iMac goes to black, but laptop doesn't detect external display

I've plugged a display port cable to both my 2011 27'' imac and my laptop (running both linux or yosemite). I run ./tdm_on.sh from the package on the imac (running debian) and the screen goes to black. However, the laptop doesn't detect any external display. 
Any idea?

I tried to boot the imac on osx (high sierra) and run cmd+f2 without any result either. 

many thanks,

Anton