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PS/2 vs USB Keyboards
Connecting Your Keyboard: USB or PS/2 Port?
Let's say you bought a fancy new keyboard for your computer and are wondering whether it's better to connect it via USB or PS/2. You'd think that the answer was simple: the USB standard is modern and constantly improved, while PS/2 is a legacy port, so surely you should use the former? There might be some truth in that, but it turns out that if your keyboard has a PS/2 connector, there's no reason not to use it. The page below will explain some of the biggest benefits of using this older connection as opposed to USB ones, as well as the other way around.
Top 3 Reasons Why PS/2 is Better Than USB
Here are some reasons why you should consider connecting your keyboard via PS/2 instead of USB:
- PS/2 is hardware interrupt-based, while USB port is polling-based. This means that when you press a key on a PS/2 keyboard, it generates a hardware interrupt immediately, whereas USB polls your keyboard many times per second (125 Hz by default, up to 1,000 Hz in "gaming" keyboards) to see if any keys are pressed. This means that PS/2 keyboards will have the lowest latency - although in all likelihood, you will never notice a difference. Perhaps more importantly, polling is more CPU-intensive, especially if high polling rate (like 1,000 Hz) is used.
- PS/2 supports so-called full n-key rollover. USB keyboards are generally limited to 6KRO (able to recognize up to six simultaneous keystrokes). Of course, it also depends on the keyboard: most cheap keyboards are just 2KRO, and it does not matter whether you connect such a peripheral via USB or PS/2 port: it will still register only 2 keys at once. Full NKRO is a feature usually found in gaming keyboards, as that's where this feature is in demand the most.
- PS/2 keyboards are more compatible with older hardware and software, which might be important for some users. To be perfectly fair though, turning on "Legacy USB" in BIOS means your modern keyboard will work in those cases as well, which almost makes this a moot point.
Are There Any Cases Where USB Connection Is Superior?
Universal Serial Bus is a much newer standard, after all
Now that we've established that a PS/2 keyboard has some real (if small) advantages over an USB one, are there cases where the latter is superior? Indeed, USB does have one important feature that its predecessor does not: it supports plug and play. This means that you can plug in or swap your input devices - such as mice and keyboards - while your computer is on, and they will be detected and according drivers will be installed automatically.
To be fair, while PS/2 is not designed for hot plugging, in many cases unplugging an old keyboard and plugging in a new one while the power is on will work just fine. However, this is not recommended, as there's a small possibility of frying your PS/2 port.
There's also the fact that PS/2 is probably going to be phased out eventually, and new motherboards will not have this connection onboard. However, it has not happened yet, even though PS/2 is more than 20 years old by now.
Finally, USB has some power-saving features, such as Selective Suspend - although, seeing how the power consumption of a keyboard is minuscule, it doesn't matter all that much.
USB versus PS/2 Keyboard Poll
How is your keyboard connected right now?
What About Your Mouse?
The situation is actually the opposite in this case. The USB port offers higher bandwidth and sampling rate, so it is the best choice for a mouse. Gaming mice are usually set to use 500 Hz or 1,000 Hz polling rate out of the box, a large improvement over the 125 Hz of PS/2. This difference in update rates can actually be felt by many users, or seen in the "Paint test" (drawing circles in a MS Paint program and inspecting them for smoothness). The smoother and more accurate mouse movement will cost you increased processor usage, although it's completely unnoticeable in a modern system with a multi-core CPU.