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I remember back in the 90s someone told me it was bad for a computer to plug and unplug a monitor or printer (LPT/parallel) into a computer while it was running.

Is there still any basis of fact to this? e.g. could a spark caused by this damage the motherboard / chipset?

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up vote 8 down vote accepted
  1. Most monitors have separate power connectors today
    (which do not connect to the PC power supply)
  2. The power on the data cable is not sufficient to cause significant sparking
  3. The data cable design makes it difficult to 'short' ground and other pins
  4. I have frequently done this with a large number of 'headless' lab machines and never seen a fault

I have a similar feeling for printers, but would like to check others who have more experience with them.

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I second that! Never had a problem either. – Snark Nov 4 '09 at 10:18
same here. wonder if this old wives' tale is leftover from the good ol' 8086/8088/286 days. (at least for monitors.) – quack quixote Nov 4 '09 at 11:16

I'll begin by stating that nik's accepted answer is wrong.

Anecdote: I blew up a monitor last year by unplugging it's signal cable while it was running, so I'd say it was still true. Also: My USB-connected printer (with it's OWN power supply) reboots my PC if plugged in while everything is running. whoops!

Spoiler alert: actual facts/science coming up....

A Ground potential difference between the two devices may occur due to the vagaries of power supply design and where the devices are plugged in. Different outlets are not necessarily on the same ground potential.

So you plug them togheter, current flows places it shouldn't and this may fry electronics at both ends. If it doesn't happen to you you're lucky. Some devices "float" their grounds, some tie their signal grounds to the chassis ( PC's are very bad at doing this, and it causes exciting issues in electrically noisy environments.)

When you turn everything on and its already plugged together, the grounds are tied together and it all runs smoothly. (For short distances, and only most of the time, it breaks in electrically noisy environments) Manufacturers who tie their grounds to mains ground are assuming this will work ( it it works great most of the time....) Our American friends miss out on a ground connection in their standard power socket, some of the time.

Equipment designed to tolerate hot-plugging while running from separate power supplies must include serious ground surge supression on connection, or isolate the devices. Most hot swappable gear allows things like disk drives to plug into a chassis. Thus there is no issue with separate power supplies, or ground equalization surges. The US military's MIL-HBK (handbook) for electrical equipment design suggest that applying 110VAC to any pin of any connector may happen, so to design for it. This is how mil-spec gear ends up really heavy, but if done right, un-killable.

Some standards are really good in the real world. (USB, parallel printer, most RS232, not included) Ethernet uses small (tiny, really) transformers to isolate the data. Security cameras installations regularly use video baluns ( transformers again) to do the same thing. Professional analog audio gear (balanced) uses... transformers. For everything else there is fibre-optic connections like TOS-link.

I hope you enjoyed this trip into the fascinating (and expensive) world of blowing up equipment by plugging it together while powered on.

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@Tim, I agree with your basics on the different electrical grounds and associated problems. The electrical designs seen today are little less reckless. Maybe, one of the reasons that most PC Power Supplies today do not host a monitor supply line (as used to be the case earlier). I have felt static-shocks more often than electrical noise related breakdowns (which probably implies the equipment grounding was quite well maintained to ground me properly). – nik Nov 16 '09 at 1:09

When I still had parallel ports, I killed some of them every once in a while by hot-plugging a printer. So I cannot recommend that.

I never killed a graphics card or monitor, but of course that doesn't mean that it cannot happen.

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