[SOLVED]
#USBWTF If you thought USB Type-C cables were a complex topic… You have no idea…
Investigating why USB-version connections differ with multiple full-featured, and tested, USB Type-C↔︎C cables.
Cable A: USB-IF *certified*, passive Thunderbolt 3/4, USB4 Gen 3, 40GiBit/s
- Actual connection: USB 3.2 Gen2 SuperSpeed+, aka USB 10GiBit/s. Yes, renamed *again*.
- R: 1000MiB/s
- W: 700MiB/s
Cable B: not-certified, same specs as A
- Actual connection: USB 4 Gen 3
- R: 5.2GiB/s
- W: 4.8GiB/s
Mystery solved.
Turns out:
- User error; aka partly my fault
- Bug in Blackmagic Disk Speed Test
I mixed up two USB-C ports, only one of which was actually USB4/Thunderbolt 4. This port was clearly labelled. I was just not paying enough attention amidst the clew of cables.
Blackmagic Disk Speed Test doesn’t always *actually* use the correct path it was set to for testing. The observed speed difference originated from the internal SSD, opposed to the peripheral.
Proper re-testing confirmed: Speed results are indeed coherent across ports, cables, enclosures and storage devices.
I’m aware that macOS doesn’t implement
- USB 3.2 SuperSpeed+ Gen 2×2
- USB 3.2 SuperSpeed+ 20Gbit/s
- USB 20Gbit/s
which are synonymous. The USB-IF just renamed the versions *again*.
So you can never get a 20Gbit/s USB-Link on macOS. Only either
- 10Gbit/s
- USB 3.1 Gen 2
- USB 3.2 Gen2
- USB 10Gbit/s
or
- 40Gbit/s
- USB4 Gen 3
- USB 40Gbit/s
- Thunderbolt 3/4
@MacLemon What's the maximum USB bandwidth that you can get out of a 40Gbit/s USB 4 port on a Mac?
If you connect it to a dock that as the the RTS5490 which provides USB 3.2 Gen2x2 (20Gbit/s) downstream, will you get the full 20 Gbit/s, or is the USB 4 host router in the Mac limited to 10Gbit/s?
@karotte You will get 40GBit/s on on a USB4 Port with matching cable and peripheral capabilities. In theory even 80/80 or 120/40 on an M5 Mac. (I only have an M1 MAX at my disposal for testing and no USB4 Version 2.0 or Thunderbolt 5 peripherals.)
USB 3.x data will get tunneled over USB4 fabric. So your Mac will do USB4 (40/40) to the hub/dock. Even if the dock and peripheral can do USB 20GBit/s, *macOS does not support that mode*. So the result will be tunneled USB 10GBit/s to the peripheral.
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