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Old 12-10-06, 01:17 AM   #1
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Join Date: Aug 2003
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Default A possible reason for a persistent black monitor.

Hiya fellow hackers,

I just posted something about this to another thread -- but I believe it's important enough to deserve it's own thread. So here it goes:

Normally, when you boot your PC, both monitors show the same (ie, the boot messages, grub menu, etc).

If it happened to you that after starting X one (or both) of your monitors, connected to a DVI plug of your video card, went black and remained black after leaving X, then the following might be an interesting observation for you.

Contrary to what I believed at first, a DVI->VGA convertor is NOT a D/A convertor. The DVI plug has pins for both, a digital signal and an analog signal. A DVI->VGA adaptor merely passes the signal from the analog signal pins of the DVI plug on to the pins of the female VGA plug. In other words: a single DVI plug can either send a signal to the digital output, or to the analog output. If you use an analog (VGA) cable, then you won't see anything unless the signal is on the analog output, and visa versa, when you use a DVI cable, then you won't see anything unless the signal is on the digital output of the DVI plug (this last remark might not be true: maybe a dvi cable passes the signal on anyway-- in that case you would only have this problem when you are using a VGA cable and the card is generating a digital signal).

I won't go into the reason a mismatch can happen - but usually it will involve the nvidia drivers not detecting a monitor at all. One way or the other, it will either not send anything to the plug (you can see that in your Xorg.0.log of course), or it can send a signal to the digital output (when it uses a DFP-0 or DFP-1 for that plug), or it can send an analog signal to the analog output (when it uses a CRT-0 or CRT-1 for the DVI plug).

At this point you can already check if this is the case for you: Look in your Xorg.0.log file to see if either CRT-* or DFP-* is used - and check what kind of cable you are using.

Now why does this result in a *permanently* black monitor? Well, if the nvidia driver can't detect a monitor, then nobody can - also not the bios. So, there is no reason to change the state that the video card is in. Once it is in the state of sending signal to the digital output on the DVI plug, it continues to do so when you leave X. Therefore, you continue to have a black monitor!

If it turns out that you suffer from this problem; try using another cable, or tell the nvidia driver that another type of monitor is connected by adding to the Device (or Monitor) Section:

Option "ConnectedMonitor" "CRT"

(or Option "ConnectedMonitor" "DFP")

(you can also be more specific and use CRT-0, DFP-1 etc, see also http://www.nvnews.net/vbulletin/showthread.php?t=82086 for examples (here I needed this because my monitor was not detected and/or detected wrongly as a DFP))

If this helped you, please add a reply to this thread.



Disclaimer: 90% of the above is guess work. In fact, I have an observation that is inconsistent with the above: I have two CRT's connected, one to the DVI plug of my FX6200. Then I start X with just one Monitor (Section) and force it to be on the DVI plug and being a DFP by using: Option "ConnectedMonitor" "DFP-0" (has to be a 0, since I only have one DVI plug).
This results in both monitors to go black: The DVI plug because it thinks a DFP is connected and it generates a digital signal while a CRT is connected, and the VGA plug, because I told the driver that there is only one monitor. However - when then I kill X - one would expect that at least the signal on the VGA plug returns... and it doesn't. BOTH monitors stay black... I have no explanation or even hypothesis for this. If anyone does - please let us know!
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