VGA Monitor Identification Bits?

Resistors, 75 ohm on each output R, G & B line to ground.

If the monitor is of the newer type (made in say about the last 13 years or so) it will have a pair of serial interface wires in the monitor cable that allow the computer, via the VGA Video BIOS, to access a 128 byte or 256 byte serial eeprom memory in the monitor to determine specific information about the monitor. This memory is known as the EDID and the typical access is via the I2C type protocol through the video adapter board.

The EDID in monitors of this era replace the old two or three "pins to ground" monitor ID bits that were used in the early part of the '90s. In the days after the multi-sync monitors were introduced and we started to see a dizzying array of video formats appear the monitor ID system rapidly became useless and thus the development of the EDID system.

ciscodsl

Impedance change.

On Sat, 17 Nov 2007 14:36:30 -0800 (PST), webpa put finger to keyboard and composed:

If the notebook recognises that a monitor is present but cannot identify its capabilities, then it is probably sensing the 75 ohm RGB terminations. If the notebook can ID the model, make, and refresh rates, then it is reading the monitor's EDID EEPROM. Note that it is the notebook's graphics subsystem which provides the power for the monitor's EEPROM, not the monitor.

See the DDC info here:

The EDID data block will appear in your registry.

I have also managed to read a monitor's EDID EEPROM using my device programmer. I made a VGA DB15-to-DIP8 adapter cable and told the programmer that it was connected to an Atmel AT24C21 EEPROM which has a "DDC1/ DDC2 Interface Compliant for Monitor Identification".

- Franc Zabkar

OK. I guess I was not clear enough: I want my computer to think a monitor is attached to its VGA port when a monitor is not connected. This is necessary because I want to be able to turn the computer's LCD "OFF" for extended periods of time while it is recording streaming audio. (The APM will ***NOT*** do this...if an app is active, the APM will not turn off internal LCD, not matter what). The FN+F7 cycle will not work unless the computer senses a monitor attached to the VGA/ SVGA connector.

BTW...None of the suggestions so far have provided a useful answer.

I'm now gravitating to buying a junk CRT monitor and stuffing the interface card into a plastic bag at the end of a short VGA cable.

Bur surely....

If it were me, I would just make an EDID circuit (as described in some of the previous replies) and attach that to the vga connector. Perhaps some high-end KVM switches also emulate a monitor and provide necessary IDs at the connector.

Regards,

Michael

OK, that sounds kinda fun. I'm going to look into that if my programmer has the ability to program that Eeprom...

Thanks for the info Franc,

- Tim -

On Sun, 18 Nov 2007 15:55:23 -0800 (PST), webpa put finger to keyboard and composed:

Adrian's suggestion should work. There are some PC BIOSes that will fail to POST if they do not detect 75 ohm terminations on the graphics card's RGB outputs. In this case I have managed to fool them by connecting 75 ohm terminations between each of the RGB outputs and their corresponding signal returns.

- Franc Zabkar

Wouldn't any easy solution be to scrap an unneeded monitor and recover the already programmed device from it and wire up a dongle for the vga connector?

Regards,

Michael

It's probably just looking for a 75 ohm load on one, or more, of the RGB output pins. It's unlikely that they designed the laptop so that it would only work with a monitor that had ID pins, or EDID. Many high end monitors use BNC inputs, and there is no ID, or EDID on a VGA to BNC cable. Simply make a terminator dongle out of a VGA connector. Andy Cuffe

snipped-for-privacy@gmail.com

Thanks to all! 100 ohm resistors from R, G, & B to their returns did the job. Computer now thinks there is a monitor attached.