Why are there no USB-C Receptacle to USB-B plug or USB-A plug adapters?

Benson Leung
Nov 19, 2018 · 2 min read

Dec 3, 2015

Why are there no USB Type-C Receptacle to USB Type-B plug or USB Type-A plug adapters?

tl;dr : Type-C spec forbids it, as such an adapter could allow a user to create unsafe conditions.

I’ve gotten this question several times, mostly from people trying to find small adapters so that they can switch their main chargers and cables to all Type-C but don’t have to buy another cable to charge their older Micro-B devices.

The answer is in the Type-C Specification, Section 2.2, with two sentences at the very end of the section.

Section 2.2 forbids passive receptacle legacy adapters

“USB Type-C receptacle to USB legacy adapters are explicitly not defined or allowed. Such adapters would allow many invalid and potentially unsafe cable connections to be constructed by users.”

Remember in my last post (https://plus.google.com/+BensonLeung/posts/hF6b228zGvy, https://medium.com/@leung.benson/what-happens-when-you-plug-two-usb-c-host-ports-together-with-a-c-to-c-cable-4f8e912f8a3d) that USB A-to-A cables are specifically dangerous and forbidden, and that the USB Type-C spec defines behavior to protect C-to-C cables from causing many of the same problems.

If C receptacle to A plug adapters exist, they allow the user to plug one in on both ends of the cable, creating an invalid cable that devolves into a USB A-to-A cable. This is why the specification specifically forbids all legacy adapters that have a C receptacle on one end.

Instead of looking for such an adapter, you should ideally have a multi-port USB Type-C charger or hub, and the right kind of cable (for example, a Legacy Device Port Adapter) that goes from a Type-C plug to a Micro-B plug instead.

For example, this cable is valid and works for charging your older phone from a newer USB Type-C charger: http://amzn.com/B00UUBRX0Y

WARNING: If you do find something on Amazon or other online stores that seems too good to be true and is the kind of adapter I am describing, be warned that it is not guaranteed to work properly (with some orientations in SuperSpeed mode failing), and certainly not safe. Do not buy forbidden adapters on Amazon.

Originally published at plus.google.com.

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