How does a Video Card detect VGA Monitor is Present?

dekodan · 09-03-2001, 04:26 PM

Hi, I have a little problem.

I have a video card with TV-out. If I plug just a TV (no vga monitor), the card outputs just to the TV right from the boot screen. I don't want it to do this because I have a software tool to initialize the TV-out from within Windows so that I can overscan the picture.

I need the video card to think that there is a regular vga monitor plugged in (although it won't be) so that the boot-up can make it to Windows and run the 3rd party TV-out program that overscans. The video card must not initialize the TV-Out from boot-up so that the 3rd party software can do so.

Bottom line question is:

What does a video card look at on the 15 pin vga connector to know that a monitor is plugged in?

- I know that it is not voltage since my vga monitor can be plugged in without power (power cord out) and video card does not initialize TV-OUT
- I know that it is not the ground pin (pin 10), I tried that.

More basics
- Pins 1,2,3 are RGB
- Pins 6,7,8 are RGB ground
- Pin 5 is DDC return (what is this?)
- Pin 13,14 are Hor and Ver Sync

Can anyone please help. I will need to make a dummy connector that simulates a monitor being plugged in.

Thanks!

Dekodan

mikep56 · 09-03-2001, 07:50 PM

Hi Dekodan,
I read on this board once that the VGA controller senses if a monitor is present by the load on it's output pins. These are the R,G, and B outputs, and the Hor and Ver Sync signals. You can fool the VGA controller into thinking a monitor is connected by wiring up a VGA terminator plug. Get a 15 pin replacement plug for monitors, and connect a 75 ohm resistor between R and R return, G and G return, and B and B return. Then connect a 600 ohm resistor between Hor and Hor return (or signal ground), and Ver and Ver return (or signal ground). This will fool the VGA controller into thinking a monitor is connected.
Good luck.

Mike

Aaron Cake · 09-04-2001, 09:51 AM

Best bet would be to find a VGA termininator. It will properly load the VGA connection and fool the card into thinking there is a monitor there. If you look up the VGA connector spec, you can see how you can make such a terminator with a few resistors (50 ohm I think...not sure).

dekodan · 09-04-2001, 11:13 PM

I checked the resistance across those pins on my VGA monitor. It is in fact 75 ohms across RGB and their return. The Horizontal Sync to ground was 500 ohms and the Vertical Sync to ground was 800 ohms.

I suspect a 600 ohm resistor will do the trick for the latter two.

I will build one of these this week and try it!

Thanks guys!

Dekodan

sebell69 · 10-10-2005, 01:16 AM

could you post a diagram of the terminator you did?

if it worked of course

sebell69

greenman100 · 10-10-2005, 01:27 AM

you do realize this thread is over 4 years old

turbocad6 · 10-10-2005, 01:43 AM

these guys probably have lilliputs buy NOW!!!!

TheLlama · 10-10-2005, 02:26 AM

Quote: Originally Posted by sebell69

could you post a diagram of the terminator you did?

if it worked of course

sebell69

Look at this website for the svga pinout: http://www.interfacebus.com/Design_SVGA_PinOuts.html

The numbering is relative to the male side of the cable (the one you will be immitating.)

Connect 75 Ohm resistors between the following three pairs of pins:
1 and 6
2 and 7
3 and 8

Connect 600 Ohm resistors between the following two pairs of pins:
10 and 13
10 and 14

That should do the trick. I haven't tested it, but it follows their discussion.

sebell69 · 10-15-2005, 01:13 PM

Thank you "thellama" for the responce and information

and greenman it dont matter its 4 years old.... thats the beauty of net forums the stuff stays .......... saves on answering the same question a few million times

thanks guys for the quick responce
sebell69

DeltaFX · 10-16-2005, 04:39 AM

Quote: Originally Posted by sebell69

Thank you "thellama" for the responce and information

and greenman it dont matter its 4 years old.... thats the beauty of net forums the stuff stays .......... saves on answering the same question a few million times

thanks guys for the quick responce
sebell69

If only all newbies could think that way

TheLlama · 10-17-2005, 01:09 PM

Quote: Originally Posted by sebell69

Thank you "thellama" for the responce and information

and greenman it dont matter its 4 years old.... thats the beauty of net forums the stuff stays .......... saves on answering the same question a few million times

thanks guys for the quick responce
sebell69

Yeah, thanks for not splitting this off into another thread. I'm glad I could help. Let me know if it works properly.

TheLlama · 10-22-2005, 05:52 PM

Sebell:
Did it work?

sebell69 · 11-08-2005, 11:39 AM

Going to buy the parts today,

we had this "small" hurricane called Wilma passby us and dump us into a post hurricane era of no electricity, no broadband, no hot water, hell I was making my coffee using a sterno can and a small bowl, camping flashback that was

Anyways I will let you know in a couple of days if it worked or not......... thank again for the help and concern.

Sebell69

TheLlama · 11-08-2005, 06:07 PM

Ugh, sorry to hear that about Hurricane Wilma. I know that the hurricane season has been terrible. I hope you haven't suffered any major losses.

Best of luck!

ttuigvc · 05-28-2009, 12:02 PM

Hey,

I'm a part of the Autonomous Robotics Club at Tennessee Technological University.

We are using an Intel Atom processor on our robot and need to boot "headless."

We found this thread, and tried it out, and IT WORKED!

Here's what we did (with the resistors we had available at the time):

Between each color and it's ground, we put one 100 ohm and two 750 ohm resistors all in parallel to simulate 75 ohms (actually closer to 78.9 ohms).

Between Hsync and SyncGnd we put two 1200ohm resistors in parallel to make a 600ohm.

We did the same thing between Vsync and SyncGnd.

That's it! It works. Best of luck to you. Thanks for the idea!

-TTU ARC

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