Thunderbolt 2 Dell M3800 with iMac 2014 in TDM

Hi,

I have just received new M3800 Dell Workstation equipped with Thunderbolt 2 port. I want to use it with my iMac 27" 2014 that should serve as external display.

I've been struggling for some hours now to do so, but no success. PC's Thunderbolt Software states that it is connected to iMac. In iMac's properties under Network,Thunderbolt Bridge I can see that port 1 Thunderbolt is connected. After PC reboot I got the same state. iMac is not entering Target Display Mode automatically as well as after hitting Command - F2. Tried combination with Fn as well.

Does it sound familiar for any of you?