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I'm using an Elgato Thunderbolt 2 Dock which has a second Thunderbolt port and a seperate HDMI port. I have an HDMI display connected to the HDMI port and a LaCie HDD connected to the second thunderbolt port.

On that LaCie HDD there is an LG ULTRAWIDE connected via MiniDisplayPort to DisplayPort.

If I only plug in the HDMI device it works. If I only connect the DisplayPort-Device to the LaCie HDD it works. If I connect both displays only the one connected first will work but whenever I connect one of the displays the MacBook goes black for a moment and comes back with only one monitor recognized.

The Elgato homepage states that there needs to be another Thunderbolt device between the dock and the DisplayPort Monitor but my LaCie IS that device.

Why don't I get a signal on the second monitor?

MacBook Pro Mid-2012
OS X 10.9.5

Connection Diagram:
MBP => Elgato Dock (with HDMI connected) => LaCie HDD => DisplayPort Monitor

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This is a really interesting question that gets to the heart of how display signals travel over Thunderbolt.

Your mac is capable of driving two "Thunderbolt displays". The special thing about the thunderbolt display is that it is capable of "pulling" the display signal out of the thunderbolt signal. This specifically is called "demuxing" (de-multiplexing). So technically, thunderbolt is capable of carrying two multiplexed display signals.

In contrast, when you use a displayport adapter on a thunderbolt chain, the bus driving that port from your mac is only capable of sending that one display signal since it is not being multiplexed on the wire.

The Elgato dock (or OWC dock) is capable of doing the same thing as a thunderbolt display, in that it is demuxing a display signal from the thunderbolt wire to provide an HDMI port. If you are using the HDMI port, you can't plug a displayport display into the other end of the thunderbolt chain -- there is no component to demux the other display signal. You COULD use a thunderbolt display in this position.

But, you have one other option! If you buy a SECOND Elgato device and add it to the thunderbolt chain, you now have a component to demux a second display signal to another HDMI port. Then you can use the two HDMI ports to drive whatever displays you want!

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The Elgato only supports two devices (one in HDMI, second in Thunderbolt socket) if the second is a Thunderbolt monitor (i.e. a not displayport monitor)

  • Martin, does that mean you cannot do a dual display using the elgato and my Mac Air? – user159512 Nov 30 '15 at 3:22
  • @Joel I just added an answer that clears this up. – NReilingh Nov 30 '15 at 3:51

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