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Thunderbolt 3 and 4 Compared

By Bob Raikes

The development of the Thunderbolt interface has been intriguing. Historically, PCs have often had both high speed and low speed interfaces. In the past, interfaces including SCSI and P.1394 (FireWire) were used to connect higher bandwidth devices, while USB was the primary connection for lower speed devices.

Thunderbolt was conceived by Intel as an alternative to high-speed interfaces and was expected to use fiber optics to deliver high speed (and used the name ‘Light Peak’). It turned out, however, that fiber was just a bit too expensive for many, so Intel went back to copper although there was a feeling that there might not be so much scope to boost speeds. It co-developed the copper-based version with Apple, the first company to put the interface on its PCs.

Initially, Thunderbolt was a proprietary interface, only available on systems using specific Intel chipsets. However, Intel made the specification for Thunderbolt 3 available on a royalty-free basis in 2017 and released it to the USB-IF (which controls the USB standard) in 2019. Now, others could build Thunderbolt-compatible silicon and that opened up the market.

Thunderbolt 3 and 4 Compared
The Thunderbolt icon and number 4 quickly identify the Thunderbolt 4 cable. Image Intel,

In 2020, Intel announced Thunderbolt 4 and the USB-IF said that Thunderbolt 4 would be an option for USB-4. These changes have made the Thunderbolt interface the ‘interface du jour’ for connecting storage devices and other demanding peripherals to PCs.

The Key Elements

So what are the key elements of the interface?

  • Thunderbolt extends the PCI interface outside the system unit allowing connections for devices right up to external GPUs that would have had to be inside the system unit before.
  • It supports multiple devices in a ‘daisy-chain’ mode – up to 6 and that’s convenient especially for multiple storage devices.
  • It allows the transmission of DisplayPort data streams to enable display connections.
  • It has support for power delivery (up to 15W in basic mode and up to 100W with the support of the USB Power Delivery standard in Thunderbolt 4). It can be used to power external devices completely or support laptop charging via a single cable.
  • In versions from Thunderbolt 3 onwards, it uses the USB Type-C connector.
  • Thunderbolt 4 is compatible with Thunderbolt 3 devices. Thunderbolt 3 is also compatible with Thunderbolt 2 devices, although you will need an adaptor as the connectors are different. (Thunderbolt 2 connectors are the same physical shape as Mini DisplayPort connectors).

The Key Differences

The key differences between Thunderbolt 3 and 4 are:

  • Thunderbolt 3 can support a single 5K monitor or dual 4K, while Thunderbolt 4 can support up to a single 8K display or dual 4K monitors as a base level – more on that later
  • Thunderbolt 4 can support up to 100W using the USB Power Delivery protocol. Thunderbolt 3 provides just 15W.
  • Thunderbolt 4 has twice the data transfer rate for storage of version 3, up from 16Gbps to 32Gbps using PCIe
  • Thunderbolt 4 has a feature that allows a keyboard or mouse connected to the port to wake up a system.
  • Thunderbolt 4 has special Direct Memory Access protection. This improves security if a system is booted from a Thunderbolt 4 device, such as a disk drive.
  • Thunderbolt 4 supports ‘Thunderbolt Alternate Mode USGB hubs and not just ‘daisy-chaining’. That allows more than just 6 devices to be connected.

Like USB Type-C, Thunderbolt supports DisplayPort ‘Alt modes’. These are modes that allow pins from the connector to be reallocated to other functions, although they can limit the performance of some aspects of the base interface. Importantly for 8K, Thunderbolt 4 supports a DisplayPort 2 alt mode that means that the full resolution of DisplayPort 2 can be carried over the cable. As we saw when we looked at DisplayPort 2,0 recently, (VESA Clarifies DisplayPort 2.0 Cable Labelling) this doubles the bandwidth available when the interface is used for storage or other bidirectional devices.

Thunderbolt_Alt_Mode DisplayPort 2.0
The Thunderbolt Alt Mode for DisplayPort uses the DisplayPort lanes for one-directional transmission, whereas the PCI Express connections are bi-directional. Using a single direction for display allows more speed.

In a typical storage application, you have to be able to both read and write data but as display data is only going one way, twice as much can be transmitted. That’s really helpful for 8K and means that resolutions right up to 16K with 10 bit 4:4:4 colour are possible. At the time of publication, DisplayPort 2.0 devices are unavailable, having been delayed in development and testing by Covid. Using the Alt mode means that the full 80Gbps of DisplayPort 2.0 is available.

USB Was Just for ‘Slow Stuff’

As we said at the beginning of this article, USB used to be just for the ‘slow stuff’ connected to the PC. It was initially conceived as a way to make it easier to connect audio devices – it was seen as a way of helping the PC take the place of a dedicated telephone on the desk. However, the USB-IF has followed a trajectory of moving up and up in performance and adding features. Several years ago, when I met with the USB-IF at CES, the organization said it really wanted to be “the only connector on the desktop”.

USB has made some excellent moves as part of its takeover of the connection world.

  • It changed to the USB Type-C connector which is becoming absolutely ubiquitous. It’s small enough for mobile devices, but powerful enough for big ones. It’s cheap enough to be used everywhere and the reversibility makes it easy for consumers.
  • It added and developed the Power Delivery feature that expanded substantially on the power that could be delivered. That enabled the connector to be used for charging a laptop as well as acting as a docking connector. A monitor equipped with enough power to charge a notebook can become a docking station using a single cable.
  • It allowed ‘Alt modes’ to enable DisplayPort, Thunderbolt, HDMI and other protocols to use the Type-C connector without having to develop new protocols.
  • It maintained backward compatibility with earlier versions of USB – specifically USB 2.0 and USB 3.2
  • The USB 4 protocol was actually based on the Thunderbolt 3 specification. The closeness of USB and Thunderbolt means that USB4 is effectively a subset of Thunderbolt 4.

A downside of the success is that it’s very hard to have a single interface that is cheap and simple enough for very low cost and low-performance peripherals such as mice or USB audio devices, while still having all the power features. To cover both ends of the spectrum, some features or levels of performance have to be optional (a challenge that has made difficulties for HDMI, for example).

Source: Wikipedia

To make things simpler, the USB-IF has a series of logos that help buyers know what level of performance they are going to get.

Source: USB Implementers Forum

What About the Real World?

In practice, standards such as Thunderbolt should allow interoperability, but the sceptics among us know it’s not always that simple. As it happens, I have a Lenovo ThinkPad notebook with Thunderbolt 3 and I invested in a dock based on that interface around two and a half years ago. It has worked well and has supported a good range of USB peripherals. I have also used it with more exotic interfaces including a Universal Audio Arrow Thunderbolt audio interface that I used for remote music performances during lockdown. The interface is entirely powered by the 15W available from the dock.

Recently, I was given an iPad Pro that also had Thunderbolt 3. Initially, I thought I might need to get a dock for that, to use it for ‘PC-like’ functions, but then I realized it should connect to my existing Lenovo dock. Amazingly, when I connected to the dock using the cable that I have on my Thinkpad, the system immediately recognised the iPad. It connected my peripherals with the display being mirrored on my monitor and my keyboard and mouse operating the iPad correctly! The iPad is not completely optimized for this, but I love being able to use the big monitor and my existing ergonomic keyboard.

This is my image of me writing this article on my iPad using a PC Thunderbolt dock and a 4K monitor. It worked!