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I have an iMac 27" with two thunderbolt ports, and a PC with DisplayPort output. In order to use the iMac as a monitor, it appears that a thunderbolt display output at the source is required (https://www.tekrevue.com/tip/ins-outs-imacs-target-display-mode/), and using a DP-mDP cable did not work (Target DisplayMode on the mac using cmd-F2 doesn't respond).
The question is, is there anything I can do on the PC side (a new graphics card, perhaps? or some sort of a converter) to make this happen?