2009 imac target display issues

[G1ose]

Hi all,

I have a 2009 27" iMac.
I've connected it to an external 4k Samsung screen. It works mostly fine initially.

I'm facing 2 issues though.
1. After sleep has been activated sometimes the external screen returns but only on half the screen. Its like it tries to cram the full image into the left half of the display
2. It doesn't seem to auto trigger the screen to turn on or off.

My macbook seems to work fine in these circumstances so I was wondering if anyone here can help me with this issue in anyway at all?
Thanks

 

[G1ose]

Hi all,

I have a 2009 27" iMac.
I've connected it to an external 4k Samsung screen. It works mostly fine initially.

I'm facing 2 issues though.
1. After sleep has been activated sometimes the external screen returns but only on half the screen. Its like it tries to cram the full image into the left half of the display
2. It doesn't seem to auto trigger the screen to turn on or off.

My macbook seems to work fine in these circumstances so I was wondering if anyone here can help me with this issue in anyway at all?
Thanks

Bump

 

[G1ose]

Bump

 

[DLary]

I thought target display mode was to allow the iMac to work as a display for another computer, not to use an external display with the iMac

 

[mikas]

Unfortunately the iMacs graphics (max. Radeon HD 4850, VRAM 512kB) are not up to the task of putting out 4K resolutions (3840x2160). Max. output resolution would be 2560x1440.
https://everymac.com/systems/apple/imac/specs/imac-core-i7-2.8-27-inch-aluminum-late-2009-specs.html

Might it be you have used a scaled down resolution with your monitor?

My thought would be, that after sleep/wake, iMacs GPU gets confused by the handsaking with samsung 4K resolution, and tries to put out all it can - maybe it tries 4K at 30Hz, or maybe something else, while the display expects 4K at 60Hz which would be the standard/normal digital signal for it. And then it fails with the picture like you describe.

A fullHD monitor/tv would work better with older Macs, and in this case WQHD (2560x1440) would fine work too.

ps. DLary is right about Target Display Mode; Use your iMac as a display with Target Display Mode

 

[G1ose]

Thanks both for the clarification.

So would lowering the output to the 4k monitor fix this do you think or do you think its just not a compatible screen setup? When it work it looks amazing.

 

[mikas]

You could try that and probably it works ok, but it can be flaky at times still.
Make sure your cable is specced as 4K compatible if you continue to use 4K after all. The cable needs to handle the bandwidth too for everything to work.