Can I use target display mode on a Mid 2011 21.5" iMac?

Currently, I have a windows PC with a 27" iMac connected to it. I am using the iMac in target display mode, and it is connected from my DisplayPort on the pc to the thunderbolt port on the mac. It works GREAT! (please don't bother with comments about how stupid this is to waste a perfectly good mac for use as a pc monitor....I really don't care, and it's not relevant to my question)

I also have a 21.5" Mid 2011 iMac that, from everything I can find online, appears to support target display mode. But using the same method, same cables, everything, I can not get it to work like the 27".

Is this possible? How can I know for sure if this can work?