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On USB-C Couplers, Extensions and Standards

A project log for Fully Analog 60% HID Cluster

Custom Human Interface Device with suite of fully-analog inputs. Keyboard, Mouse, Dual Joystick (HOSAS), Dual Throttle(HOTAT), and HOTAS.

Sp4mSp4m 11/23/2022 at 19:090 Comments

I learned something awesome this week!

TLDR: If you're using a Female-Female / Receptable-Receptacle (abbreviated R-R going forward) USB-C adapter, USB-C cables get a 'Top' and 'Bottom' side, and the connection will only work if all cables and adapters are properly aligned.

The USB-C standard [https://cdn.sparkfun.com/assets/e/b/4/f/7/USB-C_Datasheet.pdf] provides instructions that help 3rd party providers create proper USB-C cables. 

Although R-R adapters can be purchased, they violate the design standard of USB-C and cause USB-C to lose its' reversibility.

A USB-C Cable has 12 pins on each side, and in support of establishing an initial connection, those pins are mirrored so the PLUG can be inserted into the RECEPTACLE in either orientation. After a plug is inserted and the connection is initialized, the connection MAY use pins on both sides of the cable asymmetrically to facilitate high-speed data transfer.

See graphic below for USB2.0 protocol communications. The D+ and D- signals are only present on one side, and the CC and VCONN signals are not mirrored.

Once a USB-C device is connected, the PLUG and RECEPTACLE coordinate on which pins will be doing what, for a given device. This is different from SOURCE (keyboard) and HOST (computer).


If there is an R-R adapter in the middle, the pin-function coordination is established from both sides of the adapter, independently of each other.

This creates a situation where all wires in a chain need to have the same arbitrary orientation to connect, and there are no indicators when this has failed.

THEREFORE... 

If you must use an R-R connector, test the full connection to find the proper orientation of the cables that work, then mark the upright position of all your cables.

It will work, but it means your device will only connect if the cable in the proper orientation. 

It's not elegant, but it is possible.

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