Dual display iMac setup

[mikeq777]

I have two 27" iMacs and would like to use one of them as an extended display. All of the research I have done, including on the Apple website, says that all iMacs from Late 2009 onwards are capable with this feature. The two iMacs I have are Late 2009 and Mid 2011 models (see below/attached).

I have purchased a mini display adaptor cable and when I connect it between the two computers the Late 2009 iMacs screen goes blank and the Mid 2011 iMac screen momentarily shows the connection (in the display settings) between the two computers. After approximately 5 - 10 seconds the connection drops out.

I have tested both computers individually from my MacBook Pro (Mid 2014) using the Mini Display cable for the Late 2011 iMac and Lightening cable for the Mid 2011 iMac and they work perfectly.

This is very frustrating as all forums and support websites say this should work. Please help.

 

[Ledgem]

Since you didn't mention it, I'm going to ask a "step 1" question: are you just connecting the computers, or are you putting the Late 2009 system into target display mode first? (Since you mention that it worked with another system I'll guess that you're doing it the right way, but just figured I'd check.)

 

[mikeq777]

Thanks for your reply. I'm just connecting the two computers. When I tested it from my MacBook Pro I didn't need to put either computer into display mode first. The accepted shortcut is to hit Command + F2 on the screen you want to use as the extended monitor. Not even sure how you place a screen into 'target display mode' before the connection.

 

[Ledgem]

It seems Apple changed some things from the last time I used the mode - as I recall, it used to be something you had to boot into. The instructions from Apple are here. Looks like you do the connection and then press command+F2 on the Mac you want to have acting as the secondary display.

It's worth a try if you haven't done it, although I can't explain why you'd need to do it (or why it's not working) between two of your computers whereas it's working with the others you've tried.