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I'm developing an USB driver for an embedded device, which appears to Windows as a CDC (virtual COM) device. I'm using Free Serial Port Monitor and Realterm to interact with the device.

Whenever I reset the device the COM port effectively disappears from the system, even though it may be "open" by these utilities. When the device is restarted the COM port re-appears in Device Manager, as expected. However, neither Free Serial Port Monitor or Realterm can find it. My suspicion is that the port is somehow locked.

I need a tool for observing any locks on COM ports and a way of unlocking the port, short of rebooting the computer, which I've been doing (very time consuming). Does such a utility exist?

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You can try Serial Port Monitor to observ any locks on COM ports and a way of unlocking the port. The utility enables monitoring, displaying, keeping logs and studying RS232/422/485 COM ports activity. This allows you to track down a problem if there is any; it is also extremely handy when you are setting up equipment. Also it's is frendly with Windows. (

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up vote 0 down vote accepted

Disabling the device in Device Manager, and then re-enabling it, freed the device for use.

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