Interestingly, I think that the external display port which does work is currently set as the Master Display as the login screen when booting displays on the secondary monitor. I then have to change it with the software on the Mac to be the second screen, whereas usually the iMac display as always defaulted as master. Does this mean that the external port that works (and is real DP) is the first display port listed in the DCB table that you sent me?
I actually never verified if the order of DCB entries equals the order of priority in OS X, but that might be true. I don't think that the first DP in the DCB table is necessarily your working external port, it might also be an port #2, while the internal one could be port #3 or #4 (just theoretically, might be different).
When you talk about 'Active' Displayport is this TMDS? So if i change all of the connectors to TMDS?
I do believe that for the external connections on the iMac, they are using active adapters after Internet Research. The 2011 iMac allows the only a thunderbolt (not Displayport) cable to also be used as an input for a MacBook etc.
I think you're mixing some things up. "
Active DP adapters" are special adapters, which work as real DisplayPort device (they're needed for resolutions beyond 1080p or for multi-screen setups on AMD GPUs). Passive DP adapters are the usual ones which e.g. Apple is selling; they just passively pass through a TMDS signal which is generated by a "real" DP port as soon as it recognizes a passive adapter (instead of an active adapter or an real DP device).
And yes, the DP connectors somehow have to be routed through an Thunderbolt controller, but I guess (well, I hope) we won't have to care about that...
With the DCB, for example with Connector 3 (D6 5F 01 08 20 00 42 0F) which is type 0x6 (DisplayPort) which is supposed to be enabled, right, it has OF at the end which i thought meant it was disabled? Sorry if I sound stupid, I have got rather confused !! Also the ones listed in bold underneath the chart, are these the display port ones or the disabled ones or DisplayPort? I couldn't find those hash's listed underneath each connector in the chart!
Sorry, I should have explained that: The byte order is reversed, so the connector type is here: D
6 5F 01 08 20 00 42 0F
So you just have to touch the 2nd half of the first byte, nothing else.