Dear fellow Mac disassemblers,
Encouraged by posts of
MichaelDT and
GMacUser I decided to have a try and run GeForce 680M 4GB MXM 3.0b (I got one used for ~$270) on my own iMac 21.5" 2011.
I managed to get it working in OSX, but the overall results are mediocre - far from perfect. So I wish to put some discouraging words here to spare you of some time, nerves and money.
First of all my intention was to get the better GPU working in OSX, not Windows (via BootCamp). So well working card in Windows was not satisfactory. I didn't want to turn my iMac into some expensive Windows PC, didn't want to fiddle with BIOS modifications, custom fan control, etc. etc. The GPU boost on OSX was the ultimate goal.
I do confirm several facts raised by my predecessors:
- built-in display does not turn in boot menu, well there is a boot screen, accepting keyboard and so, so you can work blind, but no display
- built-in display turns only upon first boot after resetting NVRAM (Cmd+Opt+P+R) once OSX loads up (what GMacUser calls working out of the box in OSX)
- once you change something or reboot often built-in display does not turn anymore in OSX (back to point 2)
- but external Thunderbolt/DisplayPort attached display always works (yet I want to use built-in display!)
- sometimes iMac refuses to start at all, like fans spin for fraction of a second then stop (reported by some other users). I suspect there may be some initial power strain or timing problem upon boot that prevents computer from booting. After few attempts of pressing power button it stands up.
Now there are also several things that were not clear or not expressed here yet:
- even when build-in display turns on I get absolutely no possibility to control its brightness, screen stays in its full brightness, which is unacceptable, in System Preferences LCD brightness controls are GONE (but they are back when I switch back to old AMD GPU)
- card has really poor performance with its default BIOS (as GMacUser said it seems to throttle/clockdown all the time). I don't want to mod BIOS as it can likely lead to brick the card completely. Maybe it can work at full speed in Windows, but again I wanted it working well in OSX!
Now several other things that differ from what GMacUser said:
- installing card will NOT fit over HDD, when HDD is in its original position, first you need to remove plastic separator, then take out bottom of HDD from its pin holders and push it towards back of iMac making room for a card, but this is NOT yet sufficient. You need to bend bottom right HDD holder (see attached photo) otherwise it will prevent card and whole MoBo to sit in its original position and put some extra tension that can damage it in long term.
- heat-sink attachment on GPU chip itself works well, but not for GPU memory chips, that may lead to memory overheat. Note that AMD card has memory only on one (GPU sink) side that make contact with heat-sink using white thermal pads, where NVIDIA GPU has memory chips on both sides of the card, so the half of chips get nowhere to transfer their heat, and some of the memory chips on "right" side are far from the heat-sink to get a contact too
- even clocked down, NVIDIA GPU emits much more heat than AMD and I believe iMac 21.5" inch internal layout is not good enough to handle that, unless you want ODD fan running at 4000 rpm
- when you use SSD lying below ODD (like me) it will block partially air conduct from ODD fan to GPU fan
Altogether I've spent 2 days and had some good and bad moments with this. My conclusions are that GeForce 680M MXM in iMac 21.5" 2011 is barely working in OSX.
This may be partially addressed by some BIOS modifications, but I don't see how we can address inability to control screen brightness, as there is no Mac BIOS for these particular model.
Comments welcome.
