How do I make my old XP laptop (with VGA port) output a video signal?

[Ben321]

I have an old Windows XP laptop with an external VGA port for using 2 screens (the internal LCD plus an external monitor).  I want to output a signal to an oscilloscope to read out the sync and video signal waveforms, using one of these VGA breakout cables, with 5 BNC connectors on the other end https://www.amazon.com/StarTech-com-6-Feet-Coax-Monitor-Cable/dp/B000067SM6/. Unfortunately though, my laptop's VGA port won't output a signal unless it detects a monitor connected to it. Even after I press the Fn+VGA keys on the laptop keyboard to make it switch to external display, it still doesn't work. It only momentarily shifts to external and then back to internal when no external monitor is detected. Is there a way to FORCE Windows XP to command the VGA onboard hardware in the laptop, to output a signal continuously, even if no external monitor is detected? Maybe a hacked video driver? Or maybe there's an electronic component I could attach to one of the sync or video signal lines, to make it think an external monitor is connected? Any help here would be great.

[bdunham7]

4 of the pins are used to identify the monitor, simply grounding a few of the appropriate ones will probably suffice in that it will tell your laptop that you have a very old standard VGA monitor.  Refer to the Wikipedia article or google "VGA pinout DDC" for some ideas and details. 

[Ben321]

4 of the pins are used to identify the monitor, simply grounding a few of the appropriate ones will probably suffice in that it will tell your laptop that you have a very old standard VGA monitor.  Refer to the Wikipedia article or google "VGA pinout DDC" for some ideas and details.

Well the thing is I don't have access to all the pins. I have access to 5 of them, due to my breakout cable having BNC connectors for R, G, B, VSync, and HSync signals. I would have assumed that this worked with an exotic monitor, which maybe didn't use all 15 pins, and therefore the internals of the cable would have grounded the appropriate pins. Unfortunately this doesn't seem to be the case. Are the pins I need to ground one or more of the sync connectors? I was worried that shorting them to ground might damage the VGA chip in the computer. So I tried a 75-ohm resistor (I think video signals and sync signals expect a 75-ohm load) as a terminator for each of those pins (not all at once, but one at a time), and no effect. I'm kinda worried about shorting any of them to ground directly.

[JDubU]

Would these help?

https://www.amazon.com/dummy-Headless-Display-Emulator-Headless-1920x1080/dp/B075ZMVGQS

https://www.geeks3d.com/20091230/vga-hack-how-to-make-a-vga-dummy-plug/

[bdunham7]

Well the thing is I don't have access to all the pins. I have access to 5 of them, due to my breakout cable having BNC connectors for R, G, B, VSync, and HSync signals.

Methinks you have work to do!  Pins 4, 11, 12 and 15 are involved, not any of the ones connected to your BNC connectors.  If you tear apart your cable (or an extension) and grounding makes you nervous, 75R might be good enough, I don't know for sure. 

[Ben321]

Would these help?

https://www.amazon.com/dummy-Headless-Display-Emulator-Headless-1920x1080/dp/B075ZMVGQS

https://www.geeks3d.com/20091230/vga-hack-how-to-make-a-vga-dummy-plug/

First one would fit in place of my breakout cable, preventing my cable from being able to be connected, because it would take up the only VGA port. The second link looks useful, because it seems to imply that if all 3 color signals (R, G, B) are properly terminated with a 75ohm resistor, it should work. However, I'm not sure if I need to ground those other data pins (unavailable on my breakout cable) or if leaving them disconnected (my only option with my cable) would work. A few BNC T adapters, with 75ohm BNC terminators should work. One leg of the T would have the 75ohm resistor, the other would go to the breakout cable, and the stem of the T connector would become the new BNC end connector, to connect to my oscilloscope.


Or maybe that VGA dummy plug (with all the correct terminating resistors), used in conjunction with a VGA splitter cable like this https://www.amazon.com/Cable-Matters-Splitter-Screen-Duplication/dp/B005H3I38G/ would work.