My USB Type-C Cable Works as Expected, but the Advanced Cable Tester Shows Rp Errors – What Can You Tell Me about these Test Results?

Question from the Customer:

I am testing a USB 2.0 Standard-A to USB Type-C cable and I keep seeing the following error in the Advanced Cable Tester report:

  • Legacy Rp + Rd Down Check
Advanced Cable Tester Report showing resistor errors Advanced Cable Tester Report

What does this error mean and what causes it? Based on our design specifications, I believe this cable works properly, but this error keeps coming up.

Response from Technical Support:

Thanks for your question! The Advanced Cable Tester has built-in test profiles that are based on USB-IF compliance. Our Advanced Cable Tester has several preset profiles, and for special needs, supports custom profiles. One of the support cable types is USB Type-C to USB 3.1 Standard-A cables.

According to the USB specification, this cable type is required to have an Rp resistor. Based on the test results you showed us, your cable appears to have an Rd resistor instead. This is why you are seeing the test failure. If your cable intentionally does not adhere to the USB specification, custom test profiles can be created.

USB Type-C Cable Compliance and Standard Test Results

The termination resistors Rp and Rd and their switches are required by the USB specification. A cable may work correctly in one scenario, but if it does not adhere to the USB-IF standards, the standard test profiles will show error results. There are two examples:

  • Rp and Rd resistors are required for connection detection, plug orientation detection, and for establishing the USB source/sink roles.
  • Poor values against these parameters guarantee failure of the cable in optimal scenarios.

Other Considerations about Rp Test Results

There are other scenarios that could results in test errors. For example, the cable itself is not within the specification; the Rp may be on the wrong line of the cable.

  • For compliance, Rp must be on the same side as DPDM1.
  • If Rp is on the wrong side, even if the Rp value is within rage, its presence in the wrong place will result in an error: the sanity tests failed.

Additional resources that you may find helpful include the following:

If you have more questions about the Advanced Cable Tester v2 or other Total Phase products, please contact us at sales@totalphase.com. You can also request a demo that applies to your application

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