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When I use my external monitor with my laptop, the external monitor begins to display when the OS starts to load, unlike the internal monitor in the laptop, which begins to display from the beginning, including the OEM Logo and bios.
It is the same when I connect the monitor to my old laptop or my friend's laptop.
It doesn't matter at first, but recently I go an embedded machine, I want to use the external monitor to setup OS, although the monitor detected signal, but do not display anything.
The embed machine is functional, because it worked when I use my friend's monitor.
So how to make a external monitor work as the default monitor of a computer, so it can display when booting up like desktop?
Or there are differences that some monitors can and some can't?

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I think this has to do with the way BIOS reads your displays and may or may not be posible, for instance in my MultiMonitor setup, I have the HDMI output setup as default, and works fine once the OS loads up, but during bootup, the default is always the VGA monitor. You can check if your BIOS settings has any such feature.

Similarly your embedded system needs to be able to display on your external Monitor without OS driver support..

edit: this page seems to tackle the same issue.

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Bios is correct, my new HP and my older T60 thinkpad allows the bios post screen to show on my laptop screen and external monitor (hdmi or vga), once windows starts to load it turns off the laptop display and uses my external which I have set to default in windows. Older bios's do not support this. – Moab Oct 23 '12 at 2:32
One big problem is the embedded machine works with my friend's monitor without any extra setting but do not work with mine... – Sheep Oct 23 '12 at 2:53
both vga? if different display technologies, that might explain it – Karthik T Oct 23 '12 at 4:53

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