Difference between PS2 and USB @ NKRO

[Naweo]

I believe I have an ps2-adapter (The one that allows USB keyboards to be placed into that circular purple thing on the motherboard for a PS2 connection) thus I got an interest to know the differences between USB and PS2 connection on keyboards.

When I read some keyboards has limited RO over USB but NKRO on PS2, what exactly does it mean?

Does it also make multiple pressed keys repeat as:

"asdfasdfasdfasdfasdfasdfasdfasdf"

instead of :

"asdffffffffffffffffffffffffffffffffffffffffffffff" ?

Thank you, much appreciated.

I use this source as reference:

http://www.mechanicalkeyboardguide.com/guide/what-is-the-difference-between-ps2-and-usb-keyboards/

[Doesntknowwhattosay]

I'm writing on a QFR via USB and asdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdfasdf

So that's not the difference 

[dorkvader]

When I read some keyboards has limited RO over USB but NKRO on PS2, what exactly does it mean?

Usually they are talking about the limit for "full speed USB" (USB 1.1) to 6 keys + a few modifiers. If you press more than 6 keys, the exact behaviour depends on the keyboard in quesion.

So pressing and holding 123456 will result in all keys showing the key-down but not key-up. They may repeat as has been described depending on your OS. But what happens if you hold 1-6 and press 7? Some keyboards will ignore the 7 entirely (less optimal) and some will choose the more optimal way of releasing 1 and pressing 7, you will be left with effectively keys 2-7 being held down, tohugh you are in actuality pressing all 7. This is also a "choice" that a USB-to-PS/2 adapter will have to make.

In a PS/2 keyboard with NKRO, no matter what keys you press, or hold down, they will all register and be output. While there is likely little need for all of this, it's a nice solution: that no matter what you press, and no matter what combination, it'll work for sure.

Now, there's another issue about which keys (if any) send upcodes. If none of your keys send upcodes, this is usually interpreted in the adapter as it sending a downcode followed by an immediate upcode (likely to prevent strange behaviour) Mainly an odd issue with very old keyboards, and only commonly seen in IBM hardware, it is interesting to point out that this is different from NKRO.

So, on a KB that doesn't send upcodes. there is no difference between pressing and holding keys. Holding keys 1-5 down will generate "12345" and that's it with no repeat.

Usually on modern keyboards, if there is a key that doesn't send an upcode, it is the "pause-break" key.

Now on odd key-repeat behaviour:
Usually if a KB is playing games with the OS to try and implement NKRO in one of may odd ways, you can end up with odd key-repeat behaviour that can be unanticipated. This can be when the KB is emulating multiple keyboards to get a higher key rollover. When you try to press a key from both the emulated keyboards (if there are more than one) it will behave differently than expected. Please see: http://geekhack.org/index.php?topic=48158.0

Hope this helps!