HOW TO: Connect Thunderbolt devices for best performances

With current Apple Silicon Macs

All current Apple Silicon Macs (M1, M2, M3 and M4 generations) provide excellent Thunderbolt performance for professional video workflows.

Depending on the model, Macs may provide Thunderbolt 3, Thunderbolt 4, or Thunderbolt 5 connectivity. Recent systems such as the Mac mini M4, Mac Studio M4 Max, and Mac Studio M3 Ultra feature Thunderbolt 5 ports, which provide significantly more PCIe bandwidth than previous generations.

For detailed bandwidth figures and tested video channel capacities, please refer to our article: Maximum number of channels on one Thunderbolt BUS.

Thunderbolt architecture on Apple Silicon Macs

Apple Silicon Macs provide a much simpler Thunderbolt architecture than older Intel systems.

In most Apple Silicon models, Thunderbolt ports are connected to dedicated controllers, providing predictable and consistent performance. Unlike many Intel-era Macs, there is generally no need to study complex Thunderbolt BUS mappings before choosing where to connect a device.

However, Thunderbolt bandwidth is still shared between devices connected to the same Thunderbolt chain. For maximum performance, we recommend connecting high-bandwidth video devices directly to the Mac whenever possible.

Recommended configuration for best performance

• Connect video devices directly to the Mac whenever possible.
• Avoid daisy-chaining additional high-bandwidth devices behind a video interface or PCIe expansion chassis.
• Use certified Thunderbolt cables appropriate for the Thunderbolt version supported by your computer and devices.
• If multiple Thunderbolt ports are available, distribute high-bandwidth devices across different ports whenever practical.

About daisy-chaining

We do not recommend using daisy-chaining in professional broadcast workflows.

While Thunderbolt technology supports connecting multiple devices in a chain, doing so introduces additional variables that can affect overall system performance and make troubleshooting significantly more difficult.

When video devices, storage systems, displays, or other peripherals share the same Thunderbolt path, it becomes much harder to determine the source of performance issues such as:

• Dropped video frames during capture.
• Duplicated frames during playout.
• Video card buffer underruns.
• Storage read or write performance degradation.
• Intermittent device communication issues.

In a professional broadcast environment, the number of potential points of failure should be kept to a minimum. Daisy-chaining adds additional devices, cables, and bandwidth-sharing considerations that can complicate system validation and long-term reliability.

If you contact Softron support regarding performance issues on a system using a Thunderbolt daisy-chain, one of the first troubleshooting steps will be to remove the daisy-chain and connect the video device directly to the Mac. Until this has been done, it may not be possible to determine whether the issue is caused by the daisy-chain itself or by another component of the workflow.

Whenever possible, connect video devices and storage devices directly to separate Thunderbolt ports on the computer.

How many video channels can go through Thunderbolt?

Video frames are transferred between the Mac and the video hardware as uncompressed data. As a result, available PCIe bandwidth inside the Thunderbolt connection directly affects the number of video channels that can be transported.

The achievable channel count depends on several factors:

• Thunderbolt version (Thunderbolt 3, 4, or 5).
• PCIe generation supported by the video card.
• Expansion chassis architecture.
• Video format, frame rate, and bit depth.
• Driver and system configuration.

Because these variables have a major impact on performance, we maintain detailed benchmark results in a separate article: Maximum number of channels on one Thunderbolt BUS.

That article includes:

• Thunderbolt 3, 4 and 5 bandwidth information.
• PCIe Gen2, Gen3 and Gen4 considerations.
• Tested channel counts for HD, UHD and 8K workflows.
• Performance measurements using various expansion chassis and video cards.

What happens when you reach a Thunderbolt bandwidth limit?

If a Thunderbolt connection becomes saturated, several symptoms may appear:

• Frames may be dropped during capture.
• Frames may be duplicated during playout.
• Storage performance may decrease if storage devices share the same Thunderbolt bandwidth.
• Buffer underrun or overflow warnings may appear in the application logs.

Typical log messages include:

• Input: Error: Dropped X video frames
• Output: Error: Video card buffer underrun (X frames duplicated)

If these symptoms occur, review the Thunderbolt topology of your system and consult the performance information available in the Maximum number of channels on one Thunderbolt BUS article.

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Thunderbolt recommendations with legacy Intel computers

Unlike with Apple Silicon computer that have one BUS per Thunderbolt port, Intel computers could have complex configurations, with some Thunderbolt BUS shared across multiple devices.

With the 2020 iMac 27"

The 2020 intel iMac 27"  come with 2 x Thunderbolt 3 ports, and only 1 controller (or BUS).

Recommended configuration for best performances

You can plug in your video device to any of the 2 Thunderbolt port. But for best results, you should not connect anything else on the other port as because there is only one controller, the bandwidth is shared between the 2 ports. 

With the 2019 Mac Pro

The 2019 Mac Pro can have different Thunderbolt port and controllers (or BUS) numbers. Indeed some of the Graphics cards can add more Thunderbolt ports and controllers.

Note that we indicate BUS numbers in our screenshots below, but these may vary depending on your configuration. But what's important is not the BUS number, but to know that the Thunderbolt ports go by pairs, as there are 2 ports per BUS.

"Built-in" Thunderbolt ports

If you don't consider the graphics cards, the Mac Pro comes with 4 Thunderbolt ports:

• 2 connectors (1 BUS) at the back of the Mac Pro (these ports are on a PCIe card so they could in theory be removed as well):
• 
• 2 connectors (1 BUS) at the top of the Mac Pro (these are the only Thunderbolt ports that can not be removed and will always be there):
• 

Thunderbolt ports on Graphics cards

Some of the graphics cards that can be purchased with the Mac Pro can add Thunderbolt ports. These can be used either to connect Thunderbolt displays, or any Thunderbolt device.

• AMD Radeon Pro 580X: No external Thunderbolt ports
• AMD Radeon Pro W5700X:  4 thunderbolt ports (2 BUS)
• AMD Radeon Pro Vega II:  4 thunderbolt ports (2 BUS)
• AMD Radeon Pro Vega II Duo:  4 thunderbolt ports (2 BUS)

For all the cards that have Thunderbolt ports, the BUS are grouped as follows:

About PCIe usage and "Expansion Slot Utility"

It is important to note that Thunderbolt uses PCIe bandwidth. Each Thunderbolt to which at least one device is connected will allocate 4 lanes of PCIe3. 

And thus it is important to check the "Expansion Slot Utility" every time you connect a new device, in order to make sure that it will have enough bandwidth. For example:

• if you have too many PCIe cards inside the Mac, you may not have enough PCIe bandwidth to drive the additional M|Family device.
• if you connect too many Thunderbolt devices on your Mac (specially if you have 2 x Vega II (Duo), which add 8 connectors (4 BUS)). That would make a total of 6 BUS, using each 4-lane of PCIe 3.0...

Note that Thunderbolt devices will only use Pool B, so if you do a manual configuration, make sure to leave a little room on Pool B for your Thunderbolt devices.

For more information on how to handle PCIe connections on your Mac Pro, check the following article.

A few facts

• each Thunderbolt BUS where a device is connected will allocate 4 lanes of PCIe3
• all Thunderbolt BUS are allocated to PCIe pool B only. As a pool has 16 lanes, it means that you can use a maximum of 4 BUS before starting to share the bandwidth with another.
• even if the PCIe expansion utility shows that you are over 100% of allocation, it does not mean necessarily that you'll have issues. You may have issues if all Thunderbolt BUS work at their highest speed at the same time. So it is best to avoid this, but check what each one does.
• the Apple I/O card in slot 8 already allocates 4 lanes of pool B, even if nothing is connected to it. Connecting a Thunderbolt device to it will allocate 4 more lanes, but in fact if you did not connect anything in the USB ports, it will only use 4 lanes, not 8.
• when using a Thunderbolt 3 to Thunderbolt 2 adapter, 4 lanes of PCIe will be allocated, even if you only connected a display using Display Port bandwidth, and not PCIe. That's because the adapter has a Thunderbolt controller, and the macOS can not know in advance what kind of devices will be connected. It can only know what in theory could go through that port.

A few recommendations 

• each device that uses PCIe should be connected on its own BUS
• a Thunderbolt monitor (that only uses Display Port) should be connected on the Thunderbolt ports of the graphics card so the "path" is simpler.

Recommended configuration for best performances

We recommend connecting the card inside the Mac Pro in one of its PCIe slots.

If you don't have any PCIe slot available, you can connect it (if you have one of the models with a Thunderbolt box), to one of the Thunderbolt 3 connector. For best results, we recommend that  no other device is connected to the same BUS.

You should prefer the built-in ports (on the Mac Io card) over the ones on the Graphics card. Make sure also to check the "Expansion Slot Utility".

With the 2018 Mac mini

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

Recommended configuration for best performances

There has been reports of possible interference between Wifi and the Thunderbolt ports. To make sure you have no issue, it is recommended to use BUS 1, which is the father of the Power input, where the Wifi antenna is.

WARNING! When using the Mac mini in a xMac mini Server from Sonnet, you should turn off Wifi completely. Cf the following post on Sonnet's website:

Aside from that, as usual, for best results, you should not connect anything else on the same Thunderbolt BUS as the one where your M|Family device is connected. 

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 best performances

You can plug in your video 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. 

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 best performances

You should connect your video 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.

•  

Thunderbolt recommendations with legacy Intel computers

Unlike with Apple Silicon computer that have one BUS per Thunderbolt port, Intel computers could have complex configurations, with some Thunderbolt BUS shared across multiple devices.

With the 2020 iMac 27"

The 2020 intel iMac 27"  come with 2 x Thunderbolt 3 ports, and only 1 controller (or BUS).

Recommended configuration for best performances

You can plug in your video device to any of the 2 Thunderbolt port. But for best results, you should not connect anything else on the other port as because there is only one controller, the bandwidth is shared between the 2 ports. 

With the 2019 Mac Pro

The 2019 Mac Pro can have different Thunderbolt port and controllers (or BUS) numbers. Indeed some of the Graphics cards can add more Thunderbolt ports and controllers.

Note that we indicate BUS numbers in our screenshots below, but these may vary depending on your configuration. But what's important is not the BUS number, but to know that the Thunderbolt ports go by pairs, as there are 2 ports per BUS.

"Built-in" Thunderbolt ports:

If you don't consider the graphics cards, the Mac Pro comes with 4 Thunderbolt ports:

• 2 connectors (1 BUS) at the back of the Mac Pro (these ports are on a PCIe card so they could in theory be removed as well):
• 
• 2 connectors (1 BUS) at the top of the Mac Pro (these are the only Thunderbolt ports that can not be removed and will always be there):
• 

Thunderbolt ports on Graphics cards

Some of the graphics cards that can be purchased with the Mac Pro can add Thunderbolt ports. These can be used either to connect Thunderbolt displays, or any Thunderbolt device.

• AMD Radeon Pro 580X: No external Thunderbolt ports
• AMD Radeon Pro W5700X:  4 thunderbolt ports (2 BUS)
• AMD Radeon Pro Vega II:  4 thunderbolt ports (2 BUS)
• AMD Radeon Pro Vega II Duo:  4 thunderbolt ports (2 BUS)

For all the cards that have Thunderbolt ports, the BUS are grouped as follows:

About PCIe usage and "Expansion Slot Utility"

It is important to note that Thunderbolt uses PCIe bandwidth. Each Thunderbolt to which at least one device is connected will allocate 4 lanes of PCIe3. 

And thus it is important to check the "Expansion Slot Utility" every time you connect a new device, in order to make sure that it will have enough bandwidth. For example:

• if you have too many PCIe cards inside the Mac, you may not have enough PCIe bandwidth to drive the additional M|Family device.
• if you connect too many Thunderbolt devices on your Mac (specially if you have 2 x Vega II (Duo), which add 8 connectors (4 BUS)). That would make a total of 6 BUS, using each 4-lane of PCIe 3.0...

Note that Thunderbolt devices will only use Pool B, so if you do a manual configuration, make sure to leave a little room on Pool B for your Thunderbolt devices.

For more information on how to handle PCIe connections on your Mac Pro, check the following article.

A few facts

• each Thunderbolt BUS where a device is connected will allocate 4 lanes of PCIe3
• all Thunderbolt BUS are allocated to PCIe pool B only. As a pool has 16 lanes, it means that you can use a maximum of 4 BUS before starting to share the bandwidth with another.
• even if the PCIe expansion utility shows that you are over 100% of allocation, it does not mean necessarily that you'll have issues. You may have issues if all Thunderbolt BUS work at their highest speed at the same time. So it is best to avoid this, but check what each one does.
• the Apple I/O card in slot 8 already allocates 4 lanes of pool B, even if nothing is connected to it. Connecting a Thunderbolt device to it will allocate 4 more lanes, but in fact if you did not connect anything in the USB ports, it will only use 4 lanes, not 8.
• when using a Thunderbolt 3 to Thunderbolt 2 adapter, 4 lanes of PCIe will be allocated, even if you only connected a display using Display Port bandwidth, and not PCIe. That's because the adapter has a Thunderbolt controller, and the macOS can not know in advance what kind of devices will be connected. It can only know what in theory could go through that port.

A few recommendations 

• each device that uses PCIe should be connected on its own BUS
• a Thunderbolt monitor (that only uses Display Port) should be connected on the Thunderbolt ports of the graphics card so the "path" is simpler.

Recommended configuration for best performances

We recommend connecting the card inside the Mac Pro in one of its PCIe slots.

If you don't have any PCIe slot available, you can connect it (if you have one of the models with a Thunderbolt box), to one of the Thunderbolt 3 connector. For best results, we recommend that  no other device is connected to the same BUS.

You should prefer the built-in ports (on the Mac Io card) over the ones on the Graphics card. Make sure also to check the "Expansion Slot Utility".

With the 2018 Mac mini

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

Recommended configuration for best performances

There has been reports of possible interference between Wifi and the Thunderbolt ports. To make sure you have no issue, it is recommended to use BUS 1, which is the father of the Power input, where the Wifi antenna is.

WARNING! When using the Mac mini in a xMac mini Server from Sonnet, you should turn off Wifi completely. Cf the following post on Sonnet's website:

Aside from that, as usual, for best results, you should not connect anything else on the same Thunderbolt BUS as the one where your M|Family device is connected. 

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 best performances

You can plug in your video 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. 

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 best performances

You should connect your video 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.

•