How do I use both HDMI ports?

[Techstyle]

I have had a monitor plugged into 'HDMI 0' for a year or so.  Today I tried to plug a new, second monitor to 'HDMI 1'.  The second monitor showed the "Welcome to raspberry pi desktop" splash screen but would not go beyond then and when I swapped the interfaces it followed 'HDMI 1' not the monitor.  Anybody else had similar issues?

[Techstyle]

after reading some stuff online I uncommented:

Code:

dtoverlay=vc4-fkms-v3d

at the same time, I also uncommented:

Code:

config_hdmi_boost=X

 
where X is whatever the default is, what I did see is some blinking and horizontal lines on the monitor at the helm (furthest away from the Pi) - at the time I had the helm monitor on HDMI0, the helm monitor on HDMI1 and VNC to a laptop.  I didn't get time to play around with it to much but some more research leads me to believe I could play with the boost setting and also use:

Code:

dispmanx_offline=1

which would reduce the refresh rate when the GPU was congested.   

still work in progress

[gniersma]

Hi, 

i'm using the dual hdmi ports rasp pi 4 openplotter

I do have an outside monitor, it is the planar Mariner LX1501PTI / IP65 Toch Monitor. This one is build into a panel into my ship. It has proven to be very usefull so i don't want to lose this one. It is a 15" VGA monitor, with a maximum resolution of 1024 x 768 XGA 6 bit color dept, daylight and fully dimmable and single touch. So I use an VGA to HDMI convertor to connected to the HDMI1 port
Next to this i have an inside HDMI monitor on the navigation table connected tot HDMI0

this treath:  https://forums.raspberrypi.com/viewtopic.php?t=244558  helped me to get it working

Hope this helps.

G.