Yuugo wrote:

I've connected my keyboard with a standard usb cable to a standard usb port. I can press at least 8 keys simultaneously. I don't know what the max is since I've never got a way to try it.

I'll update this post later on how this stuff works, I think I've got a book that covers it lying around somewhere.

Okay here we go.

First off let us take a look at the terminology around this. It comes down to three terms that are sadly used interchangeably in the marketing of keyboards, while they are completely different things.

• Ghosting
  Blocking
  Key Rollover

Ghosting
Ghosting is, in contrary to popular belief, NOT the act of keys not being registered. It is, in fact, the opposite. Ghosting comes from an incorrect implementation of the electrical circuit of your keyboard (PM/talk to me in-game if you're interested in how).

Say you press Q and S. You'd want your computer to register just those two keys. A keyboard that 'ghosts' registers an additional ghost-key as well. In this case, most likely the letter A. If you experience ghosting, you need to go back to the store you bought that keyboard and smack them in the face with it. This should not happen these days!

Blocking
Blocking is what people are usually actually talking about when they say ghosting. This is a (bad) solution to prevent ghosting. Basically what it comes down to, is the manufacturer of your keyboard limiting the amount of key-presses that can be registered simultaneously to disable the keyboard from registering a ghost key. Depending on the construction of your keyboard, the amount of keys and the combination of what keys that can be pressed varies from board to board.

Blocking is a solution that was introduced to cut costs of the manufacturing of the keyboards, rather than solving the problem of ghosting, they implemented a (in my opinion, bad) workaround for it.

Key Rollover
This is also named N-Key Rollover, NKRO, 2KRO or 6KRO or WhatevernumberKRO. Well, they're more examples of Key Rollover, but whatever.

So, let's say your manufactured isn't a greedy bastard and actually solves the problem of ghosting by improving their circuitry. The keyboard no longer registers wrong keys and also accepts unlimited simultaneous key-presses. Great! We now have the perfect keyboard that has N-Key Rollover. Sounds like the perfect keyboard to me.
But there is another limiter on the amount of registrations that can me made at the same time. This is your connection to the computer. These are either USB or PS/2(those round plugs that used to be the standard before USB).
PS/2 transfers NKRO perfectly. If your keyboard has a circuitry that supports NKRO and uses a PS/2 plug, you should be able to press all your keys at the same time and have all of them registered fine. This means we now have a perfect keyboard!
USB on the other hand, can be limited to 6KRO. This happens because the most common USB Human Interface Device (HID) protocol used these days do not accept more than 6 simultaneous keys. So now what?

My keyboard is USB, how am I supposed to get N-Key Rollover?
First of all, don't be afraid. There are manufacturers out there that have found solutions to implement inside their boards for NKRO (talk/PM me if you're interested in how). If you're lucky you don't have to do anything.
Next is the tricky part, what if your keyboard is not already NKRO upon delivery?
The first thing you need to do is discover whether your keyboard support 6KRO. If it doesn't, you've sadly fallen victim to price cutting of the manufacturer and there is nothing you can do besides buying a different board. If it does, you can solve the USB HID limit yourself with a USB->PS/2 adapter. If your computer had an input for PS/2, this should solve your problems. Sadly, most new laptops and some minimalistic motherboards do not have an PS/2 port. If this is the case, you wil need a USB->PS/2 adapter AND an active PS/2->USB converter. This converter is specially made to support unlimited keypresses so that would solve your problem as well.



In case you're lazy: TL;DR. Seriously though, read it. At least know what you're talking about so you won't look like an idiot saying your keyboard 'ghosts'.
Ghosting: Keys that are not being pressed are still registered.
Blocking: Keys that are being pressed, are not registered.
Key Rollover: The highest amount of keys that can be registered simultaneously.

USB limit:
Got 6KRO? Get a USB->PS/2 adapter or if you do not have a PS/2 port, get both a USB->PS/2 and an active PS/2 -> USB converter
Don't have 6KRO? Welp, too bad. You're gonna have to buy a new keyboard if you want NKRO.