USB to RS232 device

[vsharma]

Hi Jan,

I have a USB to RS232 connected to a high speed hub with single TT.
After the USB-RS232 device is enumerated, I notice the "Get String Descriptor" with Product and Serial Number of the USB-RS232 device sent out repeatedly every few milli seconds.

Question: Who and why are these "Get String Descriptor" messages sent?

Also, I notice that when the application opens the USB-RS232 COM port, there is a polling frame seen on in the USB traces. These frames are seen almost every milli second.
What is the reason for these polling messages? Could the messages cause bandwidth degradation on the USB bus or have any adverse affect to other devices connected to the HS USB hub?

Please help.

Thank you.

[Jan Axelson]

The host driver is sending the message to try to retrieve the product and serial number strings. The device (adapter) should either return the strings or return STALL to indicate it doesn't support them. If the device just NAKs the data stage, it's misbehaving but causing no real harm.

The host driver polls the device so it can retrieve any data or status information the device decides to send to the host. The polling has minimal effect on bandwidth.

[vsharma]

Thank you for a quick response.

I have a Citizen printer which is a full speed device connected to the hub as well.
From our application, we are polling the printer for status every second.
We see a response from the printer and at the same time we also see a ResetTT message on the port=1 being sent from host to the device.

What is the meaning of the ResetTT?

Thanks,

[Jan Axelson]

RESETTT, also called RESET_TT, is a USB 2.0 hub-class request. See 11.24.2.9 in the USB 2.0 spec.

To answer what will probably be your next question, I don't know why the host is feeling the need to issue this request, but if the attached devices are working properly, I wouldn't worry about it.