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Comment by jauntywundrkind | Hacker News Reader

There's no current specs for dual directional USB over any cable, afaik. There very well should be.

You could present as a device and offer networking and have usb-ip advertised over multicast on that port. Easy, weekend project at most, would be dead obvious to any practitioner.

USB4 as a packetized protocol really should offer something. I do wish there were like a half speed 2.5Gbit usb4 option, that microcontrollers could have some hope of accelerated bit banging.

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According to Wikipedia, DisplayPort 2.0+ can coexist with USB data on the same cable, https://en.wikipedia.org/wiki/DisplayPort

[2.0] .. using only two lanes on the USB-C connector via DP Alt Mode to allow for simultaneous SuperSpeed USB data and video

[2.1] .. tightened its alignment with the USB Type-C specification as well as the USB4 PHY specification to facilitate a common PHY servicing both DisplayPort and USB4. In addition, DisplayPort 2.1 has added a new DisplayPort bandwidth management feature to enable DisplayPort tunnelling to coexist with other I/O data traffic more efficiently over the USB4 link.

I thought the comment was talking about both sides of the USB cable exposing devices to the other, when they said:

> Is it theoretically possible to have 2-way HID

Haven't USB hubs done this since USB 1.1?

I believe that the author is talking about plugging in a USB-C cable between your laptop and some headless system, and having your laptop send HID data, while also capturing a display output from the headless system

Isn't that exactly what a USB-C docking station does? It receives display and audio data, and acts as a USB device (keyboard, mouse, webcam). You'd need hardware that can receive DisplayPort, but it's doable.

Looks like an Alt Mode host with DP sink capability is technically allowed...? But it also looks like USB host controllers with Alt Mode support exposes internal DP source driver inputs, rather than handing you bunch of bare copper wires in Alt Mode, so I doubt it realistically has an implementation.

ref: https://vesa.org/wp-content/uploads/2019/10/USB-DevDays-2019...

Afaik this is 100% allowed by the spec. Flash drives are bidirectional (in and out). Hubs present multiple devices to the host through one cable.

IDK why it isn't done more often in one device though.

Looks like Belkin has a USB-C cable for VR headsets, which breaks out to DisplayPort (video) and USB-A (data) connectors, https://www.belkin.com/support-article/?articleNum=316883

But neither USB nor DP(or DVI, VGA...) are symmetrical interfaces. They don't work like RS232C or Ethernet ports, a product that use these standards must be designed from beginning as a host or as a peripheral(or as a special 2-in-1 gadget that can exclusively switch roles). I think that's what GP meant by dual directional.

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