Thunderbolt, USB4, DisplayPort, PCIe, etc, Oh my!

O excellent, thanks, Simon. I’m assuming that the TB3 20Gbps options represent situations where the port the cable’s connected to shares a 40 Gbps bus–something that, as OWC notes, is true of Intel Macs but not of the M1 Macs released so far, where each port is a full 40Gbps.

I don’t think I will ever need to move my TS3+ further away from my M1 Mini than .7m, but am going to check the CalDigit website to see if they note any restrictions on cable length if you’re connecting to a 40Gbps port. Seems like they should. And I guess it may be possible to use a TB4 cable if you go >.8m.

For TB3 I think the 20 Gbps vs. 40 Gbps relates to how much PCIe bandwidth TB is left with. If besides TB you’re also running DP alt mode or USB across that pipe you need to set bandwidth aside for that so then only 20 Gbps is left for TB (and in fact that’s a simplification).

Note the minimum required PCIe bandwidth changed between TB3 and TB4 and presumably for exactly that reason. They want to ensure on TB4 that even when running a display and some USB peripherals off for example a TB hub, you’re still left with ample PCIe bandwidth for “TB” devices connected to that same hub. That spec called for the new retimer/redriver chips in longer cables since the cable itself is still max 40 Gbps, unchanged since 2015.

Why the M1s so far are all 40 Gbps/TB port I guess, and have (only?) two. Not something generally discussed, thanks, Simon.

I was aware of the connection limitations of my new M1 Mini compared to my old 2012 Mini (Firewire!) and factored a dock into the cost. It’ll be interesting to see if the next round of M1 Macs have more TB4 and/or additional USB C and A ports. If not, everyone will be buying docks and loudly sharing their pain.

I also found that to connect my M1 macMini to the LG 5K monitor I needed a Thunderbolt 3 5A cable. A plain Thunderbolt 3 cable resulted in intermittent screen artefacts and an annoying whine from the display electronics. I wonder if a USB4 cable/Thunderbolt 4 cable works in this situation?

OWC says their cable is functionally identical to Apple’s cable, though it’s obviously less expensive.