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I own the official 7" touch display for the Raspberry Pi. I also own a HDMI monitor and I wonder what happens when both are connected to the Pi.

I am using Raspbian Jessie and the Pi 2B, if that matters.

  • Is there a command we can run to swap displays without rebooting? Maybe an alias that could modify the /boot/config.txt and add / remove the display_default_lcd=0 and reload a profile so you can swap between the touch and the HDMI screen at will, without a reboot? – Mitchell Tuckness Aug 31 '16 at 20:07
  • Hmm, if you change something on the boot partition, I guess it needs a reboot. Unfortunately I'm not an expert in this area and what I gave as an answer almost exceeds my knowledge about displays on Raspbian. Please delete this question, since it's not an answer to the original problem. It was meant as a comment but yeah, I know, you can't comment yet. – Thomas Weller Aug 31 '16 at 20:38
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With the following setup:

  • official 7" touch display connected via the internal connector (not USB)
  • a 1280x1024 VGA monitor connected via an HDMI-VGA adapter cable

the following happens:

  • the Raspberry boots
  • the 7" touch display is the main display
  • the VGA monitor does not go to sleep
  • the VGA monitor remains black

I have then tried the following to check if the HDMI monitor can display something:

  • sudo apt-get install fbi to install the frame buffer image viewer
  • ls /dev/fb* but that gives only one frame buffer device, which is /dev/fb0
  • fbi -T 0 appears on the 7" touch display
  • fbi -T 1 also appears on the 7" touch display

I then checked whether the VGA via HDMI monitor will be recognized as a frame buffer device in order to exclude the possibility that an HDMI monitor is never recognized as a valid FBI device:

  • shutdown
  • disconnect the 7" touch display
  • re-power the Pi

The result is the following:

  • the Raspberry boots
  • the HDMI monitor is the main display
  • the 7" touch display is black
  • ls /dev/fb* but that gives only one frame buffer device, which is /dev/fb0
  • fbi -T 0 appears on the HDMI monitor
  • fbi -T 1 gives "the operation is not allowed"

Conclusion

With minimum of effort (as described before),

  • you can technically connect the 7" and the HDMI at the same time without breaking something (given a strong enough power supply, the one used for testing has 3.0A and I had WLAN, mouse and keyboard connected)
  • you can't use both displays at the same time

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