Mini DisplayPort and DVI not working simultaneously on power up

[eyeangle]

Mac Pro 5,1
I have an Asus 27" connected via Mini DisplayPort (mini displayport to full displayport cable). And a 21" ViewSonic connected via Dual Link DVI. The graphics card is ATI Radeon HD 5870, 1024 MB. The main monitor I usually use is the Asus which I usually run from Dual Link DVI but I got the mini display port cable yesterday so I want to set up dual monitors.

- If both screens are connected at the same time and I power on the computer neither screen turns on.
- If I just connect the display port (Asus monitor) it doesn't turn on.
- If I just connect the DL-DVI (ViewSonic monitor) it turns on.
- Once I'm in Mavericks, if I then connect the Mini DisplayPort to the Asus then both monitors work.
- When both monitors are working in Mavericks and I restart then both will stay working. It's only when I shut down the computer or put it to sleep that they both stop working.

I called Apple and tried different combinations for about an hour trying to explain the problem over the phone. They've decided to send a techy out here to replace the Logic board or the graphics card. But I don't think the logic board is the problem because I had it replaced last week for a different issue so it's brand new.

Anyone got any ideas?

 

[jenzjen]

Mac Pro 5,1
I have an Asus 27" connected via Mini DisplayPort (mini displayport to full displayport cable). And a 21" ViewSonic connected via Dual Link DVI. The graphics card is ATI Radeon HD 5870, 1024 MB. The main monitor I usually use is the Asus which I usually run from Dual Link DVI but I got the mini display port cable yesterday so I want to set up dual monitors.

- If both screens are connected at the same time and I power on the computer neither screen turns on.
- If I just connect the display port (Asus monitor) it doesn't turn on.
- If I just connect the DL-DVI (ViewSonic monitor) it turns on.
- Once I'm in Mavericks, if I then connect the Mini DisplayPort to the Asus then both monitors work.
- When both monitors are working in Mavericks and I restart then both will stay working. It's only when I shut down the computer or put it to sleep that they both stop working.

I called Apple and tried different combinations for about an hour trying to explain the problem over the phone. They've decided to send a techy out here to replace the Logic board or the graphics card. But I don't think the logic board is the problem because I had it replaced last week for a different issue so it's brand new.

Anyone got any ideas?

Have you reset both the PRAM and SMC lately?

 

[eyeangle]

Yes I've reset the PRAM and SMC.

I contacted apple and they sent out a technician to have a look, after replacing the logic board (now twice in a week for two different reasons) and replacing the graphics card the problem remains. The techy said it's probably 2 things:
1) the graphics card doesn't have enough power to boot both displays on boot-up so it's shuts itself down and doesn't display either of them. (but this doesn't explain why the Asus monitor doesn't boot when only it is attached)
2) the 2m MiniDisplay Port cable is too long and anything over 1m wont work.

Apple are contacting me tomorrow about it so I'll keep you updated.

 

[ActionableMango]

Wow, those two answers don't make any sense to me.

 

[grey17]

Try a different cable. I have four mini displayport to displayport cable & adapters. Two of the cables behave oddly with unpredictable results while the other two work fine. The cable and cable adapter that work reliably are both from NEC. I have an HP monitor and an NEC monitor connected via mini displayport to displayport on an Apple HD 5770.


-- D

 

[Old-Gold]

Exact same problem as described in the OP. My cable (mini to dp) was purchased from Amazon. I will try another manufacturer.