Why Do I See Different DC Resistance Results when I Flip a USB Type-A to USB Type-C Cable?

Question from the Customer:

With the Advanced Cable Tester V2, I am testing samples of USB Type-A to USB Type-C cables from various suppliers. In many cases, I see a DC Resistance failure for “GND+SHIELD A-Side Link”. I captured the results of a single cable with normal and flipped orientations (CC is on the same side as DP/DM, or CC is on the other side):

DC Resistance Measurement of USB cable

DC Resistence Measurement of USB cable

With these two orientations, I noticed that the Expected Max (Ω) column (second column from the right in the screen captures) shows significantly different values. What can you tell me about these results?

  • Is this correct, or should the Expected Max (Ω) be the same value for each test?
  • Exactly what is this test doing?
  • Why are the measurements so different?

Response from Technical Support:

Thanks for your questions! There are shield and ground specifications that the Advanced Cable Tester v2 (ACT v2) is built to test and confirm such specifications. Following are details about what is being tested and why

Type-C Specifications with Legacy Cables

Type-A and Type-B are legacy cables. Here is information about running tests with a USB A-Side or USB B-Side GND+SHIELD Link.

The Type-C specification requires that all legacy cables have a connection between GND and the connector’s SHIELD, and that this connection is made within each connector. This is addressed in the design, and difficult to measure externally. The ACT v2 uses three measurement to check if a cable meets this requirement.

Measurements for Confirming Cable Specifications

Three measurements are taken to verify the ground-shield specifications:

  • Rgnd_wire: GND pin to GND pins, through the cable
  • Rshield_wire: Shell to Shell, through the cable’s SHIELD
  • Rgnd_to_shell: GND pin to Shell, on the A-side or B-side

When the GND+SHIELD connection is correct, the resistance of Rgnd_to_shell is substantially less than the resistance of the Rgnd_wire and the Rshield_wire.

In the case where there is not a GND+SHIELD connection within the legacy plug, but there is a connection within the Type-C plug, the path for Rgnd_to_shell is essentially the entire path through Rgnd_wire + Rshield_wire.

Pass/Fail Criteria

ACT v2 takes these three measurements, and uses the following criteria for a pass:

  • Rgnd_to_shell < (Rgnd_wire + Rshield_wire) / 2

The division by 2 provides some margin for the measurement. Please note, in cases where the shell contact resistance is far greater than the wire resistances, a false failure may occur.

Why Flipped Cables Show Different Results

The pass criterion is Rgnd_to_shell < (Rgnd_wire + Rshield_wire) / 2, which indicates that the pass criterion is dependent on the actual measurements of Rgnd_wire + Rshield_wire. Because of this criterion, two different thresholds may be applied for pass/fail for two different tests.

Using the Advanced Cable Tester V2 for Other Tests

If you need additional information, we also have a video about using the Advanced Cable Tester with various cable profiles and examples of tests to run:

 

 

You might also be interested in our recent articles about the importance of effective and accurate cable testing:

Additional resources that you may find helpful include the following:

We hope this answers your question. Need more information? You can contact us and request a demo that applies to your application, as well as ask questions about our Advanced Cable Tester and the available options, and other Total Phase products.

Request a Demo