Sam here… Before we get to the Thunderbolt busses on the New Mac Pro, here are a few things you should probably know about Thunderbolt and modern macs:

• Macs that are still Thunderbolt 1 only: Mac Mini, iMac, Macbook Air
• Thunderbolt 2 supported Macs: Macbook Pro, Mac Pro
• You can connect up to 6 Thunderbolt devices on a single Thunderbolt bus.  On a Mac Pro, you can do up to 36 Thunderbolt devices.  Note: this is very device-dependent.  In real-world testing, depending on what kinds of devices you’re trying to hook up, you may  only get up to 2-3 per bus before you start having problems.
• The Mac Pro is the only Mac that has more than one Thunderbolt Bus (it has 3, actually).  This means that even if you have more than one thunderbolt port, it doesn’t mean that you’re necessarily getting the full 10 or 20 GB/s to your device from each port.  It’s all pulling from the Thunderbolt bus that the port is connected to.  This means that if you have multiple displays (even Apple Thunderbolt Displays), these devices will affect Thunderbolt performance as you continue to add devices to your bus.

How you attach your devices to the new Mac Pro is really important as it will affect performance across all of your devices.  Here are some tips for mapping out your thunderbolt devices across the individual buses.

• Do not attach more than 2 displays to a thunderbolt bus.  If you do, expect to see problems.
• You can connect up to 6 Thunderbolt/mini DisplayPort displays (2 on each bus) to the new Mac Pro.
• You can connect up to 3 4K displays (1 each on buses #1 and #2) and a third through the HDMI port, which connects to the third Thunderbolt bus.
• On my setup, I have my ports configured this way: my two desktop monitors are on bus #1.  My Promise R8 and some additional thunderbolt storage are on bus #2.  My Ultrastudio 4K for video I/O is on bus #0, and I’ll connect additional drives/peripherals when necessary to this bus.

When you take full advantage of the Thunderbolt mapping on the New Mac Pro, you can do something like what we did at the RED booth at NAB.  We had two LG 21:9 Ultrawide monitors (3440×1440) each hooked into buses 1 and 2, as well as a large 4K Sharp HDTV (think it was the 70”… but I could be wrong) hooked up to the HDMI port.  We were able to get realtime playback of 4K Prores (playback set to High quality) in FCPX off a Promise Pegasus while simultaneously getting realtime 6k Dragon Playback in Resolve off the internal Mac Pro SSD (yes, both programs were open and playing back at the same time).  We had FCPX on one of the LG’s with the AV out to the Sharp, while resolve was open on the other LG Monitor.

And while you would never actually have a reason to do this in a real-world workflow… the fact is that you can if you wanted to, and you knew how to map your Thunderbolt ports correctly on the new Mac Pro.  We are living in interesting times.