I could swear I read somewhere that hotswapping a keyboard (changing it while the computer is still running) could kill the computer and so I have always shut down any computer I was working on that needed a keyboard change. But I’ve been wondering lately if this is really true and what the mechanism would be if it is true. How does plugging in a new keyboard kill a computer?
I’ve done this dozens of times with no ill effects. The worst result I’ve seen is that the OS no longer recognizes a keyboard, so you have to reboot–which is actually fairly common with PS/2 keyboards. USB is designed to be hot-swappable, but even PS/2 is generally safe to do so. I won’t go so far as to say it could never cause damage, but I’ve never seen it happen.
Yeah, it happens. Much more likely on old style AT DIN plugs than on PS/2 style plugs. What can result is that a small picofuse blows and the keyboard connection no longer works. Depending on the BIOS this means it doesn’t find a keyboard on boot up or doesn’t boot at all.
I’ve also seen picofuses blown from people plugging the mouse into the keyboard port and vice versa. (Which seems impossible, but it none-the-less it happens.)
If you are really determined at screwing things up, then even worse shorts can occur. But you have to be trying extra hard.
I have saved a couple MBs by replacing the picofuses (getting the parts from old scrap MBs).
I’ve killed the keyboard port hot-swapping a keyboard. I couldn’t afford a USB card and new keyboard, so that did effectively kill that computer.
'course, it was a Cyrix. It needed killing.
Back in my ‘Network Adminstrator’ days, I had one End User fry his motherboard by tugging in the PS2 cable at an odd angle. It was connected to his Dell Optiplex. All of which was under waranty though. I found out later (from his coworkers), he reclined in his chair (pictured feet propped, keyboard on lap) and fell backwards, thus giving the cable quite a tug. :smack: 
Back in the olden days (286 and 386 era) the microcontroller in the keyboard talked directly to a microcontroller on the motherboard. These two microcontrollers were connected directly together with no buffering and no safety circuitry of any kind, except for a small fuse on the 5 volt line to prevent you from totally mucking things up. They were obviously not designed to be hot swapped (plugged and unplugged while “hot” or under power). It was certainly possible to damage one or both microcontrollers doing a hot swap. The only good thing is that microcontrollers, since they tend to control things, also tend to have somewhat forgiving I/O lines. A little extra beef in the circuitry inside the microcontroller is sometimes all that saves you from disaster when you hot swap a PS/2 or AT keyboard. Also, since newer microcontrollers tend to be designed better than older ones, the chance of damaging one of the microcontrollers is lessened on a newer computer.
These days, the keyboard controller is often no longer a single chip on the motherboard. Instead, it has been integrated into the main chip set on the motherboard, so if you fry it, you might fry the entire chip set. This means that you can easily make more than just the keyboard controller portion of it stop working. You might totally fry the motherboard, effectively. Worse, even if all you do is damage the keyboard controller, it’s now inside one of those surface mounted chips with a bizillion leads soldered directly onto the motherboard. At least in the old days if you toasted the microcontroller, you could pop it out of its socket and put in a new one. No such luck on a new PC. Heck, even the fuse is usually one of those little picofuse type of things, which looks more like a resistor than a fuse to a novice, and if you don’t know how to solder you can’t replace that either.
Fortunately, damage is rare, and when things do go south it’s usually that one or both microcontrollers (the one in the keyboard and the one on the motherboard) get confused and just stop talking to each other properly, which fixes itself once you turn the computer off and back on again.
USB, on the other hand, was designed from the beginning to be hot swappable. Plug and unplug to your heart’s content. You won’t hurt anything.
FYI - If your computer still has serial and parallel ports (which are quickly vanishing from most PCs), you can safely hot swap the serial ports but not the parallel (printer) port.
When I was 12 I tried to plug in an AT keyboard after the local shop just built our family a new computer. I ended up frying the motherboard. It wouldn’t boot at all with/without the keyboard being plugged in.
The computer shop fixed it for free and since I had a leet renegade BBS running that they copied to my fat new 1GB HD, they just told me to learn something about hardware and didn’t even try to beat me with the keyboard. I then went on to get my A+ certification a few years later and learned that AT keyboards have their own capacitor in them. Who’da thunk it?
So you were the same age as this thread?
A quick update for those living in 2016: there’s no risk when hotswapping USB keyboards. If your computer still has PS/2 ports, you should probably consider upgrading.
PS/2 for life. Gotta get that N-key roll over. Nice 1000 Hz overclocked USB port. My mouse is interrupt-based so I don’t need to wait that extra 1 ms.
To answer the 12-year-old OP, no, never.
Jeez, this looks like one of the oldest zombie threads I’ve seen. I would never change a PS2 keyboard without first shutting down, just to be safe… there are disaster stories out there. USB, no problem hotswapping.
In a lighter note, does everyone remember the famous line at boot-up: «No keyboard found, press any key to continue»
Umm, see my post above.
In a long lifetime of building, using, and repairing hundreds (thousands?) of computers since 1976, I never saw one trashed by removing the keyboard or video with power-on, and I never turned it off first. Stories are just that – stories.
OOh, ooh, anecdote fight!
Look, as engineer_comp_geek stated eloquently a dozen years ago, the design of PS/2 ports has relatively little protection against hot-plug connection errors (like energizing circuitry with a floating/disconnected signal ground). It isn’t guaranteed to happen, any more than you’re guaranteed to throw snake-eyes even if you play craps for years on end. It’s just a possibility, a very real one. Frankly, I have done it once. One a 386 whitebox being used as a production Novell server in a small office, even. :mad:
Also noted earlier in the undead thread was the possibility of plugging a PS/2 keyboard into a PS/2 mouse port. It was actually quite easy to confuse the two ports, after they were both commonly mini-DIN-6 connectors only differentiated by color and position. Neither of which help if you’re groping blindly at the back of a computer.
All I can say is Thank God for USB.
What about plugging in a PS2 with computer on?
Even new, good-quality motherboards still may include a pair of PS/2 ports or maybe one that can be either mouse or keyboard.
I’ve hot-swapped PS/2 devices multiple times but I try to do it while a machine is shut down. Not because I’ve ever had damage to a machine but because most of the time the device won’t work until I reboot anyway.
Of course these days whether I have a PS/2 port or not I exclusively use USB peripherals. They’re just less hassle.
Um, like my post said I have personally seen this twice. Once someone else did, once I did :o.
So, magically, these two MBs didn’t recognize the KB but each did after I replaced their picofuses???
Think about this one very simple thing: Why do you think they put the picofuse there??? It’s to limit the damage to just the MB!
You could also Google “keyboard fuse” and find a lot of people who have dealt with this.
Not seeing something during your life doesn’t mean others have. I have never, ever seen a rattlesnake in the wild. Despite spending a lot of times living and roaming around in rattlesnake country. I would never suggest that rattlesnakes don’t exist and they’re just stories!
Here’s how it happens: You’re trying to plug the keyboard back in blind. The pins aren’t lined up. Of course it doesn’t go in. But it does go in a little. The shield on the plug contacts the case (easy to do since the plug is likely tilted due to the misalignment). If the misalignment is “just right” you have 5v going straight to ground. Pop goes the picofuse.
Posting that due to you not having seen it means it doesn’t exist is ridiculous.
(Oh, and if you think that you can’t damage a USB port by plugging something in wrong when the computer is on, you really haven’t met certain very determined users.)
To be fair, the user that is determined to hammer an upside down USB port into place is going to break something whether the PC is on or off.
When I was a computer salesman many years ago at a store that also did repairs, one customer brought in a PC that had been fried. The customer’s friend who was a “computer expert” installed a new hard drive in his computer. It was an IDE hard drive and he must have been trying to plug the power plug in upside-down because when I looked at the molex connector it had been shaved on the flat side as if whittled with a pocket knife. Apparently keying a plug so it only fits when plugged in the right way isn’t good enough for some people…
How does someone even find a thread over a dozen years old to reply to?
Can we blame The Google? Did you need to swap keyboards on your Commodore and you couldn’t Ask Jeeves any more? So this came up on ‘That Googley Thing’?