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In meetings, I often see people detaching the VGA connector from one running laptop and connecting it to another, while the projector is still on.

Is this 100% risk free, and OK by design of the VGA standard? If there's a risk involved in hot-plugging VGA, can it be removed by turning off or suspending either laptop, display, or both?

I see this being done all the time without causing disaster, so clearly I'm not interested in answers stating "we do it all the time, so it should be OK!".

I want to know if there's a risk - real or in theory - that something breaks when doing this.

EDIT: I did an internet search on the topic, and I never found a clear statement as to why it is safe or unsafe to hot swap VGA devices. The typical form is a forum question asking basically the same question as I did, and the following types of statements

  • Yes it's hot swappable! I do it all the time!
  • It involves some kind of risk, so don't do it!
  • You're some kind of moron if you think there's a risk, so just do it!

But no explanation as to why it safe or not...

Joe Taylors answer below contains a link to a forum post and answers that basically give me the same statements as mentioned above. But again, no good explanation why.

So I looked for an actual manual for a projector, and found "Lenovo C500 Projector User’s Guide". It states on page 3-1:

Connecting devices

Computers and video devices can be connected to the projector at the same time. Check the user’s manual of the connecting device to confirm that it has the appropriate output connector.

[image]

Attention: As a safety precaution, disconnect all power to the projector and devices before making connections.

But again, no good explanation.

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That warning is a CYA lawyer-mandated statement, I'd bet. –  music2myear Sep 24 '13 at 16:11

9 Answers 9

up vote 7 down vote accepted

I would think the VGA standard don't say anything about hotswapping. So it is not designed to support hotswapping (like USB or firewire). But it usually works anyway (out of lazyness done so many times myself without problem), but the fact that it 'usually works' doesn't mean it's safe. So the manufacturers of those equipments can't say it is ok because there is no guarantee it is safe.

Also you could by accident get the pins (on the cable connector) onto the grounding (of the card connector) when connecting, might be a none issue but also not. USB -connector is designed to make this impossible.

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Let's look at the signals in a VGA connector:

  • Pins 1, 2, 3: (R, G, B) 0 to 0.7V analog color signals to the monitor
  • Pins 13, 14: (Hsync, Vsync) 0 to 5V TTL synchronization signals to the monitor
  • Pin 9: 5V power to the monitor's I2C EEPROM
  • Pins 12, 15: (SDA,SCL) 0 to 5V I2C clock and data to/from the monitor's EEPROM
  • Pins 11, 12, 4, 15: (ID0,ID1,ID2,ID3) ID pins, obsolete

If we take a look at the schematic of a relatively modern LCD monitor, (LG L1733TR, taken from here), we can see several protection diodes in the input:

enter image description here

Pins 1, 2, 3: R, G, B analog signals

These three signals have protection diodes (D416-D418, in the red rectangle on top) tied to both the 5V supply of the monitor and ground. These diodes are used as TVS (Transient Voltage Suppresion) diodes, and also prevent ESD (electrostatic discharge) from damaging the analog to digital converter of the monitor.

Pins 9, 12, 15 13, 14: 5V supply, Hsync, Vsync TTL signals, I2C signals

These signals have a 5.6V zener diode each that would conduct any excess voltage to ground. (ZD410, ZD411, ZD415, ZD414: red rectangle on bottom; ZD412: red circle )

In addition, there are also resistors in series and pull-down resistors to ground in most of the signals.

On the PC side, I couldn't find any graphics card schematic. However, from my experience, I recall most graphics cards having very similar diodes for the signal lines, and even an SMD resettable fuse for the 5V supply in pin 9 (in case someone pokes at the connector with a paperclip :).

Another issue when hot-plugging is getting the voltage supply and the ground to connect first, followed by the signal pins (that's why USB, SATA and others have staggered pins). The ancient DE-15 connector used in VGA was definitely not designed for hot-plugging. However, the big ground shield of the connector or any of the ground pins is most probably going to make contact before the 5V supply, avoiding having an unwanted return through one of the signal pins.

In conclusion, I would say that although the original VGA standard did not have any provision for hot-plugging, peripherals nowadays have enough protection devices to allow it. A potential problem would occur if a very cheap manufacturer omitted these protections, but I think most devices, laptops and projectors in particular, should have no problem handling it.

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Excellent answer! –  Aaron Miller Sep 24 '13 at 14:56
1  
I will give this an arrow up just for posting schematics :D –  Joe Schmoe Jan 1 at 23:39

A VGA port is 100% "Hot Pluggable".

Here's the same question and the answers to it. All support my statement. http://forums.overclockers.com.au/showthread.php?t=539941

The only problem with continually doing it is the continual wear and tear that you are putting on the pins. If you are careful then this isn't really an issue, but letting a ham fisted 4 year old do it might not be too advisable

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8  
Forums not supporting your statement are easily found too, like our friends at experts-exchange.com/Hardware/Desktops/Q_21945114.html –  Arjan Feb 23 '10 at 17:12
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I can't see the site, what words of wisdom do our friends at EE say? –  Joe Taylor Feb 23 '10 at 18:46
7  
For any EE page: just click the link, then paste the title (with quotes) into Google, and then follow Googles hit -- voila, the accepted answers via google.com/…; But of course, we don't want to go there anyway ;-) Still, things like: "VGA and DVI cables should NOT be hot plugged" and "Is it likely that a ... video card could be ruined ..." ---> VERY!" and "The monitor could also be damaged." All without any proof whatsoever... –  Arjan Feb 24 '10 at 14:37
    
A nice little trick, that gets a +1 from me. –  Joe Taylor Feb 24 '10 at 14:48
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Thanks for your answer. +1 from me, but I have accepted Joakim Elofssons answer. It more clearly states the conclusion that I think has crystallized from this: VGA is hot-pluggable "by experience", but not by specification/design. –  Martin Bøgelund Feb 26 '10 at 10:47

This is going to be somewhat a dance of concepts, but:

I think we can say VGA is hot swap-able for a few reasons:

Windows, and in fact Linux and other operating systems have built in handlers to make hot swapping a monitor effortless, and painless without any rigamarole. As previously noted, when you plug in a monitor or change to a projector from a laptops built in display the Operating System adjust the display. If this were a safety hazard I would expect an error or warning at least once with the thousands of machines I have done this with over the years, with different OSs.

We have hot-swap devices for monitors. Their sole purpose is to hot swap monitors day in and day out. These devices are commonly called KVMs or Keyboard Video Mouse (switches).

Data integrity, this seems to me to be the largest reasons something is not hot-swappable. If disconnecting the device destroys data, then specific steps should be taken. VGA does not need to store data and the OS handles changes just fine.

The BIOS also supports hot swaps just fine.

I have a hard time imaging that all the research, licensing and and production, selling and widespread adoption between software and hardware companies would over look this if part of the spec was not to hot swap.

Finally, and perhaps where it actually gets interesting: We are not actually talking about VGA in its original sense. VGA was the standard 640 * 480 introduced by IBM in 1987. I would not be surprised if hot-swap not even considered at this time. Computers were not moved, projectors were not connected to laptops, Plug and Pray was still nearly a decade away from shipping with Windows 95 (USB sort of started the PnP train).

All that said, leads me to check with VESA's own white papers, and docs. But because I am not a VESA member I have to find their papers and then Google, find an online copy, and read... The Finding?

According to VESA:

Hot Plug Detection (HPD) is the ability of a computer system to detect the disconnection and/or reconnection of an external monitor to the video output port on the computer. HPD can only occur when the computer is on. Not all video interface specifications/standards support HPD. The VGA analog video interface (industry standard) does not support HPD.


Yes, I know that was a long post. But I thought it through and researched it simultaneously.

TL;DR

According to VESA the standard does not support Hot Plug Detection.

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Support for hot plugging and hot plug detection are not necessarily the same thing. –  Michael Kjörling Sep 24 '13 at 13:53
    
@MichaelKjörling I don't necessarily disagree with you, especially from an English / semantics perspective, but from a technical perspective I am not sure you can have hot swap supported without having a method for support the detection of a swap / change. –  AthomSfere Sep 24 '13 at 16:02
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Hot swapping can be done between two identical pieces of equipment. Hot swap detection (at least to me) means you're able to automatically act on the change in some manner. Hot swap detection implies that you are able to do hot swapping, but you can do hot swapping without hot swap detection. (Though of course, the combination makes it more reliable as well as more convenient for the user!) For a very simplistic case, consider headphones hooked to a radio receiver; the radio doesn't need to act in any way on the fact that you changed one pair of headphones for another. –  Michael Kjörling Sep 24 '13 at 18:29

There are two sides to the question : Operating system and hardware.

As regarding the operating system, the answer is that the hot-plugging of VGA is fully supported. For Windows, see for example the article Monitor Hot Plug Detection which describes the hot-plug event.

However, the periodicity of the hot-plug detection event depends on how the monitor is defined for Windows, or in other words on the display driver for the video card.

As regarding hardware, the answer is different. This is because changing to a differing electrical connection can cause electrical surges, and there are lots of reports on the Internet of hot-plugging that caused electrical surges the fried the video card, monitor or even the computer.

In the words of one answer to the post Should I turn off my PC when plug/unplug external display? :

Consult the owner's/operator's manual of the display device. Most likely the manual states that both display and PC should be powered off when connecting them. Obviously this would be the safest and no-risk method.

The capability of connecting devices while one is powered on (aka hot-plugging) is partly determined by the connector design. If the electrical ground connection(s) cannot be established before any power and then signal lines are connected then there would be the chance of damage to line receivers. Inspect a USB device connector and you'll see that the contacts are not all of the same length; note that USB devices are hot-pluggable.

Neither VGA (HD-15) or DVI connectors are designed for making ground before signal, so there could be some risk when connecting powered devices. Just because someone has done it without problems does not mean that you never will. BTW I've seen electricians work on live circuits, i.e. they don't shut power off to replace a outlet; just because it can be done does not mean it should be recommended to others.

In other words, if one is using the right-connectors this will always work. However, if one encounters a bad mismatch between connectors then anything can happen.

Another answer to the same post said :

Years ago, we had a DataTrain monitor that was misbehaving.

In a fit of frustration i unplugged, plugged in, unplugged, plugged in, unplugged, plugged in, ... the monitor cable from the video card.

The monitor never worked again.

Conclusion: The VGA port is mostly hot-pluggable, but bad things can happen if one is unlucky enough. It is mainly a matter of luck, where the dice are loaded on the side of the hot-plug working correctly, but the consequences can be rather grim when the wrong number comes up.

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VGA is not inherently hot swappable. It is not designed to be at all. What VGA is however, is analog. Your monitor isnt watching for some certain voltage on a certain pin like it would be with DVI-D, it is watching for signal. If you have a monitor plugged into a computer, then the monitor gets signal from the graphics card (assuming it's on of course) and happily displays the picture that is encoded between those pins.

There is always the possibility that some sort of electrical charge could discharge through some circuitry if you are "hot swapping" which could ruin a monitor or graphics card/chip. The same possibility also would exist if you were carrying a computer from one place to another on carpet.


TL;DR VGA is not officially hot swappable that I know of, but the OSes have been designed to handle hotswapping well.

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VGA transfers a low-voltage signal, so I think it should be safe. The best answer might be found in research data. Search for cases where hotplugging a VGA connector was the determined cause of damage. Of all the people in the world plugging and unplugging monitors and projectors for the past 20 years, there will be cases of damaged equipment if it is not safe. In my experience, I have never seen a problem.

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Unfortunately the only thing I can say about this is my own personal experience about hot-swapping VGA ports.

I have a desktop computer (so this might be more of an issue here than in laptops, as they have a default monitor) and (almost) every time I unplugged then plugged back the VGA cable, the display would freeze, and so would the computer (although I never got an conclusive proof of that, even more because I had a defective graphics board).

The thing that may prevent this from happening is the fact that laptops have one monitor by default, and that prevents the underlying system from "giving up" sending graphic signals.

I never got a problem by doing this (if you don't count the system freezing) but then again, I don't usually do this.

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easy to say yeah, fine,based on experience, but,

this guy says not a good idea if a different monitor. [1]

"Is it likely that a ... video card could be ruined ..." ---> VERY! Beyond just power issues you have drivers issues. When the computer boots it loads the drivers and sets the frequencies and voltages that the video card sends to the monitor that's attached to it. If you plug a monitor into a video card that didn't have a monitor (or had a different monitor) when the computer booted it's very likely the voltages and frequencies will be wrong for the monitor you are attaching. . The monitor could also be damaged.

[1] http://www.experts-exchange.com/Hardware/Desktops/Q_21945114.html

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Nope, VGA voltages are 0.7 V analog for the color channels and TTL for sync. Frequencies can and do differ, with resolution. That's why Windows has a "If you can't see this, we switch back to the previous resolution in 15 seconds" dialog. Too high a frequency may garble or blank the screen. –  MSalters Apr 29 '13 at 0:14
    
@MSalters another post talks of possibly damaging a VGA card. What he's saying, and I don't see how what you say refutes what he is saying, is that if a monitor -from another computer- is plugged in, then that monitor from the other computer may be using a higher voltage than the card/controller of the computer you are plugging into is currently expecting or able to take, and could damage it. –  barlop Apr 29 '13 at 4:47
    
No, the other monitor is not "using" voltages at all. VGA is driven from the source, i.e. the computer, not the sink, i.e. the monitor. Frankly, that post is even worse than the first one quoted, it shows no understanding of electricity. –  MSalters Apr 29 '13 at 7:49
    
@MSalters OK. Unfortunately we don't have anybody here that really believe this argument that hot-swapping a monitor, a different one, is a problem. Hopefully a good advocate of this view will come along to express and defend that position! My bet is with the side the side you're on, that says it's fine. 'cos my experience is it hasn't been a problem –  barlop Apr 29 '13 at 8:28

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