Whilst building a new home machine I came across this issue where the third screen in my three screen setup was only seen as an "Unknown Display". My machine has an on-board graphics card and a 2 port PCI-E riser graphics card, the monitors connected to the 2 port riser were working correctly, and the OS saw them as the LG Monitors, the third was showing as "Unknown Display". The problem this caused was that the screen's resolution should be 1440x900@60Hz, but the unknown display was only giving me the option of 1024x768@60 or lower. So to fix I used xrandr to set the monitor to a resolution I know it supported as follows:First run cvt to generate a mode, where the 1440 is the Width, 900 the Height and 60 the frequency. # cvt 1440 900 60# 1440x900 59.89 Hz (CVT 1.30MA) hsync: 55.93 kHz; pclk: 106.50 MHz Modeline "1440x900_60.00" 106.50 1440 1528 1672 1904 900 903 909 934 -hsync +vsync Now it is all set you just need to run randr to find out the name of the graphics output you are looking to set to this new mode. # xrandr ... output omitted..... VGA-1-1 connected 1024x768+0+0 (normal left inverted right x axis y axis) 0mm x 0mm 1024x768 60.00* 800x600 60.32 56.25 848x480 60.00 640x480 59.94 ....output omitted.... In my case it was VGA-1-1, so now I have this information I can apply the new mode to the monitor as follows: # xrandr --addmode VGA-1-1 1440x900_60.00 Now if you go into the "Settings" and "Display" and click on the unknown monitor you'll notice a new resolution available from the drop down. Select it and click on "Apply" and hey presto, your monitor should now be showing the correct resolution. The only issue is that the next time you reboot, the changes will be gone. To resolve this create a file called .xprofile in your home directory and add the following contents all on a single line:
Then finally make it executable with: # chmod +x .xprofile https://askubuntu.com/questions/860735/unknown-display-in-ubuntu-16-04 https://askubuntu.com/questions/754231/how-do-i-save-my-new-resolution-setting-with-xrandr https://wiki.archlinux.org/index.php/xrandr#Adding_undetected_resolutions https://www.centos.org/forums/viewtopic.php?t=62575 |
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