GUIDE: How to quickly test a #USB #TypeC A-to-C cable with a multimeter (-- or -- pinout and specification of USB-C legacy cables)
(This doubles as a quick analysis of the Nexus 6P's included A-to-C USB cable as an experimental control.)

People have seen the tests Benson does with his super-fancy "Twinkie" (Pluggable PD sniffer) and laboratory-grade LCR meter. However, in a pinch, you can do a test with a multimeter and some sewing needles to see if your cable is safe to use.

This won't test all the specification requirements of the USB-C standard, such as wire gauge, capacitance, IR drop, etc -- but it's a good place to start with simple tools.

Requirements:
(basic)
- multimeter with resistance measurement function
- two pins or sewing needles taped to probe ends
(advanced)
- a USB-C female (24-pin) breakout board
- a USB-A female (preferably 3.1) breakout board

tl;dr: measure resistance between the pins indicated in the first image. It should be in the range of [53.2 - 58.8] kR. If it is outside of that, the A-to-C "legacy host adapter cable" is non-compliant.


Determine cord type:
First, examine your cord. If you see a "SS" symbol stamped. If you do, you have a USB 3.1 cord with 9-11 wires. (Or, what claims to be one.) It should be wired up as you see listed in the table. This can also be found in [Section 3.5 Legacy Cable Assemblies].

If not, you likely have a USB 2.0 cable. You can also tell by looking into the USB-C plug. If you don't see a full set of teeth, it is likely a USB 2.0 cord with 5-6 wires.

Benson has done an amazing job describing the differences here:
https://plus.google.com/+BensonLeung/posts/HBWyVBmdVzw

Test through-connections:
If using breakouts, testing through-connections can be very important. Benson has encountered cords that claim to be 3.1, but actually have only 4 wires! (BOO! HISS!)

This includes violators like the Surjtech that connected 5V to Ground, and Ground to 5V, and fried his Chromebook Pixel! So test power and ground are wired correctly, at VERY least!
https://plus.google.com/+BensonLeung/posts/HzkGqnWcyYM
https://plus.google.com/+BensonLeung/posts/EBGMagC46fN

Test pullup resistor Rp: THE IMPORTANT BIT
What makes measuring Rp so challenging is there are no two  corresponding pins on the USB-A side. The only way you can reach it is by using a teeny tiny probe to measure from the USB-C side.

You can make one by taping a sewing needle to your multimeter probe with electrical tape. Be very careful!

Tap the other multimeter probe to the Vbus pin on the Type-A connector indicated in the first picture... it's much easier than simultaneously reaching for the Vbus pin on the Type-C plug!

Quickest way to test is try diagonal pins, fifth from the right (if the pins are on top). Then flip over the plug and repeat. Be careful not to touch the Vbus pin immediately to the right of it, or you will get a resistance of "0" and a beep from your multimeter!

Additional cautions:
Keep in mind there are TWO CC-pins in the plug. However, only ONE should be wired through. If both give you a reading, the cord is non-compliant.

Slight bit of terminology: the "other" CC line in the cord is referred to as "Vconn".... not CC2. Vconn should not be connected through. But the physical pin should exist. (Likely to ensure even wear on the device contacts, since plug direction is a 50/50 chance.)

Notes for nerds:
This is the type of attention to detail that goes into the spec! When Benson says "the specification is dense, but for a reason, and clearly spelled out" this is what he means.

Note the footnotes I've highlighted from another section of the specification [Section 4.11.1 Termination Parameters]. The USB-IF even requires different tolerance of resistors depending on if it is a C-to-C cable, or a legacy A-to-C, or yet others.

"For Rp when implemented in the USB Type-C plug on a USB Type-C to (___legacy___) USB host, a value of 56 KR +/- 5% shall be used, in order to provide tolerance to IR drop on Vbus and GND in the cable assembly."

So not only does it specify where the resistor has to go (USB Type-C plug), it also plans ahead. Unlike CC's 5V source -- that has its own independent supply, and little current so there is little IR drop --  the 5V voltage of Vbus in a USB-A adapter WILL sag as you load it, and the USB-IF anticipated it. Neato! Guestimating (I haven't done proper math), it that means that USB-C legacy adapters designed properly can tolerate voltage sag down to 4.25V.

Most manufacturers just pull components out of a bucket -- typical part tolerance is 10%. The USB-IF recognized this, so what they did was make the USB-C spec so loose that even cheap USB-C cable/device manufacturers could get by using whatever parts they have!

Unfortunately, lazy manufacturers also didn't read the warnings, and used the wrong resistors completely. (Hence Benson's original posts.) So it's a fair bet many of them didn't read, didn't care, or didn't want to spend the money on this "brown M&M clause" about using 5% tolerance resistors. (They can be 4x more expensive than the 10% ones. Ask how I know!)

Exercise for the reader:
It is possible to validate other USB-C cords, such as legacy device adapters (aka "USB-C OTG adapter", a bit of a misnomer) by using the other tables near the sections referenced above.

#USBC    
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