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PS/2 interface has existed since Windows 95 (that I can remember) and, while all another interfaces were upgraded (USB, USB2, USB3, …) or just died (such as Parallel, and the large one used for keyboards), most desktops came with a PS/2 connector until recently. Newer ones, such as USB, HDMI, FireWire, and others are just born being plug-and-play. However, PS/2 follows the same specification from more than 10 years ago (someone correct me if I am wrong) and it doesn't seems to change in a few years.

Today this is the only non-plug-and-play interface that I can remember, so there is some reason for that? What determines if a interface is or isn't plug-and-play?

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Same reason there was no sliced bread before there was. ;-> – Moab Feb 11 '12 at 2:18
    
What about RS-232 (still lives since 1962 and is not PnP), VGA DE-15 connector (still lives since 1987 and is not PnP)? So PS/2 is far not the only non-PnP interface that is widely used today. – Andrey Regentov Apr 12 '13 at 5:20
up vote 3 down vote accepted

The PS/2 interface dates from 1987. So it's a 25 year old interface, predating any plug and play OS.

One reason it's around is inertia; there's still a lot of PS2 peripherals out there. Another advantage is its simplicity - I don't need a lot of complexity in my BIOS to read a PS2 keyboard. I have a wireless USB keyboard on my old Windows PC and it takes me forever to get into the BIOS Settings to make changes. Sometimes I wish I had a PS2 keyboard to make BIOS setting easier.

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I can unplug and plugin my ps/2 keyboard whenever i want to.At least with Linux specifically latest Lubuntu prerelease.

When I plug it in its lights flash (like during computer booting) and works as before.

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PS/2 has improved since it was first introduced, although not drastically.

At the end of the day, USB and many other interfaces are designed for input/out and get faster as the standard improves and as the technology evolves.

PS/2 was designed for keyboards/mice and nothing else, there just isn't really a need to improve the technology.

That being said, A lot of modern motherboards to support plug and play PS/2 - I know that the machine I am using at the moment allows me to plug and unplug PS/2 devices when the machine is on without problems. It really depends on the chipset and other factors of the motherboard.

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You’re probably confusing PnP with hotplug. PS/2 is not hotplug and will never be, because it uses ISA signals. You should not unplug PS/2 mice and keybords while the machine is running. – kinokijuf Feb 10 '12 at 17:30
    
@kinokijuf Honestly, you are correct, I think I am mixing them up, but, doesn't plug and play mean, plug it in and it works... so, what is the difference to hot-plugging? I have plugged/unplugged loads of times on my machine - I used to do it many times to switch between a laser pointing presenter mouse and a regular one and never saw a problem. – William Hilsum Feb 10 '12 at 20:08
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According to Microsoft Michael Kaplan, keyboards are plug-and-play, but not plug-and-communicate-what-they-look-like. And hot-plug means that you can connect the device while the comp is running. PS/2 does not have that (try this at home: start computer up without a mouse, connect a PS2 mouse and observe that the computer does not see the mouse). – kinokijuf Feb 10 '12 at 20:28
    
It worked because both devices look probably like a generic „Microsoft PS/2 mouse” or something like that. – kinokijuf Feb 10 '12 at 20:30

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