How Do You Monitor and Analyze an Embedded USB Device Without Signal Degradation?

Question from the Customer:

I’m using the Beagle USB 480 Protocol Analyzer to troubleshoot an embedded device. What’s the best way to capture the signals?  The device is embedded in a system with a complicated cable setup. To avoid signal degradation, my thought is tapping off the D+/D- lines:

  1. Soldering wires to the D+/D- lines of my device cable
  2. Put a USB Type-A or USB Type-B connector on the end
  3. Plug that end into the analyzer

Do you think that would work, or is there a better solution?

Response from Technical Support:

Thanks for your question! It sounds like you’re on the right path – we have some recommendations to make sure it works for you.

The capture side of the Beagle USB 480 analyzer acts as a USB pass-through. The VBUS, GND, D+, and D- lines can be connected to either the Type-A or Type-B connectors on the Beagle analyzer using T-connections as shown below.

 Beagle USB 480 Power Protocol Analyzer Connect a Beagle Protocol Analyzer to an Embedded USB Bus on Copper Traces

Exactly how you connect to your device depends how the signals can be accessed. Most likely, you will need to cut open a USB cable. To maintain signal integrity, we recommend adding 20-40 Ohm series resistors to D+ and D- between the target system and the Beagle analyzer.  We also recommend keeping the cable as short as possible to maximize the signal.

Here’s a summary on connecting to the signal lines:

  • If the signal lines are easily accessible through a header or test pads, then connecting to it is straightforward.
  • If the signals are not easily accessible, the wires may need to be soldered directly to IC pins or copper on the PCB. If you need to do this, please be very careful and note that Total Phase cannot be held responsible if equipment is damaged.

For more information, such as whether or not you need to access the VBUS and additional information about signal integrity, please refer to our knowledge base article Monitoring an embedded USB with a Beagle USB Protocol Analyzer.

We hope this answers your question. Additional resources that you may find helpful include the following:

If you have questions about our Total Phase products, feel free to email us at sales@totalphase.com, or if you already own one of our devices and have a technical question, please submit a request for technical support.