OT: faking a VGA display

Tothwolf tothwolf at concentric.net
Thu Jan 25 12:58:30 CST 2007


On Thu, 25 Jan 2007, Jules Richardson wrote:

> Some of you here might know this :) I've got a system here where I need 
> to make it think that there's a monitor plugged into its VGA port even 
> when there isn't (long story).
>
> Plugging a real CRT into the port even when that CRT is switched off and 
> unplugged from the AC supply works, so there's obviously some way of 
> doing it.
>
> Measuring the CRT above (switched off, unplugged from AC, and unplugged 
> from the device) with respect to the VGA connector's shield gives me the 
> following readings:
>
> pin  sig      value
> 1   R        76ohm
> 2   G        76ohm
> 3   B        76ohm
> 4   NC       GND
> 5   GND      GND
> 6   GND      GND
> 7   GND      GND
> 8   GND      GND
> 9   NC       infinite resistance
> 10   GND      GND
> 11   NC       GND
> 12   DDC DAT  8.1Kohm (initially 7.6Kohm, rose at first then steadied)
> 13   HSYNC    4.6Kohm
> 14   VSYNC    4.7Kohm
> 15   DDC CLK  8.1Kohm (initially 7.6Kohm, rose at first then steadied)

> Any suggestions? Do I just need 76ohm resistors to ground on the RGB lines 
> (and possibly 4.7Kohm resistors to ground on HSYNC and VSYNC)? Or is there 
> likely something more subtle going on that I need to incorporate into my 
> "fake" connector? (Given that VGA supplies no DC out, it can't be anything 
> too complex!)

Pins 11, 12, 4, 15 are the monitor id pins 0-3. Pin 5 is the monitor's 
digital ground pin, 6, 7, 8 are the RGB grounds, respectively, and 10 is 
the sync ground.

Its been years since I did this sort of thing, but I believe I jumpered 
one or more of the id pins to ground to get mine to work. I might have 
used a resistor, but I don't remember now.

Newer monitors and VGA cards use one of the pins for digital communication 
with the monitor, but I don't remember the specifics.

-Toth



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