Is VGA port hot-pluggable?

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:

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.

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.

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.

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.