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PS/2 Question

post #1 of 8
Thread Starter 
Hello all!

I've read people say that PS/2 is interrupt based, but i've also read PS/2 polling rate is 200hz. What is correct and how does it work really?

I'm thinking about buying a Passive USB to PS/2 adapter for my Topre RealForce 105 UFB. Would this introduce any kind of lag, the adapter being passive? Or cause any other anomalies?

I really want to try PS/2 because of NKRO. I've also read that the PS/2 is slighty faster (little less laggier) than USB, is this true? I don't really care whether I would notice it or not, just would like to know pure numbers smile.gif

Thank you!
post #2 of 8
I've quoted some relevant pieces of information from the Mechanical Keyboard Guide sticky: http://www.overclock.net/t/491752/mechanical-keyboard-guide/0_30
Quote:
Originally Posted by Manyak View Post

Key Rollover (#KRO & NKRO)

NKRO is when you can press as many keys as you want at the same time, and all of them go through. This is similar to what some 'gaming keyboards' incorrectly market as "anti-ghosting", even though Logitech and Razer only apply it to the WASD cluster. Note that right now only PS/2 keyboards can exhibit full n-key rollover; though Microsoft and Ducky are just two companies who have already looked at designing NKRO over USB.


#KRO, where # = Any Number, is the key roll over of your board; and stands for the maximum number of keys you can press without experiencing any key blocking.


Many USB mechanical Keyboards are labeled as 6KRO, meaning any 6 keys can be pressed at once without the user experiencing blocking. This is generally enough for most users. Though a limited number of games may have a problem with 6KRO.

USB keyboards with 6KRO also allow for a maximum of 4 modifier keys to be used with those 6 normal keys. These modifiers include CTRL, ALT, Shift, & Super (Windows, Command, or Meta Key.)

Sometimes this also includes the FN key present on select keyboards.

Polling Rates and Response Times

While it is very useful for mice, it's just about meaningless for keyboards. Let's assume for a minute that all switches have the 5ms debouncing time of Cherry MX switches (which is being very generous). Even if you had super human speed and reflexes, every single key would be delayed by at least that much. So really, any polling rate over 200Hz (at best) is absolutely useless, and nothing but market hype. It may even be a bit detrimental, because you'd be wasting CPU time polling the keyboard unneededly. And unlike USB keyboards, PS/2 boards aren't polled at all. They simply send the signal to the PC whenever they are ready to, which causes a hardware interrupt, forcing the CPU to register that keystroke.

PS/2 or USB?

PS/2 wins on three fronts: First, it supports full n-key rollover. Second, PS/2 keyboards aren't polled, but are completely interrupt based. And third, it is impossible for it to be delayed by the USB bus being used by other devices. There are two types of USB transfer modes - the interrupt transfer mode (USB polls keyboard, when key is sensed the USB controller sends the interrupt to the CPU), and the isochronous transfer mode, which reserves a certain amount of bandwidth for the keyboard with a guaranteed latency on the bus. Unfortunately, there are absolutely no keyboards made that use the latter, because special controllers would have to be used, thus making it cost prohibitive.


So if your keyboard supports both PS/2 and USB, and your PC has a PS/2 port, there's no reason not to use it.
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post #3 of 8
Thread Starter 
Thanks for that! smile.gif

What about the other questions I asked though? Are only ps/2 mice polled but ps/2 keyboard aren't?

Can passive adapters cause any lag?

Does anyone know if Topre RF works with usb to ps/2 adapter?
post #4 of 8
Quote:
Originally Posted by Yahar View Post

Thanks for that! smile.gif
What about the other questions I asked though? Are only ps/2 mice polled but ps/2 keyboard aren't?
Can passive adapters cause any lag?
Does anyone know if Topre RF works with usb to ps/2 adapter?

RF does not work with the USB to PS/2 adapter. There are some models with native PS/2 though.

As far as the input, I don't think the keyboard is polled at all on ps/2, as it interrupts and sends input when you hit a key.
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post #5 of 8
Not exactly.

But it's irrelevant anyway since PS/2 is obsolete.

And when I see RF I think microwaves.

Here is a list of keyboards that do NOT work with the purple dongles:
Quote:
[H="3"] Incompatible[/H]
PFU Happy Hacking Professional Keyboard Series
PFU Happy Hacking Lite (USB version)
Topre Realforce
Unicomp Spacesaver
Almost all Cherry Corp Keyboards, including..
G80-8113


From the R00TW0RMed PS/2 Compatibility wiki.
Edited by ripster - 9/1/12 at 2:31pm
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post #6 of 8
Quote:
Originally Posted by ripster View Post

But it's irrelevant anyway since PS/2 is obsolete.

Really? Most every new mobo I've seen has a Ps/2 port. What leads you to say it is obsolete?
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post #7 of 8
Just be normal and use USB.
post #8 of 8
PS/2 really isn't obsolete IMO. I've never had any issues with PS/2 keyboards in BIOS's. That alone is worth them staying around.
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