Q: Thunderbolt Target Display Mode doesn't work (El Capitan)

Hello,

I have two iMacs (iMac (21.5 - inch, Mid 2011) and (iMac (27 -inch, Mid 2011). I was using the Thunderbolt to connect in Target Display Mode without any problem with a MF639ZM cable(Apple Black Thunderbolt Cable, 2M).

I actualized both of the iMac to OS X El Capitan, version 10.11.5 and... surprise!! they doesn't connect any more.

I already rester the PRAM and SMC several times in both of the iMacs. The APP Store Update is not serving any actualization, I already installed all of they.

In my iMac 21,5" the System Information has in the Thunderbolt Bus a Firmware 25.1, Port Micro Firmware Version 2.1.3. Status No Device connected.

In my iMac 27" the System information has in the Thunderbolt Bus a Firmware 22.1, Port Micro Firmware 2.1.3. Status No Device connected.

If I connect the Thunderbolt cable the Link Status in the System Information change from Link Status 0x7 to Link Status 0x1/0x2 in both of the iMac.

If I Disconnect the Thunderbolt cable the Link status in the System information change to Link Status 0x7 in both of the iMac.

In the Console, if I connect or disconnect the Thunderbolt cable I can see:

IOThunderboltSwitch<0>(0x0)::listenerCallback - Thunderbolt HPD packet for route = 0x0 port = 2 unplug = 1

IOThunderboltSwitch<0>(0x0)::listenerCallback - Thunderbolt HPD packet for route = 0x0 port = 2 unplug = 0

So, It look like the iMacs Thunderbolt ports are working and detecting the Cable connection.

But... Command + F2 doesn't switch to Target Display mode!!!  (Use all F1, F2. etc, keys as standard function Key.. is disable under keyboard).

I will appreciate any help or support to solve this nightmare.

Best Regards.