Upstream and Downstream Ports

USB4 adds some further complications when it comes to some of the technical terminology, as the USB-IF has added "upstream" and "downstream" facing ports. These terms aren't likely to appear in product information for end users, but have some implications when it comes to how USB4 devices will work. An upstream facing port is used by all devices since that's how a device connects to a host or hub. However, not all devices will have downstream facing ports as it is in many ways the same as a daisy-chained port on Thunderbolt. Just like previous versions of USB, USB4 supports hubs, something Thunderbolt didn't do until Thunderbolt 4, and downstream facing ports are slightly different from a daisy-chaining port. All USB4 hubs have to adhere to 40 Gbps data and have to support PCIe, as well as a USB 3.2 and USB 2.0, plus, strangely enough, Thunderbolt 3. This implies that USB4-based hubs will be more costly than USB 3.2 based hubs.

If this all sounds confusing, it's really not you, it's the USB4 specification. USB4 was expected to make things simpler, yet it appears to only have added even more layers of confusion due to very open-ended spec requirements. However, both Intel and ASMedia seem to have at least implemented more than the bare minimums, so on the host side, there shouldn't be any actual surprises with compatibility issues, which is also the most important part since a feature-limited host would be the last thing you'd want. Specifically to the ASM4242, there will be support for USB 3.2 2x2 (20 Gbps), something Intel doesn't support with its Thunderbolt controllers.


Note that we had to crop out and cover up certain parts of this development board at the request of ASMedia.

Devices

As for devices, it's too early to say, but as we've already seen, VLI launched its VL830 end point device, which only has USB4 with DisplayPort support from the host without PCIe support. The first impression is that USB4 support wasn't even needed here, but due to how USB4 handles parallel transmission of data and display signals, it works out as a better solution than USB 3.2 with DP Alt Mode when a high-resolution display is connected, as the USB 3.2 data speed will remain at 10 Gbps instead of having to fall back to USB 2.0 speeds, which was often the case with similar USB 3.2 devices.

The only other device we've seen is the ASM2464PD, which is a USB4 to PCIe bridge with USB PD support that we were shown by ASMedia. Although their inhouse test board was a simple USB4 to M.2 adapter, it's not limited to this and can be used to connect any current PCIe based hardware over USB4. The ASM2464PD is an early enabler for companies that can take their current PCIe based products and turn them into USB4 devices—over time, we expect to see native USB4 devices replacing many devices using the ASM2464PD and similar solutions.