USB C

  1. USB C is a physical connector. It has four high speed lanes and assorted tidbits: most importantly, power, a separate pair of wires for USB 2.0 and finally one wire to negotiate power and data mode.
  2. Everything is negotiated: which end behaves as a power provider and which end behaves as a power sink. Which end behaves as the downstream data port (host) and which one is the upstream port (device). What kind of data will be transmitted.
  3. Power: 5V 3A for legacy devices, this is always available and is the only thing that requires no negotiation merely a few resistors. Up to 60W (20V 3A) is possible with every USB C-C cable, the voltage and amperage is negotiated. 100W (20V 5A) requires a special cable. Some 5V only devices do not implement the specification properly and can only be used with an A-C cable or from an 5V only USB C charger. r/UsbCHardware/ calls these broken” for good reasons.
  4. The high speed lanes can carry USB signals, DisplayPort signals or Thunderbolt signals (in theory they could carry anyhing but these ones are used in reality).
  5. USB needs one lane to transmit and one lane to receive 5 or 10gbit per second USB data. As mentioned, USB 2.0 speed is always available, separately.
  6. DisplayPort can use two or four lanes to transmit video data. It is possible to use two lanes for DisplayPort and two lanes for USB. DisplayPort data is commonly 4.32Gbps per lane effective video bandwidth as defined in DisplayPort 1.2 (5.4gbps with overhead), more rarely it can be 6.5Gbps per lane as defined in DisplayPort 1.3 (8.1Gbps with overhead). The latter requires DisplayPort 1.4 (1.3 alone is not used in practice) support from the host which is rare because Intel integrated GPUs are DP 1.2 except for Ice Lake” and Tiger Lake” chips. Video bandwidth calculators: 1 2. Practically all USB C - DP adapters work with DP 1.4 without a problem as these adapters just negotiate the correct mode on the USB C and do not touch or even know anything about the actual DisplayPort signal.
  7. Thunderbolt is a different world, it requires special cables. It occupies all four lanes, it’s a bus with a 40gbit/s data rate. It carries a mixture of PCI Express and DisplayPort data. The PCI Express data speed is nerfed by Intel to 22gbps although many laptops with a single TB3 port can only do 16gbps. The TB3 bus does not carry USB signals, USB ports are provided by a USB root hub built into the dock’s TB3 controller. The only supplier of TB3 controller ICs is Intel. They have two generations of chips, the older Alpine Ridge only supports DisplayPort 1.2, Titan Ridge also supports DisplayPort 1.4.
  8. USB 4 is Thunderbolt 3 with a very important addition: now USB packets will be found on the bus too. This eliminates the hotplugged USB root hub for stability and much better overall user experiences. Also, it’s very likely PCIe will be able to reach 32gbps this time (or maybe even 40gbps with PCIe 4.0?). This mode, however will be optional. Everything above still applies so USB 4 ports wil be even more confusing in their capabilities.
  9. To avoid this confusion, Intel decided to name USB 4 with every feature required” Thunderbolt 4.

To run multiple monitors:

  1. The DisplayPort standard has its own thing where it can split the data coming out of a single connector to multiple displays. This is called MST and is not supported by Mac OS.
  2. Thunderbolt behaves as if there were two DisplayPort connectors and is the only way for Mac OS to run multiple monitors while plugging a single cable into the host. Plugging two cables saves you hundreds of dollars. Caveat: many laptops with a single Thunderbolt port uses an Alpine Ridge LP controller and only have one DisplayPort 1.2 worth on the bus. You can check whether your has it here even if you don’t run Linux, the components list is correct.
  3. The above was for two monitors and that’s the top of Mac capability. For Windows, some first party docks (Lenovo, Dell) have MST hubs built into them. These are much cheaper on eBay, often the cheap auctions will come without a power brick. Both Lenovo and Dell have standardized on their power bricks within the brand, so any high wattage Dell brick will work for a Dell dock, same for Lenovo. Make sure to buy them from the USA, even if it is purported as original Dell, Chinese auctions are kinda sus.

To run multiple laptops from the same monitor / USB peripherals aka KVM:

  1. I only know of one USB C switch, it’s industrial and breathtakingly expensive.
  2. There are some KVM switches which have USB C inputs and legacy outputs: iogear GUD2C04 Access Pro, Black Box USB-C 4K KVM.
  3. The cheapest solution by far is to forget USB C and use the software KVM. It detects when a USB A switch connects/disconnects the peripherals and sends the monitor a request to switch inputs. This obviously only works if a monitor has multiple inputs but most do.

To connect USB C monitors:

  1. Belkin has a VR cable which plugs into USB A and DisplayPort inputs and a USB C monitor.
  2. The Wacom Link Plus has USB A, HDMI and DisplayPort inputs and a USB C output.
  3. The Dell WD19 is an USB C hub which has a USB C downstream capable port. This is unique.
  4. TB3 docks with a downstream (chaining) TB3 port are also usable as a plain USB C port which is also DisplayPort alternate mode capable.

Footnotes:

  1. Naming is not a strength of the USB IF. 5gbps USB is called USB 3.0, USB 3.1 Gen 1, USB 3.2 Gen 1, Superspeed USB. 10gbps USB is called USB 3.1 Gen 2, USB 3.2 Gen 2, Superspeed Plus USB. We typically just call them 5/10gbps USB to avoid wading into this mess.
  2. The faster the data speed, the shorter the cable. Cables omitting high speed lanes (so only USB 2.0 and charging is possible) can be 4m long, 5gbps lane speed allows for 2m, 10gbps only allows for 1m. There are two ways to escape these limits: the cheap way where marketing will spin a story on how a cable made from the finest chinesium can surpass the spec and the expensive way where active circuitry will be added to cable to avoid the signal loss. Cable Matters has a 3M 10gbps, a 5M 5gbps and that’s it for affordable active USB C cables. Thunderbolt cables can be 0.5m for 40gbps (although some 0.8m cables have appeared recently, Plugable is recommended), up to 2m for 20gbps passive or up to 2m for 40gbps with an active cable. The active cables can only be used for Thunderbolt, not plain USB except for the Apple Thunderbolt 3 Pro.
  3. Docks touting 4k support” very, very often mean 4K @ 30Hz” because they utilize two lanes for DisplayPort and two lanes for USB 3.0 and that’s what two lanes worth of DisplayPort is capable of. In reality noone wants a 30Hz monitor so up to 3440 x 1440 @ 60Hz and 1080p @ 144Hz are typical max resolutions used with these docks.(HDMI 1.4 can only do 1080p @ 120Hz, you need DisplayPort for 144Hz). Again, video bandwidth calculators: 1 2. If you need USB 3.0 then these are the maximum without Thunderbolt (and without DisplayPort 1.4).

Avoid the following:

  1. Docks passing PD power with removable cables. You need DC input for such. More in this article. tl;dr: every cable, including the one between the hub and the device has a loss, if it’s captive and short then the loss can be calculated, otherwise you’d need to do boost power and do full power delivery renegotiation which most hubs don’t do.
  2. Magnetic cables not only violate specifications but pose immense danger to the host: they expose the pins the normal connector hides within a grounding shroud (note how DisplayPort, HDMI, USB and more has this general design) and a static discharge might fry the device.
  3. Sometimes when trying to escape the bandwidth limits you will find docks utilizing something called DisplayLink. The biggest tell of it is the ability to run video from a USB 3.0 (aka USB A port). These are good for running for office apps but not much else. Gaming will especially suck. In general, you should avoid these. Disappointment is almost guaranteed.

August 28, 2020


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