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How Do I Connect a Beagle USB 480 Protocol Analyzer to an Embedded Circuit and Ensure Signal Integrity?

Question from the Customer:

I have a product with a Coldfire processor that is connected to a peripheral device via high-speed USB. The processor and the peripheral are on the same board and the USB lines are connected by copper traces. I want to monitor the USB traffic with our Beagle USB 480 Protocol Analyzer, but since the USB signal lines between the processor and peripheral device are in copper, I cannot insert the Beagle protocol in series. Instead, it looks like I have to tap into the USB signals in parallel. For signal integrity, I believe we need some series resistors, but I’m not sure what resistance values to use or where to use them.

Also, this circuit does not provide a VBUS. I understand the Beagle analyzer needs to detect 5V to recognize the embedded device – how can I work around that?

Response from Technical Support:

Thanks for your questions! We have a Knowledge Base article that answers both your questions with detail: Monitoring an embedded USB with a Beagle USB Protocol Analyzer.


 Monitoring an Embedded System with a Beagle USB 480 Protocol Analyzer Beagle480Ports-250

Figure 1: Embedded System

Figure 2: Beagle USB 480 Protocol Analyzer


Here is a summary of what you can do to monitor an embedded USB bus:

  • First of all, the D+/D- signal path of a USB does not have to be broken to be monitored by a Beagle USB protocol analyzer. The VBUS, GND, D+, and D- lines can be connected to either the Type A or Type B connector on the Beagle analyzer with T connections to the traces.
  • You will probably need to cut open a USB connector and connect the wires to the signal traces, or if they’re available, to a header or signal pads.
  • For the signal integrity, we recommended using 20 – 40 ohm resistors to the D+ and D- signals between the target and the Beagle USB 480 analyzer.


You are correct - the Beagle USB 480 analyzer does need to detect 5V on the VBUS to recognize the host. There are two ways to work around this issue:

Additional resources that you may find helpful include the following:

We hope this answers your questions. If you have other questions about our protocol analyzers or other 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.

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