Second monitor - Dell and iMac 27 5 Retina I have an iMac 27" 5K Retina (late 2015) with 32 GB, AMD Radeon R9 M395x 4096MB.   I have just bought a second monitor, Dell Dell 27" UltraSharp UP2715K.   I am running macOS Sierra 10.12.1.   

Can someone help me......   When I connect my iMac to the monitor with the DisplayPort Cable (Mini-DP to DP) the Dell Monitor simply is black screen and on its way to shut go in power save.....   

When I use 1 of the DisplayPort Cables along with DisplayPort Cable (Mini-DP to Mini DP) I only get 4K on my Dell...   

What am I doing wrong? Am I not able to use this second monitor?
Your iMac can support up to two displays at 4096 by 2160 pixels, or one display at 5120 by 2880 pixels.

In other words, to use the second display in addition to the built in display they both need to run at 4K. You can't run them both at 5K.

Your iMac's Thunderbolt digital video output also supports up to two Thunderbolt displays, but these must also be run at 4K.

You can view the full specs here.