USB 2.0 Loopback Plug transceiver errors

[bigfoot]

Hi,

my company purchased the USB 2.0 Loopback Plug (Firmware v2), and I ran some tests using the USB2Test (v1.1 (1000)) tool.

In some device configurations I observed lots of "device transceiver errors" I don't understand showing up:

* Loopback Plug is connected to root hub.
* External USB 2.0 Hub is connected to root hub.
* USB mouse is connected to external USB 2.0 hub.

This leads to a ~2:1 ratio "transceiver errors" : "operations" when running the "Loopback" test with 100% load.

Where do these errors come from? I use shielded USB 2.0 (full/high speed) cables; communication with the mouse (low-speed INT transfers) supposedly runs via USB 2.0 split transactions to the external hub, which I don't suspect of generating all these errors (should I?).

Kind regards,
bigfoot (Horst Schirmeier)

[Ian (PassMark)]

"Device transceiver errors" are triggered when the USB transceiver on the USB 2.0 loopback plug detects what it considers to be an error. These are events at a low level that may lead to the packet being retransmitted, or they may not. They do not represent data errors at the application level. These errors are normally not visible to the user, but are displayed to help identify potential problems, for example, poor quality cables, cables that are too long, or system internals with insufficient electrical shielding with high electrical interference on the bus. Errors that may cause a device transceiver error are:
o Bad PID
o CRC error
o Bit stuff error
o Extra bits in a packet
o Full Speed EOP ending on K
o Loss of high-speed valid before EOP (truncated packet)
o Overflow (host clock running too fast or device clock running too slow)
o Token larger than 3 bytes
It should be noted that device transceiver errors are not an indication that the USB port does not comply with the USB Specification.

In many cases they are not errors as such, however we report them as high levels like you are seeing may warrant further investigation. A typical reason for this is poor quality cables, internal as well as external. It is possible that another USB device on the bus (like a mouse) could trigger the USB chip we use to flag an error.

I would repeat the test without anything else on the bus, and add devices incrementally to determine the difference.

It should be noted that unless there are "Verification errors" as well, a "device transceiver error" should not be considered serious - but rather just that as an indication that you might like to consider further investigation - like changing cables or using one of the (really expensive) analyzers.

Best regards,
Ian