The number of displays you can use with Elgato Thunderbolt™ Dock depends on the power of your computer’s graphics system, the type of displays you’re trying to use, and whether you're using a Mac or PC.
The following is an exhaustive listing of how you can use up to two monitors with Elgato Thunderbolt™ Dock in various configurations.
To check to see if your Mac can support two monitors via Thunderbolt™, see Apple Technote http://support.apple.com/kb/ht5219#18
See the end of this article for a more technical explanation of why such limitations exist.
The Only Monitor is a DisplayPort Monitor
Mac:
You can use one DisplayPort monitor with the Elgato Thunderbolt™ Dock, by connecting it to the Thunderbolt™ port.
This will work as long as no other monitor is connected to the HDMI port - if an HDMI monitor is connected, it will take precedence over the DisplayPort monitor, and the DisplayPort monitor will not work.
PC:
You can use one DisplayPort monitor with Elgato Thunderbolt™ Dock, by connecting it to the Thunderbolt™ port.
This will work as long as no other monitor is connected to the HDMI port - if an HDMI monitor is connected, it will take precedence over the DisplayPort monitor, and the DisplayPort monitor will not work.
The Only Monitor is a HDMI Monitor
Mac:
You can use one HDMI monitor with Elgato Thunderbolt™ Dock.
PC:
You can use one HDMI monitor with Elgato Thunderbolt™ Dock.
The Only Monitor is a Apple Thunderbolt Display
Mac:
You can use one Apple Thunderbolt Display with Elgato Thunderbolt™ Dock.
PC:
You can use one Apple Thunderbolt Display with Elgato Thunderbolt™ Dock.
One HDMI Monitor and One Apple Thunderbolt Display
Mac:
When using a powerful enough Mac, you can connect an HDMI display and an Apple Thunderbolt Display to Elgato Thunderbolt™ Dock at the same time.
PC:
When using a powerful enough PC that has two Thunderbolt™ ports, you can connect an HDMI display and an Apple Thunderbolt Display to Elgato Thunderbolt™ Dock at the same time.
Please Note:
On PC motherboards, only the boards with two Thunderbolt™ ports provide each port with two display signals. In that setup, you would run one Thunderbolt™ cable from your PC to Elgato Thunderbolt™ Dock, but it would be carrying two display signals.
One HDMI Monitor and One DisplayPort Monitor - WILL NOT WORK
Mac:
When using a Mac, you can’t connect an HDMI monitor and DisplayPort monitor to Elgato Thunderbolt™ Dock at the same time. Only one monitor will work.
PC:
When using a PC, you can’t connect an HDMI display and DisplayPort display to Elgato Thunderbolt™ Dock at the same time. Only one monitor will work.
Please Note:
Any video adapter that uses Thunderbolt™ or DisplayPort counts as a DisplayPort monitor for the sake of this article.
Any adapter that uses HDMI counts as a HDMI monitor the sake of this article.
That means you can't use a HDMI monitor (or adapter attached to the HDMI port) and a DisplayPort (or Thunderbolt™) adapter at the same time. Only one monitor connection would work.
For example:
• A DisplayPort (or Thunderbolt™) to DVI adapter can't be used at the same time as a HDMI monitor
• A DisplayPort (or Thunderbolt™) to VGA adapter can't be used at the same time as a HDMI monitor
• A DisplayPort (or Thunderbolt™) to HDMI adapter can't be used at the same time as a HDMI monitor
Other combinations using various standards also won't work.
Simply put, you can't use a DisplayPort and HDMI connection at the same time - only one will work.
Two Displays Via Two Thunderbolt™ Docks
Mac:
You can connect an HDMI display, and then connect the Elgato Thunderbolt™ Dock to another Elgato Thunderbolt™ Dock via a Thunderbolt™ cable.
Then, the second Elgato Thunderbolt™ Dock can connect to a display via Thunderbolt™, HDMI or DisplayPort.
PC:
When using a powerful enough PC that has two Thunderbolt™ ports, you can connect an HDMI display, and then connect Elgato Thunderbolt™ Dock to another Elgato Thunderbolt™ Dock via a Thunderbolt™ cable.
Then, the second Elgato Thunderbolt™ Dock can connect to a display via Thunderbolt™, HDMI or DisplayPort.
Please Note:
On PC motherboards, only the boards with two Thunderbolt™ ports provide each port with two display signals. In that setup, you would run one Thunderbolt™ cable from your PC to Elgato Thunderbolt™ Dock, but it would be carrying two display signals.
This setup won't work on Macs in Bootcamp.
One Thunderbolt™ Dock Can Only Run Two Displays At A Time
Mac:
The Thunderbolt™ signal from a Mac has up to two display signals in it. Therefore, no more than two monitors can be connected to Elgato Thunderbolt™ Dock at the same time.
PC:
The Thunderbolt™ signal from a PC usually only has one display signal in it.
The exception is a PC that has two Thunderbolt™ ports. Only PC motherboards with two Thunderbolt™ ports provide each port with two display signals. In that setup, you would run one Thunderbolt™ cable from your PC to Elgato Thunderbolt™ Dock, but it would be carrying two display signals.
No more than two monitors can be connected to Elgato Thunderbolt™ Dock at the same time.
Exceptional Cases
1) DisplayPort monitors will not function if connected to the Thunderbolt™ port of an Apple Thunderbolt Display.
See Apple Technote http://support.apple.com/kb/ht5219#20
2) Any monitors cascaded after an Apple Thunderbolt Display won't work.
Why Are There Restrictions On How Many Displays Can Be Used?
Thunderbolt™ has PCIe and DisplayPort signals multiplexed together.
There can be up to two DisplayPort signals available on the Thunderbolt™ bus, depending on the Mac or PC.
When the DisplayPort signals come from the computer over Thunderbolt™, and travel into the Elgato Thunderbolt™ Dock, one of them is demultiplexed. That DisplayPort signal is ready to be used.
The demultiplexed DisplayPort signal is translated by the Elgato Thunderbolt™ Dock into HDMI. That leaves one DisplayPort signal still multiplexed in the Thunderbolt™ signal, for later use in the Thunderbolt™ chain.
That second DisplayPort signal can only be demultiplexed by another Thunderbolt™ device, like the Apple Thunderbolt Display, or by another Elgato Thunderbolt™ Dock.
If you connect a DisplayPort monitor to the Thunderbolt™ connection when the HDMI port is in use, the DisplayPort monitor will not light up. In this case, only Thunderbolt™ signaling is available at the Thunderbolt™ connection, which the DisplayPort monitor does not understand.
When you try to connect two non-Thunderbolt™ monitors to the Elgato Thunderbolt™ Dock, you can either get a signal over the HDMI port, or via DisplayPort, but not both at the same time.
If you choose to use a HDMI device, then one multiplexed signal remains, associated with the Thunderbolt™ port, available for Thunderbolt™ devices only.
If you choose DisplayPort (via the Thunderbolt™ port), then the Thunderbolt™ bus is terminated, when it comes to display signals. The DisplayPort monitor will work, but the HDMI port will not.