Monitor only recognises the signal when started first

Question

My monitor, an old Toshiba 47wlg66, will display what my boards output (whether it's my old Raspberry Pi 1 A or new Raspberry Pi 4 B) only if it was powered first. Otherwise it will remain black..

It is troublesome because I power the 2 at the same time, and the Pi's must be quicker to send their first signal because in that case the screen remains black and I need to power off and on the Pi for the monitor to recognize the signal.

I usually use dietpi, but have seen this behaviour with Raspbian as well. Is there a way to fix that, or at least some kind of signal I could send from a script that would tell the monitor to start listening as it does whilst booting?

Answer 1

Bullseye update There is now an option in raspi-config Display Options to set VNC Resolution for headless operation.

The method below prevents other config.txt options from working.

---

You need to force HDMI output in /boot/config.txt (earlier models defaulted to composite, the Pi4 does nothing).

    # uncomment if hdmi display is not detected and composite is being output
    hdmi_force_hotplug=1

This requires a default resolution to be set in raspi-config (at least on the Pi4 - earlier models defaulted to VGA resolution).

It is also possible to set resolution manually, but in raspi-config it is easier to select a resolution supported by your monitor.

Mine is (this selects a different resolution if the wide screen Acer is detected) -

#DVI DMT (35) RGB full 5:4, 1280x1024 @ 60.00Hz
hdmi_group=2
hdmi_mode=35
[EDID=ACR-Acer_KA210HQ]
#DVI DMT (82) RGB full 16:9, 1920x1080 @ 60.00Hz
hdmi_group=2
hdmi_mode=82
[all]

NOTE don't just blindly copy the values above - set defaults which are supported by your monitor, so it will work if the monitor is connected.

Answer 2

Is it because the Pi is defaulting to a low resolution? You can try setting the default resolution (within the configuration) to match the resolution of your monitor. I forget what the default resolution is but it is much less than monitors typically are - it's probably set to target the small touchscreens.