COM Port 'Access Denied' Error in Mission Planner — Only Fix Is Rebooting

ArduCopter
1

Hi all,

I’m running into a persistent and incredibly frustrating issue with Mission Planner and COM ports. I consistently get this error when trying to connect to my flight controller:

Access to the port ‘COM6’ is denied.

at System.IO.Ports.InternalResources.WinIOError(Int32 errorCode, String str)
at System.IO.Ports.SerialStream..ctor(String portName, Int32 baudRate, Parity parity, Int32 dataBits, StopBits stopBits, Int32 readTimeout, Int32 writeTimeout, Handshake handshake, Boolean dtrEnable, Boolean rtsEnable, Boolean discardNull, Byte parityReplace)
at System.IO.Ports.SerialPort.Open()
at MissionPlanner.Comms.WinSerialPort.Open()
…

This happens even when:

  • Mission Planner is the only program open
  • No other cables or serial devices are connected
  • I’ve confirmed (a hundred times) that nothing else is using COM6
  • I change to a different USB Port on my computer

I can be working fine with my FC, then unplug it to reboot after changing parameters — and as soon as I plug it back in, I get the “Access Denied” error. The only reliable fix is restarting my entire computer, which is really annoying to do every time I disconnect my FC or relaunch Mission Planner.

Things I’ve tried:

  • Ensuring no background apps are running
  • Unplugging and replugging multiple times (rarely works)
  • Changing USB ports and cables
  • Checking Device Manager — COM port shows up fine
  • Closing everything else in task manager
  • Running Mission Planner as admin

At this point I’m out of ideas. Any suggestions for solving or debugging this would be hugely appreciated.

Thanks in advance.

2

Use this method to find which process is locking your serial port

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