Davinci Resolve, I Mac and External Monitor HP

I have problems trying to connect an external monitor for Davinci Resolve 12. I work in iMac 2104 (Yosemite). I have Ultra Studio Mini Monitor and I have had connected some TV and everything was fine. Now I want to connect HP monitor (1980x1920) and I can't. If I connect the monitor directly to the computer, I see it and I can arrange it. But when I connect it through the Mini Monitor, there appears a note on the monitor that I have to change the mode to 1920x1200. But now the monitor is not recognizable by the computer. Do I have to change the monitor because of some incompatibility of what to do?!

Yep, monitoring through a video I/O device needs a video monitor. Your HP is not 1920 x 1080 (or anything that exists as video format).

Monitor should work with your Mac, 1920x1200 is not an issues. When you connect it to your Mac over DislayPort it should be detected at its native resolution and work fine. If you want to use it for Resolve as full screen then don't worry about 1200 height, your video will be still 1:1 pixels, you will just see black bars at the top and bottom.

Andrew, he wants to monitor via a video I/O device, as it is recommended for Resolve.

Sure it'll work when connected to the computer directly.

Yes, it works when I switch it directly to the computer but not via Mini Studio.

I think I found the reason of the problem - the incompatibility is because of the different Hz of the monitor and the video out. The monitor works only on 60 Hz. When I changed it with other 1920-1200 monitor who works on 50 and 60 Hz, everything went better.

If you are connecting to mini monitor then as you found out it may not work with some formats. 50Hz may not work, interlaced may not work. Only higher-end PC monitors support custom refresh rates (except 60Hz) as ell as have better support for resolutions like 1920x1080, e.g. 1:1 mode (and built in deinterlacer). You need to check this before buying PC monitor.