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Silly question perhaps ... I'm told that PS2 technology dates back to the early 1990's or thereabouts. By and large, motherboard manufacturers still support PS2 technology/ports instead of providing an additional couple of USB ports for keyboard and mouse. Is there any distinct advantage that a PS2 port has over a serial/USB port?

Not that I have anything against manufacturers continuing to support PS2 (+: My Logitech First mouse dates back nearly a decade and has seen 3 computers already

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I have USB keyboard and mouse, but due to some misconfiguration in my BIOS, I'm only able to use the keyboard after the OS have finished loading. So, I can't press F2 to correct the BIOS... the solution? use one PS2 keyboard. –  woliveirajr Sep 29 '11 at 14:21
I'll go w/AndrejaKo's answer; the rationale being that IMHO a USB controller handling multiple devices might hang or something or the other - perhaps when there're devices connected communicating across different USB versions (for instance). Uh ... basically AndrejaKo's mention of USB version, and Barlop's mention of reliability both struck a chord –  Everyone Sep 29 '11 at 15:28
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4 Answers

up vote 2 down vote accepted

Yes! Some operating systems just won't work with USB keyboards or mice. Furthermore some operating systems may not have right USB drivers available during installation which could cause problems with OS installation. Sure, such problems are rare now (but could resurface due to USB 3), but for device manufacturers it pays to have compatibility with older standards.

Now on the hardware side the benefits of PS/2 are clear. The required drivers are much simpler to work with and can be loaded at much earlier stage than USB drivers so PS/2 can be helpful when restoring damaged operating systems which "hang" during certain part of boot process. Another point is that PS/2 KVM switches are easier to design and cheaper to manufacture. Finally the main point is that PS/2 connector itself is relatively small, not very difficult to use and it is sufficient for keyboard and mouse use. As far as response times are concerned, this answer claims that PS/2 actually has an advantage over USB devices.

Another "benefit" for PS/2 is that it is deeply entrenched, old and proven standard and therefore has support of manufacturer "inertia". As we can see when analyzing computer history, it took some time for USB to replace existing serial and parallel ports as well as certain proprietary connectors even in situations where it had significant speed and usability advantage. In current situation from what I can see, USB doesn't have that many advantages when used to connect mice and keyboards to desktop computers where the only major disadvantages of PS/2 are that the connector isn't as easy to plug in as USB and official hot-plugging support which has been mostly back-ported into current systems which use PS/2.

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so if an OS hangs in the boot process, how does a ps2 keyboard help? –  barlop Sep 29 '11 at 13:32
@barlop Some operating systems (mostly Unix-like) have several boot levels so you can boot system with minimum required drivers and services and then troubleshoot system further. In some situations, the lowest boot levels may not include USB drivers at all. Having a PS/2 keyboard helps in such situations, because you don't have to go digging for a serial console (if you have a serial port). –  AndrejaKo Sep 29 '11 at 13:37
re the answer you link to, I have found cases where I type and letters don't appear quickly, probably when its happend its been more with USB –  barlop Sep 29 '11 at 14:53
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No. In fact, PS/2 ports are worse than USB or serial, because they don't support plugging a device in without a reboot, and on many systems you have to get them the right way round or they don't work. In fact, PS/2 ports are only provided for compatibility with old keyboards and mice these days. They are becoming less and less common as USB and wireless keyboards and mice get more common.

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"Serial" ports don't exactly support hotplugging, either... –  grawity Sep 29 '11 at 13:02
It can be hard to fry serial or ps/2 ports. Most chipsets these days will allow you to switch keyboard and mouse, but it used to be you had to plug the keyboard in first - mice could damage a keyboard port - I never had one go, but I always plug in the keyboard first. –  Broam Sep 29 '11 at 13:04
By 'hotplug' I meant the autodetection feature. (As for the other meaning, USB 2.0 ports are fork-resistant.) –  grawity Sep 29 '11 at 13:07
in fairness, i've never had any damage from ps/2 hotplugging. or plugging in, or out, in any order, at any time. Sometimes if it wasn't plugged in at boot time it wont work when you plug it in, but that's the worse i've had. –  Sirex Sep 29 '11 at 13:18
USB is becoming common only because notebooks need to be thinner. Almost the only another reason is WOW! USB! Never had that! should be cool! –  Nakilon Apr 16 '12 at 14:16
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From what I read on this page so far it seems a lot of people are misinformed. The following is information on KEYBOARDS ONLY. I have no idea about PS/2 for mice. The info is from http://www.overclock.net/t/491752/mechanical-keyboard-guide#post_6009418:

Key Blocking & Ghosting

Ghosting is when you press two keys on the keyboard, and a 3rd key - which you didn't press - gets sent to the PC as well. This is very rarely seen on even the cheapest modern boards, because manufacturers have the habit of limiting the rollover so that ghost keys are always blocked.

Key Blocking is as simple as it sounds; you experiencing it when you reach your maximum key roll over. So if you press 2 keys, and the third key is blocked on your board; then you just experienced blocking because your keyboard is only 2KRO.

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.

xKRO, where x = 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.

Key Bouncing

All types of key switches - including rubber domes - do this. When you press a key, the switch "bounces" on and off very quickly as it sets into place. This causes keys to register multiple times for each press. Because of this, keyboards need to implement some sort of debouncing delay - so that once you press a key, the controller waits a certain amount of time before registering a keypress. As an example, Cherry MX switches need 5ms of debouncing time, while rubber domes need longer (exactly how long depends on their quality).

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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If a keyboard or mouse doesn't work in a USB port, plug a keyboard or mouse into a ps2 port.

If a computer only has 2 USB ports, or even 4.. then 2 ps2 ports are particularly useful. I have almost never had to plug/plug ps2 it seems so reliable. USB i've had to though maybe largely because i've plugged them into a USB hub.

When you plug a ps2 keyboard in as soon as power goes to it, e.g. if it's off and you turn the computer on. or if you've been "risky" and plugged it in while it's on as a test, then the lights flash, power goes to it. When plugging a USB keyboard in you don't get that indication. (I suppose you could tap num lock etc but you tend not to get immediate indication)

A negative, is apparently ps2 is only not hot swappable, but doing so can damage the motherboard http://www.computing.net/answers/hardware/ps2-mouse-amp-keyboard-hot-plug/24664.html
that was old news so maybe has changed..

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