What is the difference between thunderbolt and displayport iMacs when used as monitors?

Solution 1:

The following options will work for you:

• old iMac as a display for MBP, MBA and new iMac. Use a mDP cable.
• new iMac as a display for MBA. Use a TB cable.

Solution 2:

From what I understand, the devices need to have the same connector technology, i.e. a Mini DisplayPort device will talk to another one, and a Thunderbolt device will talk to another one, but you can't cross connector technologies—no Mini DisplayPort on one side, Thunderbolt on the other.

Solution 3:

The matrix is simple - matched icons on the ports is a winning combination.

Video in requires matching cable and port on the sending mac. (If all don't match, it won't work.)

Display mirroring (where an iMac can receive video input) only works thunderbolt to thunderbolt or displayport to displayport

The backwards compatibility between the formats only works when a displayport monitor is connected to a thunderbolt mac.