# VGA Ground Connections

Discussion in 'Beginner Electronics' started by =Ray=, Oct 20, 2005.

1. ### =Ray=Guest

On the VGA pinout, I see that there are five grounds: one for R,G,B, Sync
and a generic ground. Is there any real difference between these, and can
I safely combine them?

Thanks,
=Ray=

2. ### DBLEXPOSUREGuest

They are all tied to chassis at the video out. Electrically the same point.

3. ### AlexanderGuest

That's not always the case.
The generic ground is the one which makes a connection between the case and
another case so both equipmrnts have the same (earth) potential.
The other grounds are specific for each connection. And are connected to the
ground of the input buffer/amplifier.
Most of the time you can connect them together. But as you know with
transmission lines an HF signals, there can be a difference in potential on
the same line.
For this (and some other reasons) the ground may not always be the same.

Alexander

4. ### NoozerGuest

I would say no...

Generic ground is the chassis ground to ensure that the monitor and PC
chassis are at the same ground potential.

The other grounds are relative to the signals to pair with...

An exaggerated example:

The red output could go from 5v to 12v and ground to chassis could read 4v
to give a good 1v to 8v range to the signal.
The blue output could go from 2v to 9v and ground to chassis could read 1v
to give a good 1v to 8v range to the signal.
The green output could go from 8v to 15v and ground to chassis could read 7v
to give a good 1v to 8v range to the signal.
The sync output could go from -3v to 4v and ground to chassis could read -4v
to give a good 1v to 8v range to the signal.

Or for an even more abstract example:

You're driving your car at 41km/h. The car in front of you is going 40km/h.
You hit that car. How much force was in the impact? 1km/h!
You're driving 101 km/h. The car in front of you is going 100km/h. You hit
that car... How much was the impact? 1km/h!
Now your driving that same 101km/h and hit the car going 40km/h. What's the
force here? 61km/h!

Tying grounds together could lead to a bigger voltage differential than was
originally designed for.

5. ### DBLEXPOSUREGuest

Well, just checked 15 different monitors while on my smoke break,
NEC,SONY,KDS,MISUBISHI, and a few others. All signal grounds on the VGA
where common to chassis....

6. ### AlexanderGuest

I never said that it never was the same, I just pointed at the possibility
that according to the standard it doesn't need to be.

Also a good explanation why is given by Noozer.

7. ### DBLEXPOSUREGuest

With most monitors using a common ground approach, wouldn't designing a
video card that uses different signal grounds for each channel be opening up
the door to problems such as ground loop noise or hum in your video? In the
component video world,(broadcast,consumer video) black is .75V (or
7.5%)Red,Blue & Green if these all came from channel paths with different
reference grounds, seems to me you would have to have extra circuitry to
compensate or you would end up with a strange color cast in your blacks.

Radio gear with different signal and chassis grounds is confusing enough
(even though done for good reason) having a devise with four different
reference points would be difficult at best to have to repair or
troubleshoot via signal tracing methods. Engineers hardly ever think about
the poor guy who has to fix there creations so I suppose a device like this
does exist somewhere.

8. ### Rich GriseGuest

Well, yes and no. ;-) (sorry, couldn't resist) - the grounds on the R, G,
and B lines are the shield of a coax, so shouldn't connect to anything,
other than the shield of the coax inside the monitor. The sync ground is,
of course, the return path ("signal ground") for the sync signal, and the
generic ground is, well, a ganeric ground. Those two you could probably
"safely" tie together, but I don't know if it would affect the signal
quality.

What are you trying to do anyway?

Thanks,
Rich