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I recently changed my port number for the arduino USB from COM8 (what it defaulted with) to COM256 (attempting to see if that will work).

However, whenever I try to upload it gives

'avrdude: ser_open(): can't open device "\.\COM256": The system cannot find the file specified.'

Is there a way to fix this or do I reset the port back to COM8?

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  • 3
    What operating system? How did you change the port number? Why did you do this? Commented Aug 17, 2024 at 6:56
  • why do you say that post 8 is standard? ... that would be COM8, not just 8 Commented Aug 17, 2024 at 15:26
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    I did not say that port 8 is standard. I just said that it is what the computer assigned when I plugged in the Arduino. Commented Oct 7, 2024 at 21:53

1 Answer 1

Reset to default
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In short, disconnect and reconnect the board after you change the COM number.

I tested with Windows 10. But this part of Windows has not changed is a very long time.

In testing I changed an UNO R3 to port 256 through Device manager, device properties, Port Settings, Advanced..., COM Port Number and it gave the error mentioned when trying to upload.

Only changing the port number down to 99, 31, and 9 made no difference. However, physically disconnecting and reconnecting the board after each instance of changing the COM number allowed it to upload again, even when that was COM256.

I haven't looked into why reconnecting the board matters where Device Manager listing reflected the change immediately. But I'm also not surprised.

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  • I reuploaded it and it works! I kind of did what you suggested. Commented Aug 19, 2024 at 0:41

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