I have a graphics card for a laptop whose driver kills itself if it runs for longer than two minutes with no external monitor attached. To solve that, I decided to make a sort of dummy plug that would be detected as a monitor by PC.

I have managed to find out what impedances video card expects for most of the pins, but for H-Sync and V-Sync, Google brought up numerous conflicting answers, mostly from people who are guessing.

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    \$\begingroup\$ Are you sure that it is the lack of a load on the signals, and not a lack of response on the serial monitor ID line, which causes the shutdown? \$\endgroup\$ – Chris Stratton Mar 19 '13 at 18:19
  • \$\begingroup\$ @Chris Stratton I'm not. I know that some cards do detect monitors by loading of the signals and monitor ID bits. Anyway, resistors are cheap and if they don't work, I'll probably just add a small microcontroller to provide the EDID. \$\endgroup\$ – AndrejaKo Mar 19 '13 at 18:24

According to the VESA Plug and Display standard, which uses compatible electrical signaling for analog video, the host is supposed to detect a minimally-capable monitor by looking for 75 Ohm termination on the video lines. Anything beyond basic functionality is determined by reading EDID information over the I2C lines.

Because EDID is so old, the video card may not even bother checking for a monitor without it. You could probably fool the video card into believing a monitor with any desired capability is present with an appropriately programmed I2C EEPROM or microcontroller.

  • \$\begingroup\$ I'll accept this answer because of link to VESA standard. The standard says that the Synchronization Signal lines should have 2 kΩ to +5 V DC. \$\endgroup\$ – AndrejaKo Mar 19 '13 at 22:25

Why isn't more likely to be something like bad capacitors on the inverter for the LCD display? For external monitors this is a very common symptom - I have a monitor on the bench that pretty much runs for a minute or two then the backlight goes black (but you can still the LCD pixels being driven if you look hard)

  • \$\begingroup\$ According to the question, it is not the monitor, but the graphics source that fails. And does so in a controlled way. \$\endgroup\$ – Ben Voigt Mar 19 '13 at 21:11
  • \$\begingroup\$ It is indeed the graphics card that is problematic. As far as I can see, it tries to enter some sort of power saving mode which results in BSOD. \$\endgroup\$ – AndrejaKo Mar 19 '13 at 22:21

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