iMac (late-2012) cannot detect DELL U2515H external monitor

Hi there,

I have iMac late 2012. I tried connecting it to my Dell U2515H monitor via thunderbolt-DisplayPort cable. It did not work. Monitor cannot detect signal at all. TechSpec of my iMac tells me it supports 30 inch with max resolution 2560X1560 top.

The monitor works fine connecting to my rMBP 2013. can somebody help.

thanks!