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Windows application fails to open virtual COM port for a USB device RRS feed

  • Question

  • We designed a micro-controller based embedded device with USB device stack with CDC ACM class. Using Windows 7 as host. I created an inf file with usbser.sys. When board is connected to a PC, Win 7 device manager enumerates the board as a COM port. But when application tries to open COMx port, I see from USB analyzer that CDC class related baudrate/handshake related messages are sent from PC and embedded device sends response. But win 7 application reports 'semaphore timeout' error. I dont need baudrate negotiation or other handshake messages. I tried to set hardware/software handshake off when COM port is opened. Still Windows keeps sending. Is there a way not to send those messages from PC and move to data transfer. Failure is not consistent and sometimes it open port successfully and communication works well. I think there was some timing issue device is not doing correctly. Any help appreciated to either bypass CDC handshake message or to give more time or some hint to fix on device. I am not doing a modem type device and I want to use simple virtual com port. Thanks.
    Tuesday, July 1, 2014 4:39 PM

Answers

  • just accept the messages and acknowledge them appropriately (essentially lie about changing the setting)

    d -- This posting is provided "AS IS" with no warranties, and confers no rights.

    Tuesday, July 1, 2014 6:03 PM

All replies

  • just accept the messages and acknowledge them appropriately (essentially lie about changing the setting)

    d -- This posting is provided "AS IS" with no warranties, and confers no rights.

    Tuesday, July 1, 2014 6:03 PM
  • Thanks Doron. I am trying to lie but there is something wrong that Vista or Win 7 usbser.sys dont like - either something in the content or I believe timing issue. This worked with Linux PC, I was told. So I am just trying to see if I can bypass that stage I dont really need. It works sometimes but most of the times it wont work - says 'semaphore timeout' when opening com port. It waits for many seconds before error. So difficult to believe device not having time to respond. I have control over device software and win 7 application. I appreciate any hints on what could go wrong at CDC level packets. 
    Tuesday, July 1, 2014 8:13 PM
  • I didn't find any Microsoft documentation on this except telling to follow USB standards. Any details on usbser.sys I missed.
    Tuesday, July 1, 2014 8:15 PM