6 Mini DisplayPort to Dual-Link DVI Adapter on a Mac Pro
I have a Mac Pro (2013) connected to 6 Apple Mini DisplayPort to Dual-Link DVI Adapter (Apple original). According to Apple spec this is supported. But for my setup it works only every other reboot.
The problem is that i often get only 5 displays after rebooting and it's consistently the one on Port 4 (Channel 2). If i go and unplug the port and plug it back in it will get detected again until next reboot.
Since i'm in a lucky position to have more than those 6 Adapters i even swapped around to rule out faulty adaptors.
Then i tried to reducing the displays to 4 but even then i got only 3 displays occasionally. This time not consistently on port 4 though.
When in next testing stage i reconnected the missing 2 displays using standard passive Mini DisplayPort to DVI Adaptors (Apple Brand). I had to make sure that one of these cheapskates is in Port 4 and everything works as required. Even using 5 active and one passive in Port 4 worked.
I took the time to reduce to 3 active adaptors and again i lost one display on every other reboot. It was seemingly always the last one in a chain. Swapping around with keeping the same ports used and the problem stuck to that port. Avoiding that port on the next round of tests moved the problem to another port. I couldn't write up anymore which port-precedence i found out because it was confusing me sometimes but one thing is clear: Port 4 is the "last" one in the chain.
Out of curiosity i plugged in a Mini DisplayPort to VGA (Yes that old analogue) into Port 4 with no screen attached !!! This must have had an effect on the detection chain as somehow the internal chips of the adaptor create a link even though no display is connected to VGA. I could connect 5 full featured active Adaptors and none failed during numerous reboots.
As interesting as all of these findings are i want to know why? I've got my solution (5 active Dual-Link DVI + 1 standard DVI on Port 4) but it cost me valuable time. I wanted to follow Apple guidelines.
Oh and for the major OS version question i feel coming up: My setup required 10.10.5 and is currently running that way. But my tests were made with a virgin 10.10.4 installation and later rechecked with a 10.11 and i found no dependency to OS version.