HOW TO: Connect Thunderbolt devices for best performances

With the 2013 Mac Pro

The 2013 Mac Pro has six (6) Thunderbolt 2 ports that you can use. Below are some useful tips to know.


Choosing the right Thunderbolt port / BUS

The 2013 Mac Pro has six Thunderbolt ports but "only" 3 busses, which are as follows:


© Apple Inc.


You should connect devices that use a lot of bandwidth on different Thunderbolt busses. Also if you are using the built-in HDMI output, prefer to use busses 1 & 2 as the built-in HDMI output is also on Thunderbolt Bus 0.


Recommended configuration for the M|Family

The M|80, M|62 and M|44 use a lot of bandwidth, so you should connect your M|Family device alone on one BUS, he should not share the bandwidth with another device. See the diagram below of one recommendation.


Of course, this is one possible configuration, and you could connect the M|family device on BUS 1 and your storage on BUS 2, that would make no difference. But avoid BUS 0 that can share the bandwidth with the Display.


With the 2017 iMac Pro

The iMac Pro comes with 4 Thunderbolt ports, and 2 controllers (or BUS) which are mapped as follows:


© Apple Inc.


Recommended configuration for the M|Family

You can plug in your M|Family device to any of the 2 Thunderbolt Controller/BUS. But for best results, you should not connect anything else on the same Thunderbolt BUS as the one where your M|Family device is connected. 


How many channels can go through one Thunderbolt ?

When we receive the video frames from the video card, we receive them as uncompressed. So the available bandwidth in the Thunderbolt is really important as it will tell how many channels of uncompressed video we can receive through one Thunderbolt cable and it depend on a lot of variables. Among which:

  • if using a PCIe card, it will depend on if the card is a PCIe gen 2 or gen 3, and the number of lanes it uses.
  • the device and expansion chassis itself.

So for example with the M|Family we can reach maximum:

  • in Thunderbolt 2 with the Sonnet Echo Express SEL: 1.200MB/s
  • in Thunderbolt 3 with the Sonnet Echo Express SEIII: 2.100MB/s

Knowing this, and how to calculate how much data an uncompressed frame uses, you can make calculations of how many channels of uncompressed video can go through the Thunderbolt. Here are a few examples.

First with the Deltacast cards in the Echo Express SEL (Thunderbolt 2):

  • 1080p25 -> 8bit = 11 channels   /  10bit = 8 channels
  • 1080p30 -> 8bit = 9 channels   /  10bit = 7 channels
  • UHDp25 -> 8bit = 2 channels   /  10bit = 1 channel
  • UHDp30 -> 8bit = 2 channels   /  10bit = 1 channel

And with the Deltacast cards in the Echo Express SEIII (Thunderbolt 3):

  • 1080p25 -> 8bit = 20 channels   /  10bit = 15 channels
  • 1080p30 -> 8bit = 17 channels   /  10bit = 12 channels
  • 1080p50 -> 8bit = 10 channels   /  10bit = 7 channels
  • 1080p60 -> 8bit = 8 channels   /  10bit = 6 channels
  • UHDp25 -> 8bit = 5 channels   /  10bit = 3 channels
  • UHDp30 -> 8bit = 4 channels   /  10bit = 3 channels
  • UHDp50 -> 8bit = 2 channels   /  10bit = 1 channel
  • UHDp60 -> 8bit = 2 channels   /  10bit = 1 channel

 Note that:

  • This only tells how many channels you can "receive" on your Mac using one single device and Thunderbolt cable. You could receive more if you use multiple Thunderbolt devices, connected to different controllers/BUS on your Mac.
  • This does not tell you how many channels you will be able to encode, which depend on your CPU and GPU.
  • Of course you will need the video card that support
  • These are calculated theoretical values, and we advise that you test in your workflow as results can vary.


About daisy-chain

We do not advise that you daisy chain anything behind your M|Family device as it will have to share the bandwidth with other device, and we already use a lot. However if you lack Thunderbolt connectors and really have to, you could try and consider this:

  • Apple recommends that you place the device that ask the most bandwidth first and then "smaller" devices after.
  • In theory the Display Port signal is separated from the PCIe signal, so you could connect a monitor after, but prefer not to use a monitor with too much resolution, so it does not use too much bandwidth still.
  • Also, the input and output bandwidth are separated, so again in theory you could connect a storage device if you only write to it, but not read. (so it does not use the "input" bandwidth that the M|Family needs).


What happens if you reach the limit ?

There will be 2 consequences:

  • In the Activity and Logs window, you will see a log saying: "Video Card buffer underrun". This is because we can not get the frames quick enough from the video card.
  • As we can't retrieve the frames quick enough from the video card, we will duplicate the corresponding video frames and drop audio, so there will be drops in both audio and video in the recorded file.
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