Raspberry Pi Forums

Posted: **Fri Apr 22, 2016 12:07 pm**

If I connect to my RPi3 via SSH and command a reboot when the HDMI cable is unplugged or if I pull the power line and reinsert it then when the RPi has started up and I reconnect the HDMI monitor cable there is no picture and the monitor goes to sleep.

Questions:
1) Must there be a live monitor connected to the RPi3 when it reboots or power cycles in order for it to send any data to the HDMI?

2) Is there any way to command the output to start on an RPi3 which has booted with no HDMI cable plugged in? I have connections both via SSH (PuTTY) and TightVNC to the RPi3 and both works just fine. It is just the monitor connected to HDMI that remains blank.

Posted: **Fri Apr 22, 2016 12:39 pm**

hdmi_force_hotplug=1
hdmi_drive=2
in the correct /boot/config.txt
or change the display type to hdmi via tvservice

Posted: **Fri Apr 22, 2016 1:13 pm**

RaTTuS wrote:
Code: Select all
hdmi_force_hotplug=1
hdmi_drive=2
in the correct /boot/config.txt
or change the display type to hdmi via tvservice

I had a look at the file /boot/config.txt and it contains a large number of settings, all of which are commented out except the last line:

Code: Select all

# Enable audio (loads snd_bcm2835)
dtparam=audio=on

Of course I do not use audio at all so that makes no sense...
Anyway, I did both of the above changes and now I have video on HDMI if I connect the cable after boot.
But the image is nowhere like the resolution I get when I have the cable present on boot.
It looks like it is 640x480 or similar...
Should be 1920x1200
So I commented the line again:

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hdmi_drive=2

And after reboot I connected HDMI but it is still low-res.

Posted: **Fri Apr 22, 2016 1:19 pm**

https://www.raspberrypi.org/documentati ... fig-txt.md
1980x1200 is a weird size
1920x1200 is possible better

Posted: **Fri Apr 22, 2016 1:21 pm**

The raspberry does not support display hotplugging.
It reads the display data from the moitor on boot and that's it.
If there's no HDMI/DVI monitor, it defaults to video out.
This has been manually overridden with the hdmi_force_hotplug=1
Now it defaults to 640x480 resolution.
You can override this with hdmi_group & hdmi_mode settings.
To find out which mode to use, use the tvservice command.

Regards
Aydan

Posted: **Fri Apr 22, 2016 2:03 pm**

Now I tested this config.txt setting:

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# uncomment if hdmi display is not detected and composite is being output
hdmi_force_hotplug=1
# uncomment to force a specific HDMI mode (this will force VGA)
#hdmi_group=2 means digital monitor interface
hdmi_group=2
#hdmi_mode=71 means 1920x1200  85Hz
hdmi_mode=71

But now there is again no display at all when I connect HDMI cable after reboot.
I have a DELL 24" 1920x1200 monitor and the HDMI cable plugs into the monitor via an HDMI to DVI adapter.

Posted: **Fri Apr 22, 2016 2:53 pm**

Aydan wrote: To find out which mode to use, use the tvservice command.

I tried the command:

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tvservice -m DMT
Group DMT has 2 modes:
           mode 4: 640x480 @ 60Hz 4:3, clock:25MHz progressive
  (prefer) mode 68: 1920x1200 @ 60Hz 16:10, clock:154MHz progressive

So then I changed to 68 and rebooted without the monitor plugged in.
Now it actually does work to plug in the cable afterwards!
Thanks!

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