I get transceiver errors with USB2 testplug (USB2+USB1.0 devices plugged to a HUB)

[claverie]

Hi,

Please forgive me if I am posting this into the wrong place.

I am currently measuring the performances of a USB connection integrated into a medical device. This integration is composed of 2 devices USB1.0 and one device USB2 connected to a HUB.

For this purpose I use the USB2 test plug with the associated test program.

Whenever I benchmark the pipe by replacing the USB2 device by the test plug, I get fairly good results (average of 320Mb/s). However, when I perform a verification test I can see many device transceiver errors (8 errors/packet transmitted!).

I am wondering why the bandwidht seems not affected by this situation. I have also read on this site that the following situation "Token larger than 3 bytes (this may occur when using the USB 2.0 Loopback plug behind a hub, with Low/Full speed devices connected to the hub)" can lead to transceiver errors to be reported.

Are these errors real or are they a normal behavior whenever a HUB mixes USB1 and USB2 data ?

I have noticed the following facts :
- Off-the-shelves products present the same behavior
- USBTrace is not seeing any transceiver errors in this situation
- If I disconnect all USB1 devices, the errors are gone

Thanks for you help and you clarifications,

best regards,

[Ian (PassMark)]

The PassMark USB 2.0 Loopback plug flags tokens greater than 3 bytes as possible errors (among a range of other errors). This is most likely a Token larger than 3 bytes indicator, a SPLIT, which may occur when using the USB 2.0 Loopback plug behind a hub, with Low/Full speed devices connected to the hub. It should be noted that device transceiver errors are not an indication that the USB port does not comply with the USB Specification. Rather that further investigation is requried.

I would suggest you try 2 tests,
(1) test the USB ports directly conencted to your system, and
(2) remove the USB 1.x devices from behind the hub and re-test.

If you don't get the problem in these scenarios then this is normal behavior when connecting USB 1.x and 2.0 devices behind a hub.

The USB 2.0 specification describes this:
"USB defines a special token for split transactions: SPLIT. This is a 4 byte token packet compared to other normal 3 byte token packets. The split transaction token packet provides additional transaction types with additional transaction specific information. The split transaction token is used to support split transactions between the host controller communicating with a hub operating at high speed with full-/low-speed devices to some of its downstream facing ports. There are two split transactions defined that use the SPLIT special

token: a start-split transaction (SSPLIT) and a complete-split transaction (CSPLIT). A field in the SPLIT special token, described in the following sections, indicates the specific split transaction.

"