Can an imac 11,2 be used as external display?

Hello,

Target Display Mode with an iMac

In our 2011 iMac review I mentioned that you could use the new iMac as an external display if, and only if, you had a Thunderbolt cable connecting it to a 2011 MacBook Pro. When I reviewed the iMac, you couldn't buy a Thunderbolt cable. Now you can.

The process is pretty simple. Just connect any 2011 MacBook Pro to any 2011 iMac via a Thunderbolt cable. Hit Cmd + F2 on the iMac's keyboard and boom, your brand new iMac is now a secondary display for your MacBook Pro. Note that you need a modern Apple keyboard for this to work. Older Apple keyboards and some non-Apple keyboards won't send the right key code and will refuse to activate target display mode on the iMac.

http://www.anandtech.com/show/4489/promise-pegasus-r6-mac-thunderbolt-review/8

Trouble is, it doesn't mention 21.5", & you don't mention which year 21.5" yours night be.

http://www.tuaw.com/2011/05/03/target-display-mode-missing-in-action-on-new-imac s/

http://reviews.cnet.com/8301-13727_7-20060517-263.html

Only a guess here, but I doubt HDMI will work at all.

Might give this a try...

ScreenRecycler...

http://www.screenrecycler.com/ScreenRecycler.html