO is
unlimited rollover; USB keyboards have this when the host USB
controller has told them to switch from boot to normal mode.
Well...maybe. Operating systems are supposed to issue that mode switch
near the end of their boot sequence. It's fairly common for broken
firmware in cheap keyboards to fail to interpret the mode switch
correctly. Less commonly, obscure bugs may prevent the OS from
issuing the mode-switch command when it should.
Often, failure to switch out of boot mode goes unnoticed because
actual 7-key combinations are very, very rarely invoked even by
twitch gamers.
Those are minimums, not a maximum. 2KRO keyboards can, and often do,
support lots of three-key and longer combinations. Just not all of
them - and for modern uses, the problem is worst when there are WASD
cluster has collisions.
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A different problem that affected some older Unicomp products is that they
powered up in normmal USB mode rather than boot mode. This meant they
couldn't be used at all until the boot phase of some operating
systems was over.
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When a keyboard has limited rollover, the worst that can happen is
some keystrokes in a multi-key press fail to register. Ghosting is a
nastier failure mode: if a keyboard is susceptible to it, a
multiple-key press will generate spurious "ghost keystrokes" that
weren't in the sequence you pressed.
If this sounds like a different definition of "ghosting" than you've
heard before, that's because sloppy usage of the term "anti-ghosting"
in keyboard marketing literature has misled a lot of people about what
"ghosting" is.
When vendors talk about "anti-ghosting", what they actually mean
is that they support more than 2-key rollover, but only for certain
groups of keys like the WASD cluster and shift or control modifiers
that they expect to be used in combinations.
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Model F keyboards, both old and new, have unlimited rollover
and never ghost. For model Ms, it's more complicated. They
never ghost, but their rollover is limited. Most variants only support
2-key rollover. However, modifier keys like Shift, Control, and
Alt have their own traces so they don't count against that limit.
As you can see from this diagram, the WASD cluster that many
modern games use as arrow keys does *not* have its own traces.
That's because the Model M was designed well before that convention
developed in the 1990s.
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The new Unicomp Mini-M is an exception. It reworks the traces on its
sensor membrane to eliminate possible collisions with frequently-used
keys, including not just the modifiers but the WASD cluster as well.
Then it repeats a trick used in many gaming-oriented keyboards to
support more than the boot-mode USB limit of 6-key rollover. That is
to stuff two USB controllers into the board handling different sets of
leads from the keyboards.
With two USB controllers, you get better than 6KRO even if the
operating system fails to issue the keyboard mode switch when it
should.
According to Unicomp, the Mini-M can correctly report combinations as
long as 10 simultaneous keys. But not all such combinations; some
will still drop keys. It would take a much more fundamental redesign
of the sensor membrane to entirely solve that problem.
Mine does whenever I think about this ...
