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Why Do I See Phantom Transactions When I am Using a 7-Port USB Hub with the Beagle USB 480 Protocol Analyzer?

Question from the Customer:

I am using the Beagle USB 480 Protocol Analyzer. Normally I have no problem, but I don’t understand the readings with this setup, which includes a 7-port USB hub as shown below.

 Layout for Beagle Protocol Analyzer, USB Hub and Target Device
Figure 1: Block Diagram of the Setup with the USB Hub

The transfers with device address 05 do not seem appropriate here, as the device under tracing has the bus address of 06. As device 05 shows LS transfers, it appears that the USB keyboard and the mouse are also connected through the same hub. Also, I’m seeing Collapsed Orphaned errors on the Data Center Software screen and the tdc file shows “phantom transactions”. Can you tell me why I am getting these readings?

I am using basic settings with the Data Center Software.

Device settings:

  • USB 2.0 Bus Speed:  Auto Detect
  • USB 2.0 Capture Mode: Real-time

Capture Mode:

  • Aggregate

Response from Technical Support:

Thanks for your question! Orphaned packets can occur when a device is connected through a path that does not pass through the Beagle USB 480 analyzer, such as shown in the figure of your setup. The Beagle USB 480 analyzer and the Data Center Software mark these indexes as errors (Collapsed Orphan), as a notification for the developer to look at these indexes.

 Using the USB 480 Protocol Analyzer to capture USB 2.0 bus data

Figure 2: Beagle USB 480 Protocol Analyzer


About your  phantom transactions reported in the tdc data file - when you connect a low-speed USB keyboard or mouse to the USB hub, which can be internal or external to the computer, the host sends packets (Dev 05) to the keyboard through the hub.  When the hub gets the packets from the host, the hub retransmits these packets downstream to all the devices that are connected to the hub, including the Beagle USB 480 analyzer and the target device of your setup - the USB audio board.

Due to the broadcast nature of USB, all devices downstream from the host see the packets transmitted by the host.  However, upstream traffic is only seen by devices directly in the line of data communication between the host and device, in this case the keyboard, hub, and host.  In this set up the Beagle USB 480 analyzer is not in the path between the keyboard and host.  Thus, the Beagle USB 480 analyzer sees the downstream packets from the host (phantom transactions) but does not see the response from the keyboard resulting in orphaned packets.  For more details, please refer to the USB Device Settings section of the Data Center Software User Manual, and to chapters 8 and 11 of the USB 2.0 Specification document.

Additional resources that you may find helpful include the following:

We hope this answers your question. 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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