4

I've searched for a USB-C hub, but I couldn't find any. There are hubs that have C upstream and A downstream or even the one attached to a MacBook that has C up, A down and something that looks like another C down, but in reality it's only for charging. Bottom line: they don't offer anything I could not get by using a C->A adapter and a regular, non-C hub.

What I mean by a "USB-C hub" is something that has at least two C ports, and does not remove any features that would be present if devices were connected directly, that is fully negotiable and has exchangeable roles.

Is such a hub even possible?

  • Your new title, was broken english, I attempted to fix that problem. – Ramhound Oct 21 '16 at 17:27
  • Can you suggest a use case with a hub that switches port roles? How do you envision the desired functionality? – Ale..chenski Oct 21 '16 at 17:41
  • @AliChen Two phones, a pendrive and a charger. Phones swap who is the master, pendrive is always a slave while a charger keeps everything powered. – Agent_L Oct 21 '16 at 17:46
  • Then you are talking about sharing a set of devices between two hosts. There are hubs that can do this, see superuser.com/questions/1128622/pc-switcher-for-usb-hubs/… – Ale..chenski Oct 21 '16 at 17:57
  • @AliChen I meant that while one phone is serving as slave, it's available to the current master as another slave. Everything is connected all the time, just the role of master is variable. Not a KVM-style switch. – Agent_L Oct 21 '16 at 18:03
6

The all-Type-C hub is certainly possible, and eventually will dominate. Examples of "Hybrid-A-A-C-C = >C cable" hubs do exists.

The main problem here is in additional cost of Type-C port. Also, the purely C hub is largely useless these days, since the number of devices with Type-C connectors is still miserable on the market.

The Type-C port must have the polarity-detection mode, lane muxing, and full VBUS power control, because the VBUS switching (must be always OFF when no cable is attached) is mandatory, in contrast with the regular USB, where "ganged" wiring of VBUS is allowed. Even in classic USB the hubs who implement full power control on VBUS are selling in $40-$50 range.

Until design houses as GL, VIA, TI, or Microchip came up with hub ICs that natively support the lane muxing, CC detect/handshake, and VBUS control, the C-C-only hubs will be expensive to make, and therefore harder to find. Enhanced Power Delivery increases the cost dramatically. But I guess this all is a matter of time.

Bi-directional hubs are not possible in principle, the USB was designed as host-centric architecture with star topology, where hubs are expanders of the preconceived topology. Hubs are not a set of wires or switches/muxes, they cannot "interrupt" any OTG negotiations. A USB hub is a pretty sophisticated communication processor, which has a local and independent port control management, with deep elasticity buffers, with re-timing and re-sampling of data flows between upstream port and downstream ports. But they can't route anything from one downstream port to anther downstream port "across" the hub.

Theoretically, however, it is possible to design a device (SoC) that would have multiple OTG/DRP ports, where the CPU would provide any type of cross-communication, but this will be a new type of USB device, "super-hub", or "super-dock", or something.

  • 2
    The term "reversible" is usually construed as "polarity reversible", that's why I missed your point. If you mean reversing the role between Host and Device, then the right term is DRP (dual-role port), sometimes referred as OTG (older form of DRP). The DRP functionality of hubs is not defined, mostly because it is difficult to find any reasonable/meaningful configuration topology of connections. However, there were attempts to change this, see superuser.com/questions/1122096/… – Ale..chenski Oct 21 '16 at 17:11
  • 1
    No, the DRP is not a default nor mandatory mode. Type-C ports define three distinct categories - device, host, and DRP. To be a DRP, a lot of extra intelligence is required. – Ale..chenski Oct 21 '16 at 17:39
  • 1
    This is the same with basic Type-C - the CC1 pin state defines the role of a port. If both (port and cable) CC pins have 5.1k pull-down, they are both devices. If both have pull-ups - both are hosts, and nothing happens then. A DRP port alternates its CC pin state 15 times per second. A USB hub is a device, and does not substitute anything. – Ale..chenski Oct 21 '16 at 17:52
  • 1
    Yes, you can connect two OTG devices, and they will work fine. The roles however (who becomes the host and who becomes a device) will be determined at random. Read Type-C specifications, Figure 4-11. How the negotiation happens, see above, it is periodic advertisement to be host or device. – Ale..chenski Oct 21 '16 at 19:12
  • 1
    I said nothing about type-B. The discussion is about Type-C. – Ale..chenski Oct 21 '16 at 19:19

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.